NUREG-2170

A Risk-Informed Approach to 

Understanding Human Error 

in Radiation Therapy

Office of Nuclear Regulatory Research

NUREG-2170

A Risk-Informed Approach to 

Understanding Human Error 

in Radiation Therapy

Office of Nuclear Regulatory Research

Manuscript Completed: September 2016

Date Published: June 2017 

Prepared by:

J. Wreathall1, W.S. Brown2, L. Militello3, S.E. Cooper4

C. Lopez3, C. Franklin4

1The WreathWood Group

2Brookhaven National Laboratory

3Applied Decision Science, LLC

4U.S. Nuclear Regulatory Commission

S.E. Cooper, NRC Project Manager

iii

ABSTRACT

A critical part of the U.S. Nuclear Regulatory Commission (NRC) mission is licensing and 

regulating the use of byproduct materials, including in radiation therapy. More than half of the 

reported misadministration events in the medical use of byproduct materials in radiation therapy 

are attributed to human error.

In support of the NRC recommendation to continue to apply and extend a risk-management 

framework (as discussed in NUREG-2150 [1], A Proposed Risk Management Regulatory 

Framework), this report discusses an approach to improve patient safety that is based on human 

reliability analysis (HRA), traditionally part of probabilistic risk assessments (PRAs). Using HRA 

and related disciplines, four key elements for improving patient safety are highlighted: 1) improved 

understanding of human error in radiation therapy; 2) improved ability to anticipate errors;

3) effective strategies for supporting humans in managing complexity; and 4) methods to evaluate 

and monitor the effectiveness of corrective actions. This report also includes reviews of studies 

conducted by the NRC using human reliability analysis methods and recommends the continued 

application of these methods. Successful safety improvements from commercial aviation and 

nuclear power industries are examined and offered as an example of the feasibility of this 

approach. It is important to emphasize that target audiences for this document include, but are not 

limited to, decision makers, regulators, designers, and external vendors engaging with the 

risk-informed framework.

v

FOREWORD

It is widely recognized that human error is an important contributor to significant events for a 

variety of complex technologies. Studies of various technologies have examined the accident 

record and developed estimates for the contribution from human error. For commercial aviation, 

industrial applications, and nuclear power, these estimates typically range from 70 percent to 

80 percent. Among the complex technologies that the U.S. Nuclear Regulatory Commission 

(NRC) regulates are commercial nuclear power and the use of byproduct materials in radiation 

therapy. As such, the human error contributions for these two technologies have been of particular 

interest to the NRC. 

For commercial nuclear power, the 1979 event at Three Mile Island Unit 2 led to dramatic 

changes, including an intensified concern for how nuclear power plant (NPP) operators are 

supported in responding to NPP events. Responding to concerns about the safety of commercial 

nuclear power, the NRC implemented new requirements, including improvements in control room 

design, emergency operating procedures, and operator training. To support this effort, the NRC 

needed human factors engineering staff to review and inspect such changes. Also, as the NRC 

transitioned to a risk-informed agency, research and development has occurred in the area of 

human reliability analysis (HRA) (which incorporates understanding from multiple disciplines, 

including human factors engineering and psychology) in order to support broader scope 

probabilistic risk assessment (PRA) studies. Consequently, the NRC is widely recognized as one 

of the leaders in identifying and addressing human error for commercial nuclear power.

The history for radiation therapy has been different. Although notable events have occurred (for 

example, the overdose of a patient at Indiana, PA, in 1992 [2]) or the widely publicized series of 

misadministration’s of radioactive iodine seeds for treatment of prostate cancer over several years 

at the Philadelphia Veteran’s Administration Medical Center [3], with increasing mention of 

"human error" in conferences, journal papers, and workshops, it is clear that the need for attention 

to the issue has registered within the healthcare community. However, as evidenced by the 

Institute of Medicine’s (IOM’s) report in 2000, To Err is Human [4], and its follow-up in 2012 [5], 

the improvement in quality in much of the health care system is “still proceeding at a glacial pace.”

This report lays out simply and clearly, for both experts and non-specialists, how to understand 

human error and anticipate future errors. It summarizes the NRC research to date on 

understanding radiation therapy from an HRA and human factors engineering perspective, and 

identifies unique and critical differences between therapeutic uses of radioactive materials and 

other complex technologies that are important to reducing human error. This report also aims to 

identify different approaches in other countries such as France where previous accidents led to 

very large scale consequences for patients. In France, the reports show how the French regulator 

has undertaken collaboration with radiotherapy professionals to improve the management of the 

safety and quality of the care given to patients. Finally, this report provides risk-informed 

strategies and recommendations, building on the success stories in commercial nuclear power 

and other industries, for effective human error reduction in radiation therapy.

Again, based on past experience in several industries and with several technologies and 

endeavors (e.g., NASA, commercial aviation, nuclear power, Yucca Mountain waste repository, 

chemical processing), it is known that implementing a risk-informed framework or regulatory

strategy comes with challenges. In particular, the authors are familiar with the learning curve for 

effective technology transfer between HRA/PRA experts and experts working in specific applied 

technologies. However, the NRC staff knows how to highlight the benefits of using a risk-informed 
vi

process that can make existing jobs (e.g., designing, regulating, and using licensed equipment 

and associated procedures) more effective and efficient.

Because the licensing and regulatory responsibilities for radiation therapy are distributed among 

multiple agencies and organizations (of which the NRC is only one), the challenges are likely to be 

greater than those typical of other industries or technologies that have transitioned to a risk 

informed perspective. However, the success stories in human error reduction summarized in this 

report are evidence that the effort and resources definitely can provide a positive return.

Consequently, it is the staff’s sincere hope that those of us in the HRA and PRA disciplines will be 

called on to help make the next steps in reducing human errors in radiation therapy.

Richard Correia

Director, Division of Risk Analysis

Office of Nuclear Regulatory Research
vii

TABLE OF CONTENTS

ABSTRACT............................................................................................................................... iii

FOREWORD ..............................................................................................................................v

TABLE OF CONTENTS ........................................................................................................... vii

LIST OF FIGURES ....................................................................................................................ix

LIST OF TABLES......................................................................................................................ix

EXECUTIVE SUMMARY ...........................................................................................................xi

ACKNOWLEDGMENTS...........................................................................................................xv

ACRONYMS............................................................................................................................xvii

1 INTRODUCTION....................................................................................................................1

1.1 Why Human Error is a Problem .........................................................................................1

1.2 Addressing Human Error....................................................................................................4

1.3 The Role of The NRC ........................................................................................................6

1.4 How This Report Develops an Understanding of Human Error .........................................7

1.5 Organization of This Report ...............................................................................................7

2 SIGNIFICANCE OF HUMAN ERROR IN RADIATION THERAPY.........................................9

2.1 The Age of Human Error....................................................................................................9

2.2 NRC Studies of Error in Radiation Therapy .....................................................................11

2.2.1 Overview of NRC Research in Radiation Therapy: The 1990s 

to the Present Day...............................................................................................11

2.2.2 Summary of Insights from Early NRC Research in Radiation Therapy................12

2.3 Examining Trends: Current Research..............................................................................14

2.4 Summary..........................................................................................................................17

3 PERSPECTIVE FOR UNDERSTANDING HUMAN ERROR AND 

ANTICIPATING FUTURE ERRORS ....................................................................................19

3.1 Understanding human capabilities ...................................................................................19

3.1.1 Human Error is NOT random ...............................................................................19

3.1.2 Human Error is Not the “Cause” of a Mishap.......................................................19

3.1.3 Human Error can be Predicted.............................................................................20

3.2 Identifying System Vulnerabilities ....................................................................................21

3.3 Summary..........................................................................................................................22

4 SUPPORTING HUMANS IN MANAGING COMPLEXITY....................................................23

4.1 “Black Box” Aspects of Automation..................................................................................23

4.2 Repetition and Automaticity .............................................................................................24

4.3 User Interfaces: Mode Errors...........................................................................................25

4.4 User Interfaces: Data Entry..............................................................................................26

4.5 Independent Verification for Noticing Errors ....................................................................26

4.6 Prospective Memory ........................................................................................................28

4.7 Summary..........................................................................................................................29
viii

5 SUCCESS STORIES IN USING A RISK PERSPECTIVE ON HUMAN 

ERROR: ARE THE CORRECTIVE ACTIONS WORKING?.................................................31

5.1 Improvements in Managing the Contributions of Human Error to Risk ............................31

5.2 Improving the Effectiveness of Error and Risk Reduction................................................34

5.3 Significance for Reducing the Risks from Human Error in Radiation Therapy.................36

5.4 Safety Developments from the Nuclear Safety Authority in France .................................36

5.4.1 Why Notifying the Regulator is Important.............................................................36

5.4.2 The Notification Approach in France....................................................................36

5.4.3 The Notification Criteria........................................................................................37

5.5 Summary..........................................................................................................................37

6 HUMAN RELIABILITY ANALYSIS APPROACH TO RISK MANAGEMENT 

IN THE MEDICAL USES OF BYPRODUCT MATERIALS...................................................39

6.1 Overall Approach to Risk Management ...........................................................................39

6.2 Framework for Risk Management for Patient Safety .......................................................40

6.3 Recommendations Summarized......................................................................................43

7 SUMMARY AND CONCLUSIONS.......................................................................................45

8 REFERENCES.....................................................................................................................47

APPENDIX A ROOT CAUSE ANALYSIS .........................................................................A-1

APPENDIX B ERRORS IN REMOTE AFTERLOADING BRACHYTHERAPY ..................B-1

APPENDIX C ERRORS IN TELETHERAPY .....................................................................C-1

APPENDIX D ANALYSIS OF RADIATION THERAPY EVENTS FROM 2001 

THROUGH 2010: ERROR CATEGORIES AND SAMPLE EVENTS...........D-1

APPENDIX E SUMMARIES OF EXAMPLE ABNORMAL EVENTS..................................E-1

APPENDIX F PROCEDURAL EVENT ANALYSIS TOOL (PEAT) FOR 

UNDERSTANDING HUMAN ERROR AND IMPROVING 

PERFORMANCE......................................................................................................F-1

APPENDIX G REFERENCES FOR APPENDICES ...........................................................G-1
ix

LIST OF FIGURES

Figure 1-1 Summary of an analysis from The New York Times .....................................................3

Figure 1-2 Four key elements for improving safety ........................................................................4

Figure 2-1 Causal factors events reported in NMED’s second quarterly report for FY 2013..........9

Figure 2-2 Early depiction of the Swiss Cheese Model ................................................................10

Figure 2-3 Later version illustrating organizational influences on defenses in depth....................10

Figure 2-4 Timeline of NRC studies of human error in radiation therapy events..........................12

Figure 2-5 The model of sustainability and effectiveness of corrective actions ............................17

Figure 5-1 Safety performance in the aviation industry ................................................................32

Figure 5-2 Rates of significant events at U.S. commercial nuclear power plants.........................33

Figure 5-3 Performance evaluation ..............................................................................................34

Figure 5-4 Double loop learning ...................................................................................................35

Figure 6-1 Generic form of risk-informed decision making...........................................................42

Figure A-1 Root causes for all of The Joint Commission sentinel events, 2010 through

2012, 2nd quarter......................................................................................................A-5

Figure A-2 Root causes for The Joint Commission sentinel events involving 

radiation overdoses...................................................................................................A-5

Figure A-3 Causes assigned to Abnormal Events from FY2008 through FY2011......................A-7

Figure F-1 The Boeing PEAT Process for Aviation Errors in Using Procedures ........................F-2

Figure F-2 The Model of Human Performance used in Boeing’s PEAT Process .......................F-2

LIST OF TABLES

Table 2-1 Summary of error distributions for each treatment modality......................................15

Table A-1 The Joint Commission’s definition of a sentinel event.............................................A-2

Table A-2 The Joint Commission’s categories of root causes and sentinel events .................A-3

Table A-3 NRC criteria for a reportable event..........................................................................A-6

Table B-1 Contributing and mitigating factors for error in RAB from NUREG/CR 6125...........B-1

Table C-1 Contributing and Mitigating Factors for Error in Teletherapy from 

NUREG/CR 6277 ....................................................................................................C-1

xi

EXECUTIVE SUMMARY

The mission of the U.S. Nuclear Regulatory Commission (NRC), quoted from NUREG-2150 [1], is 

to “License and regulate the Nation’s civilian use of byproduct, source, and special nuclear 

materials to ensure adequate protection of public health and safety, promote the common defense 

and security, and protect the environment.” A critical part of this mission is licensing and regulating 

the medical use of byproduct materials, including in radiation therapy. This report is intended to 

support NRC staff in understanding the role of human error in the medical use of byproduct

materials. The overall goals are to highlight the importance of the NRC role in healthcare and to 

provide a human reliability analysis (HRA) framework for characterizing human error and 

developing meaningful corrective actions in a healthcare context.

A report published by the Institute of Medicine 

in 2000, To Err is Human [4], brought national 

attention to patient safety and medical errors,

reporting that between 44,000 and 

98,000 people die each year as a result of 

preventable medical errors.

Unfortunately, in spite of efforts to reduce the 

number of medical errors and reduce risk 

factors, the Institute of Medicine’s more recent 

report [5] noted that, while some progress has 

been made in specific processes (e.g., surgical 

mortality rates), the improvement in quality in 

much of the health care system is “still 

proceeding at a glacial pace.”

In fact, a new estimate of the number of 

preventable adverse events (i.e., harm resulting 

from an error) leading to premature death put the lower bound (based on medical records) at 

210,000 per year and suggested a minimum two-fold adjustment be applied to that number “to 

compensate for the known absence in the medical records of errors of commission and the 

inability... to detect errors of omission.” [6, p. 127]

Examination of radiation therapy where the NRC has oversight responsibility reveals similar 

themes. Bogdanich [7] authored a series of articles examining radiation therapy errors between 

2001 and 2009, highlighting a troubling recurrence of similar errors over time and across events, 

including miscalculations and misuse of positioning equipment resulting in wrong dosages and 

wrong sites. While many of the events reviewed by Bogdanich did not involve the same specific 

modalities of treatment that are the scope of the NRC’s regulatory actions, many of the underlying 

causes are similar.

In this report, the authors outline an HRA perspective on radiation therapy and propose four related 

elements for improving safety: 1) improved understanding of human error in radiation therapy; 

2) improved ability to anticipate errors; 3) effective strategies for supporting humans in managing 

complexity; and 4) methods to evaluate and monitor the effectiveness of corrective actions.

The illustration above shows these four elements as a progression in which, for example, 

understanding human error in radiation therapy is required in order to anticipate future problems.
xii

Each of these elements is discussed below, with a mention of which report section provides the 

supporting discussion.

Significance of human error in radiation therapy

As shown in the figure below, more than half of the reported misadministration events in the 

medical use of byproduct materials in radiation therapy are attributed to human error (in data from 

the Nuclear Material Event Database). In spite of the prevalence of human error as a cause, 

investigations into contributing contextual factors and trends in error events over time and across 

facilities are limited. The NRC, however, has conducted a series of studies that provide:

• important examples of HRA in practice, including examination of error trends in 

misadministrations [8, 9, 10, 11];

• strategies for disseminating an HRA perspective [12, 13]; and

• an examination of the effectiveness of corrective actions [14].

Understanding human error and anticipating future errors

Section 2 presents a summary of these studies. Each represents important foundational research 

illustrating HRA methods. The authors contend that an extension of this foundational research 

would lead to improved safety in radiation therapy.

Section 3 focuses on an improved ability to anticipate errors. HRA integrates multiple disciplines 

(including human factors and ergonomics, cognitive and behavioral science, engineering, design, 

and operations) and provides a risk focus by virtue of its connection to probabilistic risk 

assessment (PRA). Understanding human capabilities provides insight into predictable aspects of 

human performance, as well as system vulnerabilities that are likely to lead to error. The practical 

application of this perspective is illustrated by a study and associated paper that reviewed

misadministration events to identify risk significant human actions [12].

Looking across these incidents, researchers were able to identify specific circumstances likely to 

result in misadministration, overexposure of workers to radiation, or exposure of members of the 

public to radiation.
xiii

Supporting humans in managing complexity

Effective strategies for supporting humans in managing complexity are discussed in Section 4.

Complexity and error are inherent in healthcare and in many of its processes of treatment. Rather 

than focusing on eliminating error, HRA seeks to identify common features of error events and to 

examine trends for aspects of the system or the environment that increase risk in a predictable 

way. This knowledge also highlights leverage points for designing systems and technology to 

support humans in managing complexity. The authors began with an examination of radiation 

therapy events and then integrated these findings with vulnerabilities identified in other industries.

Six aspects that increase the risk for error are discussed, namely:

1) “Black box” aspects of automation,

2) Repetition and automaticity,

3) The role of user interfaces in mode errors,

4) The role of user interfaces in data entry errors,

5) Independent verification for noticing and correcting errors, and

6) Prospective memory demands.

For each aspect, the authors describe the vulnerability or increased risk and also discuss 

strategies for better supporting human capabilities and thereby reducing risk. In addition, risk 

measures, through the HRA approach proposed, allow different strategies to be prioritized by the 

amount of risk contribution or reduction.

Success stories: Are the corrective actions working?

Application of an HRA or a risk perspective has proved successful in many industries, including civil 

aviation and commercial nuclear power. In Section 5, the authors summarize the dramatic success 

these industries have had in reducing error, in spite of estimates indicating that human error 

contributes to between 70 percent and 80 percent of civil and military aviation incidents [15] and that 

similar rates of human error contribution to adverse events occur in the nuclear power industry [16].

One of the key lessons learned from the remarkable success of these industries is the need to 

constantly monitor and evaluate the effectiveness of corrective actions. It is not enough to 

implement corrective actions based on accident analysis. It is only after the corrective actions have 

been in place that the positive and/or negative impacts of the change can be assessed.

A human reliability approach to risk-management

NUREG-2150 [1] includes the NRC’s recommendation to continue to extend and apply a riskmanagement framework to the oversight and regulation of medical use of byproduct materials. The 

efforts described in this report are consistent with the risk-management framework presented in 

NUREG-2150. Although challenges remain, the HRA perspective and methods detailed in this report, 

especially those noted in Section 6, are important components of a risk-management framework.

Summary and conclusions

Error will occur despite best intentions, careful design, and skilled personnel. From an HRA point 

of view, there is a dual aim: to reduce the likelihood of error and to increase the likelihood of 

recovery from error before negative consequences occur. HRA offers:

1) A perspective for understanding human capabilities,

2) Strategies for identifying system vulnerabilities or “error traps,”
xiv

3) Methods for analyzing errors when they do occur, and

4) Recommendations for developing effective corrective actions.

An HRA approach also advocates a process in which an organization deliberately articulates 

safety-related goals and evaluates corrective actions to determine whether they are, in fact, 

contributing to increased safety.

The NRC has been conducting human reliability studies about the use of byproduct materials in 

radiation therapy since the early 1990s. Transitioning from research to operations is not trivial in 

any industry, and perhaps more challenging in healthcare than in many others. For example, the 

NRC’s scope of regulations is limited to the use of byproduct materials in medicine, while the U.S. 

Food and Drug Administration (FDA) regulates the design of medical devices. Additionally, it is 

the NRC's policy not to intrude into medical judgments affecting patients, except as necessary to 

provide for the radiation safety of workers and the general public. In spite of these and other 

challenges, the remarkable success in other complex industries such as aviation and commercial 

nuclear power are powerful examples of the feasibility of this approach.

The authors offer two recommendations to better incorporate a human reliability approach in the 

use of byproduct materials in radiation therapy.

1. Human errors should be recognized as important contributors to radiation therapy events. 

Strategies for reducing such errors are best addressed by a combination of disciplines, 

including the medical and health physics fields, HRA, human factors, and cognitive 

science. In particular, HRA can be used to integrate inputs from multiple disciplines and 

provide a risk-informed perspective, allowing prioritization of potential fixes and balancing 

of competing needs.

2. Historical experience in addressing human errors in technologies such as commercial 

nuclear power and aviation has shown that, while there are multiple approaches to 

incorporating “human factors” into an error reduction effort, not all approaches are equally 

effective. The NRC has in-house experience on these “more effective approaches” 

(predominantly in the Offices of Nuclear Regulatory Research and Nuclear Reactor 

Regulation) that could assist or guide human error reduction efforts for radiation therapy.
xv

ACKNOWLEDGMENTS

The authors acknowledge the many colleagues and subject matter experts who have made 

valuable contributions to this report. The authors are especially indebted to:

• The technical advisors in the NRC’s Office of Nuclear Material Safety and Safeguards 

(including those in the former Office of Federal and State Materials and Environmental 

Management Programs (FSME)) who participated in an extensive technology exchange and 

helped us to gain an understanding of radiation therapy, namely Dr. Donna-Beth Howe, 

Ronald Zelac (retired), Robert Ayres (deceased), Cynthia Flannery, and Dr. Patricia Rathbun 

(retired). Also, FSME management (past and present) for working closely with us to develop a 

substantive User Need Request and providing staff support during times of critical staff 

shortages, including Christian Einberg and Sandra Wastler.

• Two members of the medical field who are advocates for a systems or risk-informed 

perspective for reducing human error: 1) Rollin J. “Terry” Fairbanks, M.D., Director of the 

National Center for Human Factors in Healthcare and of the Simulation Training and 

Education Laboratory (MedStar SiTeL) as well as a professor and specialist in emergency 

medicine, and 2) Robert L. Wears, M.D., professor and specialist in emergency medicine.

• Colleagues in the NRC’s Office of Nuclear Regulatory Research (RES) who have provided 

continuous interest, review comments, and a sympathetic audience during times of difficult 

technical problems, especially Stephen Fleger. While with the NRC, Julie Marble (now with 

John Hopkins) was invaluable in helping to get this most recent research started. In addition, 

Dan Hudson provided much needed project management support when this most recent 

project was initiated.

• NRC RES management (past and present) who helped this project maintain some visibility 

and importance, including Christiana Lui, Richard Correia, Scott Newberry (retired), Jose 

Ibarra (retired), and Sean Peters.

• Anthony Delamotte, a French ASN (Nuclear Safety Authority) nuclear safety and radiation 

protection engineer assigned to the NRC, provided important input on how French and other 

European regulatory agencies are addressing human error in radiation therapy.

• The Brookhaven National Laboratory’s (BNL’s) Principal Investigator throughout the effort, 

John Lehner, for facilitating BNL staff and contractor support and contract management.

xvii

ACRONYMS

AO abnormal occurrence

ASEP Accident Sequence Evaluation Program

ASN Autorité de sûreté nucléaire or Nuclear Safety Authority

ATHEANA A Technique for Human Event ANAlysis

CAST Commercial Aviation Safety Team

DRF Dose rate factor

FDA U.S. Food and Drug Administration

GSR gamma stereotactic radiosurgery

HDR high dose rate (brachytherapy)

HRA human reliability analysis

IATA International Air Transport Association

ICAO International Civil Aviation Organization

INPO Institute of Nuclear Power Operations

IV intravascular (brachytherapy)

LDR low dose rate (brachytherapy)

NMED Nuclear Material Events Database

NRC U.S. Nuclear Regulatory Commission

PEAT Procedure Event Analysis Tool

PRA probabilistic risk assessment

RAB remote afterloading brachytherapy

RIDM Risk-informed decision making

RIRIP Risk-Informed Regulation Implementation Plan

RPP Risk-Informed and Performance Based Plan

THERP Technique for Human Error Rate Prediction

1

1 INTRODUCTION

A critical part of the U.S. Nuclear Regulatory Commission’s (NRC’s) mission is to license and 

regulate the medical use of byproduct materials, including in radiation therapy.1 This is a key 

component of the larger mission to “license and regulate the Nation’s civilian use of byproduct, 

source, and special nuclear materials to ensure adequate protection of public health and safety, 

promote the common defense and security, and protect the environment” [1, p. 2-1]. In fact, safety 

in the context of the medical use of byproduct materials raises challenges not found in the more 

predictable environment of nuclear power. The goal of this report is to support NRC personnel 

(both staff and management) in understanding the role of human error in the medical use of 

byproduct materials. In particular, the aim is to highlight the importance of the NRC’s role in 

healthcare and to provide a framework for characterizing human error and developing 

risk-informed corrective actions in a healthcare context to reduce the likelihood of error.

1.1 Why Human Error is a Problem

The report published in 2000 by the Institute of Medicine entitled To Err is Human: Building a 

Safer Health System brought national attention to patient safety and medical errors [4]. The report 

estimated that between 44,000 and 98,000 people die each year as a result of preventable 

medical errors and set ambitious goals for reductions in the number of errors. Among the 

strategies recommended by the report were 1) creating an environment that encourages 

organizations to identify errors, evaluate causes, and take appropriate actions to improve 

performance in the future, and 2) applying what has been learned in other industries about ways 

to prevent error.

Unfortunately, in spite of efforts to reduce the number of medical errors and reduce risk factors, 

the Institute of Medicine’s more recent follow-up report [5] noted that, while some progress has 

been made in specific processes (e.g., surgical mortality rates), the improvement in quality in 

much of the health care system is “still proceeding at a glacial pace.” In fact, a new estimate [6] of 

the number of preventable adverse events leading to premature death put the lower bound (based 

on medical records) at 210,000 per year and suggested a minimum two-fold adjustment be 

applied to that number “to compensate for the known absence in the medical records of errors of 

commission and the inability... to detect errors of omission.”

In 2009, medical errors involving a treatment modality regulated by the NRC became front-page 

news when The New York Times reported on events at the Veterans Affairs Medical Center in 

Philadelphia [3]. During a period of six years, prostate cancer patients who were prescribed low 

dose rate (LDR) brachytherapy treatment received substandard treatment. The treatment involves 

implanting small radioactive seeds into the prostate to treat the cancer. Ninety-seven of the 116 

treatments completed during the timeframe in question resulted in medical events2 due to 

incorrect placement of seeds. The incorrectly placed seeds resulted in higher than expected does 

to organs and tissues outside the area the physician intended to treat, including healthy organs 

such as the bladder and rectum. An investigation by the NRC found that a serious lack of safety 

culture at the facility allowed the medical events to continue for so long. Normal safety measures, 

 1 The NRC is responsible for licensing and regulating the use of byproduct materials (e.g., materials made radioactive in 

a reactor). The NRC has no authority in radiation therapy involving accelerators.

2 The term "misadministrations" was changed to "medical events" in 2002. This report uses "misadministrations" only 

when describing events prior to 2002.
2 

such as peer review and regular use of equipment to measure whether patients received the 

appropriate dose, were not in place at the Philadelphia facility. 

A subsequent investigation undertaken by The New York Times [17] used records from the New 

York State Department of Health (which monitors radiation therapy) to examine the prevalence and 

causes of errors. The analysis revealed that between January 2001 and January 2009, there were 

621 mistakes in radiation therapy (both source and accelerator based) in the State of New York.

Most mistakes had two or more causes (see Figure 1-1). Errors included wrong dosages or even 

the wrong patient treated. The identified causes included miscalculations and misuse of positioning 

equipment. The recurrence of similar errors over time and across events is particularly troubling.

Leape, who is considered the founding father of the study of medical errors and the leader of the 

specific study that developed the data quoted in the first Institute of Medicine (IOM) report, 

published an important journal paper [18] in which he points out that “errors are usually 

discovered only when there is an incident—an untoward effect or injury to the patient,” and that, 

when they do come to light, the corrective measures are likely to be “directed toward preventing a 

recurrence of a similar error, often by attempting to prevent that individual from making a repeat 

error. Seldom are underlying causes explored.” As a consequence, Leape holds that “... although 

the individual may learn from a mistake and change practice patterns accordingly, the adjustment 

often takes place in a vacuum. Lessons learned are shared privately, if at all, and the external 

objective evaluation of what went wrong often does not occur.” [18, p.1852]

The effect of all this is to make it difficult to detect trends and promulgate effective 

countermeasures. The tradition of covering up errors, blaming an individual and recommending 

additional training or admonishing the individual to be more careful is not conducive to increasing 

patient safety. 

It is important to note that these problems are not limited to healthcare in the United States.

Recent information from the National Quality Board in the United Kingdom highlights similar 

issues and proposes that a “wider understanding of Human Factors principles and practices will 

contribute significantly to improving the quality (effectiveness, experience and safety) of care for 

patients.” [19, p. 5].
3 

Figure 1-1 Summary of an analysis from The New York Times
4

1.2 Addressing Human Error 

Drawing from a human reliability analysis 

(HRA) perspective, four key elements to 

improving safety are proposed (Figure 1-2):

1. Improved understanding of human 

error in radiation therapy (Section 2)

2. Improved ability to anticipate 

errors (Section 3)

3. Effective strategies for supporting 

humans in managing complexity 

(Section 4)

4. Methods to evaluate and monitor 

the effectiveness of correction 

actions (Section 5)

Each element: 1) builds on the previous one, 2) incorporates HRA approaches (and related or 

underlying disciplines), and 3) has been demonstrated to contribute to improved safety. In addition, 

a fundamental starting point for achieving all four elements described above (which has proved 

effective in other domains) is the use of a systems level approach or perspective, as opposed to a 

solely person centered approach, as a basic premise. Within the framework of a systems approach, 

the following HRA concepts can be used to understand and predict human error: 

• A basic understanding of HRA and human factors is that many human errors are predictable 

and preventable. 

• Human error is most often the result of latent conditions within a system which have gone 

unnoticed or which are undetected by the user until the use of the system triggers an 

unexpected response or interaction (e.g., system fails to function); such situations are

sometimes described as “error traps” [20].

• The person who makes the error is usually the last link in a chain of error traps that have been 

present in the system without users being aware of them until conditions in the environment 

make it possible for them to impact performance.

• It is simple during root cause analysis to “blame, shame, and retrain” the person who erred.

However, it is more effective to analyze the system to determine what pre-existing factors set 

up the event. In order for that to occur, designers and system reviewers need to ask what can 

go wrong with the system; that is, look at the entire process (not just the person involved) from 

a “failure mentality.”

One way to explore a system with a systems-level approach or perspective is to use the concept 

of “defense in depth” as is done in commercial nuclear power. In this approach, all parts of a 

“system” (e.g., the overall design, the design of the user interface, training, procedures, and 

supervision) represent “barriers” or defenses against a possible event.

Figure 1-2 Four key elements for improving safety
5

For most systems, a secondary or even tertiary defense lies between any failure and the resulting 

adverse event.

However, adapting the defense-in-depth approach for use in healthcare has its own challenges.

The technology involved in nuclear power production is not as varied as that associated with 

radiation therapy. Furthermore, in medicine, each individual patient is different. For example, 

tumors occur in different locations, patients are different sizes, and new applications for 

equipment are constantly introduced. Also, medical treatment problems are often unexpected,

require rapid response, and services of different medical disciplines must be coordinated.

Healthcare providers must multi-task and are frequently interrupted, so the flow of the task and 

thought process are disrupted. These characteristics are often not supported in the technologies 

and procedures that are used, and thus patient safety can be degraded. So, while the 

defense-in-depth approach has been highly successful in the commercial nuclear power industry, 

adaptation will be required to generalize this approach to healthcare.

This report recommends an approach that involves characterizing errors in a way that highlights 

the potential systemic issues from which the errors originated. This requires an understanding of 

human cognition in the context of complex systems. Multiple fields of study focus on errors in this 

manner. This report focuses on four fields that have different academic and applied roots, yet 

have evolved complementary methods and principles. Although these fields overlap, each brings 

a different emphasis:

1. The field of HRA seeks to identify and articulate human caused risks, often with the intent 

of quantifying the probability of specific errors [16]. 

2. The field of cognitive systems engineering focuses on identifying system-level factors that 

increase the likelihood of error and developing corresponding design changes to 

ameliorate vulnerabilities to error [21].

3. “Human factors” is a broader term that encompasses a number of human centric 

approaches to accident investigation and the design of robust systems [22].

4. Resilience engineering has emerged in recent years with the intent of creating a paradigm 

shift, moving away from legacy perspectives and focusing on how people and 

organizations “recognize, adapt to and absorb variations, changes, disturbances, 

disruptions, and surprises” [23].

This report draws on tools, techniques, insights, and understanding from all four of these fields.

HRA is traditionally performed to support a more broadly scoped study, probabilistic risk 

assessment (PRA), by addressing the “as operated” portion of the facility, equipment, process, or 

other analysis focus. As noted above, the performance of HRA requires integration of multiple 

disciplines, including human factors and ergonomics, cognitive and behavioral science, 

engineering, design, and operations. HRA also has been used separately from the larger PRA 

study, either quantitatively or qualitatively, in order to provide:

1) A risk focus (which facilitates prioritization of goals such as the safety of the public, workers, 

and patients that need to be addressed), and

2) A means of integrating and prioritizing the results of various analysis tools (e.g., from 

engineering, failure modes and effects analysis (FMEA), and human factors task analysis) with 
6

respect to risk and human performance drivers, potentially effective design improvements, and 

effective improvements in training, procedures, and administrative controls.

Consequently, this report has adopted an approach that can take advantage of both the risk 

prioritization and multidisciplinary benefits that an HRA informed analysis can provide. Also, 

because HRA integrates all of the four fields above and a risk focus, the term “human reliability 

analysis” or HRA is used throughout this report for simplicity and brevity.

1.3 The Role of The NRC

The NRC makes decisions regarding the licensing and application of medical devices that use 

nuclear isotopes (radiation therapy). Note that the NRC only reviews the clinical application of the 

use of nuclear isotopes; it does not review the treatment of medical conditions or the prescription 

of medicine. In order to make these decisions, the NRC advocates a risk-informed decision 

making process, which means that risk information, along with other information (i.e., costs and 

benefits) are included in the decision making. As such, the purpose of this report is to explore and 

discuss human error in radiation therapy, using risk-informed perspective so that latent errors can 

be detected and removed before the circumstances for an event can occur.

In addition to the NRC, it is important to be aware of the role of the U.S. Food and Drug 

Administration (FDA) in regulating radiation therapy and other medical devices. Patient safety is a 

complex issue that crosses multiple disciplines and processes and requires an integrated, 

comprehensive approach to improvement. The Federal Food, Drug, and Cosmetic Act [24] 

requires a reasonable assurance of safety and effectiveness before a device can be marketed. 

The FDA enforces this requirement. The 510(k) process [25] is used to approve devices that are 

rated as having moderate risk to patients; this includes many radiation therapy devices. 

As a result of the attention focused on errors in medicine, the FDA has increased efforts to 

investigate the devices used for treatment of patients and patient safety strategies. The FDA’s 

guidance on incorporating human factors engineering into risk management [26] describes how 

designers of medical systems can use human factors principles to design systems that will 

support decision making by the user, allow the medical professional to understand what is 

occurring in the systems, and reduce the potential for human error. More recently, the FDA issued 

draft guidelines for applying human factors and usability engineering to optimize medical device 

design [27] that closely resemble NRC's NUREG-0700 [28]. Although this new guidance is not yet 

in effect, it reflects the FDA’s current approach to human factors, encouraging manufacturers to 

conduct thorough testing in simulated environments to identify risks and vulnerabilities before a 

device is released for use. 

The FDA approach is consistent with NRC direction. A continued focus on the critical role of HRA 

is expected in both the 510(k) review process and the NRC licensing and regulation of the use of 

byproduct, source, and special nuclear materials in radiation therapy.
7

1.4 How This Report Develops an Understanding of Human Error

An HRA approach requires a shift in perspective as well as the application of appropriate 

methods. This report will focus primarily on communicating an HRA perspective relative to errors 

in radiation therapy. Throughout this report, the reader is referred to relevant literature for 

guidance on implementing specific human reliability methods. The following sections will:

• Explore and discuss human error in radiation therapy

• Provide an HRA perspective for understanding human error and anticipating future errors

• Discuss strategies for supporting humans in managing complexity

• Examine success stories in using a risk perspective on human error (e.g., are the 

corrective actions working?)

• Describe an HRA approach to risk management in the medical use of byproduct materials

In each section, the authors offer relevant examples. Additional details and examples can be 

found in the appendices.

1.5 Organization of This Report

Section 1 of this report makes the case that human error is an important issue in healthcare and 

that the NRC has an important role in addressing this issue. Section 2 contains a discussion of 

human error in radiation therapy specifically, and recounts the foundational studies that the NRC 

has conducted to better understand human error and corrective actions in this context. Section 3 

communicates a perspective for understanding human error and for anticipating future errors.

Section 4 highlights situations in which human error is more likely to occur, characterizing these 

as leverage points for better supporting humans in managing complexity. Section 5 reviews 

success stories from the aviation and nuclear power arenas in reducing error and discusses the 

concept of two-loop learning to continually assess the effectiveness of interventions in achieving 

improved safety. Section 6 integrates concepts introduced previously in the report to advocate for 

an HRA approach to risk management in the medical uses of byproduct materials. Section 7 

summarizes the main points of this report and draws conclusions.

9

2 SIGNIFICANCE OF HUMAN ERROR IN RADIATION THERAPY 

Over half of the misadministration events reported in the Nuclear Material Event Database 

(NMED) involving the use of byproduct materials in radiation therapy are attributed to human error 

(see Figure 2-1). The figure is taken from the report of the second quarter of fiscal year 

(FY) 2013 [29], but it is worth noting that the percentage of events attributed to error varies little 

over time3

. Although many of the reportable events do not result in harm to patients or healthcare 

staff, those that do can have devastating effects [30, 7]. One goal of the inspection and reporting 

process is to aid the NRC and the radiation therapy industry in recognizing and preventing errors 

before catastrophic events occur.

Figure 2-1 Causal factors events reported in NMED’s second quarterly report for FY 2013

Given the large proportion of reportable events classified as being caused by human error, it

follows that an improved understanding of human error represents a significant opportunity for 

improving safety.

2.1 The Age of Human Error

The approach of investigating and classifying events simply as “human errors” is an outdated 

approach that was popular in the 1960s and 1970s. Hale & Hovden describe this era of safety 

management as “the age of human error” to distinguish it from the earlier period of focusing only 

on hardware faults [31]. Using tools such as root cause analysis; it identified only a limited set of 

contributing factors for the event that could be improved. Appendix A provides a description of 

how root cause analysis has been used in healthcare and power plant settings.

 3 Data do not include a record of how many procedures are conducted during each period. Although we know what 

percentage of events reported are classified as human error, we do not know what percentage of procedures result 

in a reportable misadministration event.
10

In the age of human error, accidents like those at Three Mile Island Unit 2 in1979 were explained 

in terms of human fallibility and how they were caused. In this age, the goal was to develop an 

understanding of the factors that lead to increased likelihood of errors occurring, such as the 

influence of human machine interface design, the effectiveness of training, use of procedures, and 

so on. This allowed designers and facility owners to develop designs and practices that reduce 

the occurrence of human errors. 

In the1980s the focus for understanding accidents moved to the area of safety culture and 

management, particularly as a response to the Chernobyl accident as well as lesser (at least in 

scale) such as the sinking of the ferry Herald of Free Enterprise [32] and a series of major railway 

accidents in the UK. While the topic of safety culture and management influences has been 

included as a factor in healthcare safety programs (for example, it is listed in The Joint 

Commission’s list of root causes; see Appendix A for more detail), few approaches to developing 

effective responses to these types of events are included. 

As a result of knowledge gained from these 

evolutions, a new paradigm emerged that in 

many ways integrated the previous 

perspectives. Perhaps the archetypal model 

of this era is the so called “Swiss cheese” 

model of accident causation [33, 34, 35] that 

shows how it is the combinations of factors at 

multiple levels of the organization each with 

its own sets of vulnerabilities that are 

necessary to cause accidents. In order to 

respond to events using this model, there is 

considerable need to evaluate and audit the 

different kinds of factors at the multiple levels 

in order to uncover the gaps or “holes” in the 

“Swiss cheese.” This can be a demanding 

and resource intensive effort. Figure 2-2

shows an early version of the Swiss cheese 

model of accident causation. Figure 2-3

shows another version highlighting the need 

for “defenses,” a key driver of the defense-indepth approach described in the introduction 

of this report.

Hollnagel and others [23] have described the 

emergence of the next era of safety analysis, 

which is aimed at understanding the 

complexity of events rather than treating them 

as combinations of independent failures.

No single commonly accepted term has yet 

emerged to refer to this perspective, though 

the terms Resilience, Complex Adaptive 

Systems, and The Adaptive Age are used to 

describe it.

Figure 2-2 Early depiction of the Swiss Cheese 

Model

Figure 2-3 Later version illustrating organizational 

influences on defenses in depth
11

Its characteristics include:

1. The goal, unlike those of all of the previous eras (which relied on continuously analyzing 

events and learning from them), is to anticipate and prevent or mitigate the circumstances 

that can lead to accidents so that accidents do not happen;

2. The recognition that successful safety performance is the result of positive adaptation, as 

opposed to the elimination of bad behaviors (thus, lessons can be learned from both 

successful performance and failures); and 

3. The realization that understanding accidents and other events requires models based on 

how work is actually performed (as opposed to being based on the designer’s intentions).

Currently, healthcare, in general, appears to be preoccupied with safety management principally 

at the second era and to a lesser degree the third era, the eras of human error and of 

management. For example, the website of the U.S. Agency for Healthcare Research & Quality 

(the Federal Government agency responsible for improving patient safety) hosts a preponderance 

of tools for improving patient safety that are focused on improving individual human performance.

Similarly, the Institute for Healthcare Improvement (a major not-for-profit organization aimed at 

improving patient safety) indicates an emphasis on skills improvement for human performance.

However, there is also an interest in the “high reliability organizing” approach on the part of 

healthcare organizations [36, 37]. This approach represents a kind of midpoint between an HRA 

perspective and a management perspective; in theory it is about the desirable attributes of a 

highly reliable organization, but in practice these attributes are primarily related to the attitudes of 

people (e.g., a preoccupation with failure).

The NRC, in contrast, has conducted a series of studies from a human reliability perspective, 

focusing on strategies for understanding misadministration events in the context of work as it is

actually performed, seeking to understand what factors increase the likelihood of error. This work 

is discussed in the next section, immediately below.

2.2 NRC Studies of Error in Radiation Therapy

Over the last 20 years, the NRC has conducted a series of studies to better understand the 

significance and nature of human error in radiation therapy events (see Figure 2-4). These 

careful studies illustrate NRC efforts to predict and prevent error in radiation therapy, 

highlighting the significance of human error and demonstrating the use of human reliability 

methods in radiation therapy.

2.2.1 Overview of NRC Research in Radiation Therapy: The 1990s to the Present Day

In the early 1990s, the NRC undertook studies of evolving medical treatment modalities involving 

radioactive material. These included investigations and analyses of misadministration events in 

NUREG/CR-6088 [8] and human factors evaluations of remote afterloading brachytherapy (RAB) 

in NUREG/CR-6125 [9, 10] and teletherapy in NUREG/CR-6277 [11]. The initial studies of 

misadministration in the early 1990s illustrate the range of errors that fall into the category of 

human error. Through an exploration of direct and contributing causes of the events, the authors 

also discuss the effectiveness of corrective actions. Starting in 2005, the focus shifted from 

individual analyses to strategies to capturing HRA perspectives and methods for reviewers and 

inspectors through the development of a job aid [38] and associated training materials [39], then 

presenting this work at technical conferences [12, 13]. This job aid built on the 1990s studies, 
12

offering examples and articulating strategies for recognizing factors likely to increase vulnerability 

to human error.

Figure 2-4 Timeline of NRC studies of human error in radiation therapy events

Efforts to disseminate HRA perspectives and methods to the radiation therapy community and 

across the NRC continued in 2010 and 2011 with a series of papers highlighting success stories 

such as redesigned features of the gamma knife to reduce specific error types [40], and analysis 

of incident reports to identify vulnerabilities [41].

The most recent publication goes a step further, examining the effectiveness of corrective 

actions [14]. Although understanding human error and identifying system-level vulnerabilities are 

critical steps toward increased safety, it is not enough. Corrective actions that effectively reduce 

error are necessary if real improvement is to be realized.

2.2.2 Summary of Insights From Early NRC Research in Radiation Therapy

The relevant findings from the early NRC research are described immediately below, along with 

discussions of ways the findings can be used to improve understanding of human error and 

effects on radiation therapy safety. The analyses reported in NUREG/CR-6088 [8] are helpful.

This work formed the basis for this report, which includes NRC research from the early 2000s up 

to the present day. Section 2.3 discusses the most recent research and associated insights.

NUREG/CR-6088 [8] details a series of investigations of a representative sample of 

misadministrations reported during 1991 and 1992. The authors identified direct and contributing 

causes for each of seven events. These included:
13

• deficiencies in policies and procedures (cited as direct causes of all of the events 

investigated and as the initiating cause of two of them)

• inadequacies in training and experience (direct causes of five of the seven events)

• changes and unique conditions (contributing causes in all but one of the events)

• organizational structure/culture (contributing cause)

• hardware incompatibilities (contributing cause)

• workplace design (contributing cause)

• labeling (contributing cause)

To learn more about specific aspects of misadministrations, the authors compiled a database of 

misadministrations that were reported between 1987 and 1992. They found more evidence of the 

causes listed above as contributing to misadministrations, adding inadequate communication to 

the list. They further noted that data entry and software errors were dominant causes of such 

multiple misadministrations, in which more than one patient was affected as a result of the same 

underlying cause. While these multiple misadministrations were infrequent, they had the potential 

to impact a large number of patients.

This report is helpful in defining the categories of causes of misadministrations. Many of the 

causes listed above involve human actions, thus introducing the potential for human error. It is 

important to note that the authors found the corrective actions narrow in scope because they dealt 

only with the factors directly related to the cause of an event. These narrow corrective actions 

would be more useful in preventing the same type of event from recurring, but may not be as 

effective at preventing other related events. In addition, the authors recognized that an 

examination of more general trends for identifying corrective actions might have larger-scale 

benefit (i.e., benefits for human actions beyond those directly tied to the events that occurred).

NUREG/CR-6125 [9, 10] describes a human factors evaluation of remote afterloading 

brachytherapy (RAB). In this study, a task analysis was used to decompose each of the functions 

comprising RAB (patient preparation, treatment planning, treatment delivery, post-treatment, and 

quality assurance and maintenance) into individual steps. For each critical step, the authors 

describe and evaluate practices associated with the step, and suggest and evaluate alternative 

techniques that might be employed to reduce the likelihood of misadministration. They offer three 

primary strategies for reducing error: decreasing the likelihood of human error, increasing the 

detectability of human error, and limiting the consequences of human error.

An example of a critical task associated with RAB is the entry of the treatment plan into the 

treatment system. The vulnerabilities associated with this task include the need to manually enter 

the treatment plan into the treatment console, calculations that are not easily visible or verifiable 

by the user, and difficulties in identifying differences between the initial directive and the final 

parameters. The authors suggest two strategies to mitigate the vulnerabilities associated with the 

task: 1) the direct electronic transfer of the treatment plan to the treatment console and 2) better 

feedback from the system. See Appendix B for full descriptions of all the critical tasks, associated 

vulnerabilities, and mitigating factors.

NUREG/CR-6277 [11] describes a human factors evaluation of teletherapy undertaken around the 

same time as the RAB evaluation described above, using a similar approach. The main functions 

that were articulated include clinical evaluation, therapeutic decision, target volume localization, 

treatment planning, treatment, periodic evaluations during treatment, and follow-up evaluation.

Specific steps that had a high likelihood of producing an error were identified and labeled as 

critical. The vulnerabilities associated with each of these steps that can lead to errors were 

articulated, along with ways that these vulnerabilities could be mitigated.
14

For example, one of the critical tasks relates to setting up the equipment and treatment 

environment. The vulnerabilities associated with this particular step include high workload and 

rapid pace of treatment schedules, understaffing, poor layout of the workstation, and presence of 

distractions and competing tasks (e.g., having to answer phones). The authors suggested that 

these vulnerabilities could be mitigated by scheduling patients in a more evenly distributed 

manner, improving staffing levels, redesigning the workspace, and using computerized record and 

verification systems to check deviations from the prescriptions. See Appendix C for full 

descriptions of all the critical tasks, associated vulnerabilities, and mitigating factors.

Both of these studies looked critically at the key steps involved in the administration of specific 

types of radiation therapy. They identified critical steps during which an error would most likely 

result in a misadministration. This type of analysis is important in building an understanding of the 

contexts in which errors are likely to occur.

2.3 Examining Trends: Current Research

The authors of the present report analyzed relevant abnormal occurrences (AOs) found in 

NUREG-0090, Reports to Congress on Abnormal Occurrences. Specifically, the analysis 

examined events associated with radiation therapy from 2001 through 2010 [41] in terms of risk 

and the nature of events that have occurred. These events were associated with the high dose 

rate (HDR) brachytherapy, gamma knife, intravascular (IV) brachytherapy, and low dose rate 

(LDR) brachytherapy modalities of treatment. AOs include incidents or events having a moderate 

or more severe impact on public health or safety, including moderate exposure to or release of 

radioactive material. Although the reports do not include a great level of detail about each event, 

the analysis revealed that specific errors recur with specific technologies.

Events were analyzed based on whether they involved a wrong dose or a treatment to the wrong 

site, and what types of errors caused the event. These error categories include:

• data entry/planning software errors, involving an issue with inputting data or the software 

interface

• equipment errors, in which equipment did not function properly

• target errors, in which the treatment site was not properly identified before treatment 

administration

• verification errors, in which verification processes that were in place did not catch errors

• procedural errors, in which inadequate procedures directly led to unsafe conditions

• visualization errors, in which the target was not clearly visualized either before or during 

treatment administration

• patient movement errors, in which the patient’s movement during treatment resulted in 

an error

Because many of these events were complex, several fell into multiple error categories. Also, 

because the analysis was conducted solely from the event narratives rather than first-hand, 

in-depth investigation, it is possible that other errors contributed to the events. For example, there 

were probably many more verification errors than appear in the analysis, because facilities usually 

employ multiple checks and verifications of medical treatments before, during, and after 

procedures.

Table 2-1 below shows the distribution of errors in each category for each treatment modality that 

was analyzed, including a frequency count and percentage within each modality. As the table 

shows, different treatment modalities seem to be prone to different types of errors, probably 
15

because of the different procedures needed to implement each type of treatment and the different 

types of equipment used. HDR brachytherapy had more data entry errors than any other 

category (14), while the errors associated with gamma knife were almost evenly distributed 

among target, verification, and equipment errors. Most of the errors associated with 

IV brachytherapy were equipment errors (4), and the most common errors associated with LDR 

brachytherapy were target errors (18) followed by visualization errors (10).

Table 2-1 Summary of error distributions for each treatment modality

HDR 

Brachytherapy Gamma knife IV Brachytherapy

LDR 

Brachytherapy

Data entry 

errors

14

67%

2

14%

1

14%

1

3%

Equipment 

errors

5

24%

3

21%

4

57%

3

8%

Target errors 0

0%

4

29%

1

14%

18

49%

Verification 

errors

2

10%

4

29%

0

0%

1

3%

Visualization 

errors

0

0%

1

7%

1

14%

10

27%

Errors 

caused by 

patient 

movement

0

0%

0

0%

0

0%

3

8%

Procedural 

errors

0

0%

0

0%

0

0%

1

3%

(Note: Percentages represent the error percentage within that modality. For example, 67 percent of the errors in HDR 

brachytherapy were data entry errors. See Appendix D for more details about each error category and the events that 

fell within each.)

This analysis highlights the importance of understanding the underlying reasons and context for 

errors so they can be correctly classified. For example, an event might occur because the doctor 

placing small radioactive seeds for LDR brachytherapy inserted them in the wrong spot (the 

bladder instead of the prostate). This could be classified as “human error.” A deeper investigation, 

however, might reveal that the seeds were placed incorrectly because the visualization equipment 

that was used was misleading or made the patient’s anatomy difficult to distinguish. This 

additional distinction as a visualization error or equipment error suggests a different approach to 

reducing risk. Rather than a reprimand and additional training for individuals, the problem might 

be more effectively addressed on a larger scale by improved technologies and/or procedures to 

support visualizing seed placement.

Understanding error trends over time and across treatment modalities is vital to understanding 

where errors are likely to occur and what corrective actions are more likely to be effective in 

preventing future adverse events. Capturing and recording events is an important first step in 

conducting this type of analysis.
16

The NMED database (e.g., Reference 29) represents a powerful repository for examining trends.

The NMED quarterly and annual reports further summarize events that have occurred, as well as 

their causes and corrective actions. However, many of the events listed are attributed to “human 

error” (see Figure 2-1) with very little context about why they were classified that way [29].

Without the context, it is difficult to implement corrective actions that truly address the source of 

the error. For example, a review of nineteen events associated with HDR brachytherapy reported 

in NMED for the period from 2009 to late 2011 [14] revealed that all of the reports identified 

“human error” as the root cause; however, the reports indicated the following corrective actions:

• new or modified procedure—100 percent

• staff retrained—56 percent

• person reprimanded—11 percent

• new management plan—6 percent

• new equipment—6 percent

The high percentage of reported events identifying human error as a root cause and the 

implementation of seemingly ineffective corrective actions (note that each event was usually 

followed by more than one corrective action) reveals a worrisome tendency.

When considered in the context of Manuele’s safety decision hierarchy [42], the weakness of the 

current approach to corrective action becomes clear. Manuele’s hierarchy has six levels, ranging 

from the most effective at reducing risk (#1) to the least effective at reducing risk (#6):

1. Eliminate hazards and risks through system design and redesign

2. Reduce risks by substituting less hazardous methods or materials

3. Incorporate safety devices (fixed guards or interlocks)

4. Provide warning systems

5. Apply administrative controls (work methods, training, etc.)

6. Provide personal protective equipment.

Of the corrective actions listed above as responses to events caused by human error, the one 

most frequently used is also one of the least effective types of intervention: administrative control.

This analysis suggests that although human error is listed most frequently as a causal factor of 

adverse medical events, the corrective actions that are most often implemented are not ideally 

suited for either mitigating the effects of human errors or preventing them from causing adverse 

events in the first place.

More recently, human factors researchers created a model of sustainable and effective corrective 

actions following root cause analysis in healthcare [43]; the model is illustrated in Figure 2-5. The 

authors of this journal paper carried out a qualitative analysis of 344 root cause analysis 

investigations, conducting interviews with frontline staff one to five years after interventions were 

implemented. Findings are consistent with Manuele’s hierarchy in that solutions aimed at the 

individual, such as counseling or disciplinary action, were minimally effective and had little 

sustained impact. Changes at the institutional level (e.g., creating a new pediatric inpatient 

pharmacy) and enhancements to information technology infrastructure (i.e., improved usability 

and improved interoperability) were found to be highly effective and to have high sustainability.
17

Figure 2-5 The model of sustainability and effectiveness of corrective actions

2.4 Summary

The series of NRC sponsored studies reviewed in this section help to characterize the nature of 

human error in radiation therapy. Studies ranging from in-depth analysis of an individual modality 

to broader examination of error types across modalities highlight the complexity of error in 

radiation therapy, but also illustrate HRA approaches for understanding error and strategies for 

developing effective corrective actions.

19

3 PERSPECTIVE FOR UNDERSTANDING HUMAN ERROR AND 

ANTICIPATING FUTURE ERRORS

After discussing the nature of human error in radiation therapy, the attention is turned to HRA as a 

perspective for understanding human error. An important aspect of HRA is taking a systems view; 

that is, considering humans, technology, and the environment as interdependent components of a 

system. Complex systems rely on humans for resilience in the face of adverse events or 

oversights. These large systems rely on humans to recognize novel events, technology 

malfunctions, and errors in order to quickly adapt and recover before catastrophic events occur.

An HRA perspective focuses on understanding human capabilities and identifying system 

vulnerabilities as a means to anticipate error.

3.1 Understanding Human Capabilities

As discussed in Section 2, “human error” is listed as the cause of many incidents, but this superficial 

label offers limited insight into root causes and strategies for appropriate corrective actions. The 

term human error often connotes random, unpredictable events that cannot be anticipated or 

prevented. HRA offers a different perspective, some precepts of which are given below.

3.1.1 Human Error is NOT Random

Because error reports are often superficial and focus primarily on the events immediately before 

an incident, most human errors appear to be idiosyncratic. By examining the context in which an 

error occurs, however, factors influencing human performance become visible. Human error is 

more likely to occur when people must contend with competing demands, interruptions, and 

technology with poor usability, fatigue, or any number of other contextual factors [12]. An HRA 

approach advocates for an investigation of errors that includes contextual factors, along with an 

examination of similar errors that occur in different locations at different times. This type of 

analysis reveals trends and systemic issues that contribute to human error and often suggests 

interventions likely to increase human reliability. Section 3.2 describes recent efforts to identify 

system vulnerabilities in radiation therapy.

3.1.2 Human Error Is Not The “Cause” of a Mishap

Although many mishaps are attributed to a single root cause, most are the result of an 

accumulation of factors and acts. Reason [44] posits multiple levels of failure, including 

organizational influences, unsafe supervision, preconditions for unsafe acts, and the unsafe acts 

themselves. These have been described in the context of the “Swiss Cheese Model” in which 

individual parts of the system are depicted as slices of Swiss cheese (see Figure 2-2 at the 

beginning of Section 2). Errors occur all the time, but few result in any real harm because of 

redundancy and checks. Real failures occur only when all of the holes in each of the slices 

momentarily align, permitting a “trajectory of accident opportunity” [33]. In this instance, a hazard 

passes through holes in all the defenses resulting in a failure. Thus, while human error might have 

been among the immediate causes of an incident, several other factors had to fail as well.
20

3.1.3 Human Error Can Be Predicted

There is no question that human behavior is highly variable. An understanding of human 

cognition, however, reveals important patterns that allow us to predict situations in which human 

error is most likely to occur. Some examples include:

• In the context of accomplishing real world tasks, people tend to engage in 

goal-directed, practical, and economical behavior. People tend to work toward goals 

based on a mental model of the situation, using whatever time and resources are 

available. People are also highly adaptive, often seeking innovative strategies to 

accomplish goals efficiently even as the context and constraints change. Understanding 

these often undocumented strategies can highlight important behavior patterns that are 

highly adaptive most of the time, but increase vulnerability to error when something 

unexpected occurs.

• Furthermore, work environments such as healthcare, nuclear power, and aviation 

prescribe processes that people tend to follow. This allows people to follow predictable 

paths, maximizing routines and automatized actions to minimize mental effort. In fact, 

these highly practiced, smooth, seamless actions are a hallmark of expert performance.

However, while this means that human behavior in these environments is fairly 

predictable, the tradeoff is that, when in this “automatic mode,” people are less likely to 

interact with the environment mindfully. In other words, they are more likely to see what 

they expect to see and to miss important anomalies that might indicate a problem.

• As people gain expertise in an area, they develop sophisticated pattern-matching 

skills that allow them to recognize typical situations and interpret problems. These 

skills allow experts to see things that are invisible to the novice. Experts build up an 

experience base that allows them to make use of heuristics and develop expectations about 

future events. These strategies allow experts to effectively and quickly adapt to a range of 

contexts, manage different types of complexity, and move forward in spite of uncertainty.

Understanding expert strategies also provides insight into vulnerabilities or situations in 

which even experts may be misled by heuristics.

While these human tendencies are highly adaptive most of the time, each one can cause 

susceptibility to certain types of error. Human errors are not isolated breakdowns, but rather are 

the result of the same processes that allow a system’s normal functioning. Take the example of 

the common dissociative phenomena that are often experienced when driving to a very familiar 

location. Many people forget details of the drive because they have done it so often. Most of the 

time, it does not matter if the driver was not paying very close attention. The driver is able to safely 

navigate the route while mentally reviewing the events of the day, preparing for an upcoming 

appointment, enjoying radio programming, or any number of activities. However, occasionally an 

unexpected event will occur that could prove catastrophic to a driver who has slipped into 

mindless, automatic driving behaviors.

Section 3.2 includes examples and a discussion of how these typically highly adaptive strategies 

can be vulnerable to error in certain circumstances. Identifying the circumstances that increase 

the likelihood of error is a core component of HRA.
21

3.2 Identifying System Vulnerabilities

Working from the premise that human error is not random; an HRA perspective recommends 

strategies for identifying system vulnerabilities that lead to predictable types of errors. To illustrate 

this approach, consider a project conducted in 2003 to investigate the effects of human error on 

the risks associated with medical use of byproduct material for the NRC [12]. Medical events 

associated with selected modalities of treatment were reviewed: HDR brachytherapy, teletherapy, 

gamma stereotactic radiosurgery (the “gamma knife”), and IV brachytherapy.

These modalities were selected because they involve high dose rate sources so there is very little 

margin for error (or opportunity for recovery) for the safety of the patient because the treatments 

involve short durations and very small volumes. In addition, because of the source strengths, the 

clinical workers, family, or others near the patient can also be at risk.

The authors began by reviewing the events to identify risk significant human actions. Seven risk 

significant human actions were identified across the four modalities. These were:

• data entry

• use of planning software

• miscommunications (of prescription or source orders)

• selection of sources

• calculations of source strength and location

• source handling and installation

• verification of planned actions

A next step was to identify specific circumstances having the potential to lead to unintended 

events such as misadministration, overexposure of workers, or exposure of members of the 

public. These included circumstances in which:

Source activity can change. Changes in source activity tend to happen very slowly as the 

source decays over time, culminating in an eventual replacement of the source, which might 

represent a significant change in source activity. If calibration tasks are not completed as 

scheduled or are not completed accurately, an incorrect level of source may be assumed in 

treatment planning, resulting in an incorrect dose delivery to the patient.

Material can be supplied by others. Handoffs of any sort tend to be vulnerable to error. In 

radiation therapy, unit conversion errors are of particular interest because of the likelihood of 

harm. The possibilities of errors associated with unit conversion are most likely to arise when 

operators order radiopharmaceuticals. In addition, misidentification and mislabeling can occur 

when multiple doses are prepared and transported.

Treatments can be performed by personnel other than the prescribing physician. When 

treatment is carried out by someone other than the prescribing physician, two types of errors are 

more likely. First, misidentification of patients is more likely to occur when the person 

administering the treatment is not acquainted with the patient. Second, orders that are vague or 

not transmitted accurately may result in therapeutic doses being administered when diagnostic 

procedures were intended.

Source configurations (e.g., physical dimensions, activity) can vary. For some modalities, 

the source can have multiple configurations, such as the different seed types used in low dose 
22

rate brachytherapy. This variability introduces the possibility of incorrect selection of source 

configuration (e.g., seed type), including errors owing to lack of familiarity and mislabeling.

Untrained personnel may encounter radioactive sources. For some modalities such as 

brachytherapy, the source is small and portable enough that it may be dropped without being 

noticed while being inserted or become dislodged during the treatment.

Treatment planning can be computer-aided or partially or fully automated. Although 

automation offers the opportunity to reduce some error types, new types of error are often 

introduced. These can include number transpositions and other data entry errors, inappropriate 

default values, or the presence of multiple draft treatment plans.

Checks can become ineffective owing to repetition or production pressure. In many 

radiation therapy processes and procedures, multiple clinicians are required to review the 

treatment plan before it is administered in order to notice and correct any errors before they are 

implemented. However, the repetitive procedures and need to work quickly and efficiently can 

actually reduce the effectiveness of these cross-checks.

These circumstances were involved in one way or another in large numbers of the events 

reviewed. Despite the differences in actions performed, the purposes for which they are carried 

out, and the environments in which they are conducted, the events have some common features.

This examination of predictable error-prone tasks and circumstances reveals potential leverage 

points for both system and technology design in the context of radiation therapy. In Section 4, 

generalizable elements are discussed that make a system vulnerable to error and challenge 

humans’ ability to operate it reliably. Within each, we provide examples from radiation therapy 

events and suggestions for supporting humans in managing these types of complexity.

3.3 Summary

Understanding human capabilities and identifying system vulnerabilities are important elements of 

recognizing potential error traps. By focusing on how humans function in complex environments 

and on specific circumstances and system factors that are likely to disrupt or degrade human 

performance, it is possible to anticipate errors before they occur. The next section takes this 

discussion a step further, highlighting factors that increase the risk of error and serve as leverage 

points for improved system design.
23

4 SUPPORTING HUMANS IN MANAGING COMPLEXITY

Complexity is inherent in the healthcare system. Complexity and error are parts of the system that 

will never go away. Rather than focusing on eliminating complexity (or error), the focus should be 

on supporting the human in managing this complexity. From an HRA perspective, the authors’ 

intent was to identify common features of error events and to examine these trends for aspects of 

the system or the environment that increase the risk of error in a predictable way. Authors began 

with an examination of radiation therapy errors documented in NMED and then integrated these 

findings with vulnerabilities identified in other industries. Discussed below are six aspects or 

vulnerabilities that increase the risk for error:

1. “black box” aspects of automation,

2. repetition and automaticity,

3. the role of user interfaces in mode errors,

4. the role of user interfaces in data entry errors,

5. lack of independent verification for noticing errors, and

6. prospective memory demands.

These vulnerabilities become leverage points for improvement to the system, as discussed below.

Appendix E contains a longer description of each event cited in this section.

4.1 “Black Box” Aspects of Automation

The impact of increasingly sophisticated automation, both positive and negative, has been well 

documented in the context of healthcare [45, 46], as well as with automated cockpits [47, 48] and 

spacecraft incidents [49]. There is a constant tension between goals of simplifying workflow for the 

human (e.g., through reduced need for calculations, filtering of low priority data, or automation of 

repetitive, predictable tasks) and encouraging the operator to stay engaged in the task and aware 

of the current system state. As automation becomes increasingly sophisticated in pursuit of 

simplifying workflow, often the operator becomes further removed from the task, transitioning from 

an active role to a more supervisory and passive role. Not only is it more difficult for the operator 

to maintain engagement in the task from this supervisory perspective, but the system becomes a 

black box, making it harder to detect and notice errors and anomalies with fewer direct cues about 

how the task is being accomplished [50].

These “black box” issues are compounded in radiation therapy where automation purposefully

keeps the practitioner at arm’s length from the process in order to limit radiation exposure.

Typically, in other domains, the “distance” is an unwanted side effect of automation. The fact that 

operators cannot actually see or feel the device that is transporting the source is a particular 

problem in brachytherapy; there is no true compensatory feedback feature.

For example, in one intravascular brachytherapy event (AS 04-09 in Appendix E), an error 

resulted because the operator could not see a kink in the delivery catheter. In this case, the kink 

was not substantial enough to affect the flow of sterile water used to send and retrieve the source 

train. Thus no sensors detected the problem, no error messages or alarms appeared, and the 

operators had no way of detecting that the actual dose did not travel the full length of the catheter.

The kink was discovered the next day during medical physics quality checks.

Increased automation has raised concerns in Gamma Knife teletherapy devices as well. In new 

versions of the device, automation has eliminated the opportunity for the most frequently reported 

errors such as forgetting to change the collimator helmet and incorrect shot coordinates and 
24

treatment times. However, at the same time, the reduction in the amount of practitioner interaction 

with the treatment mechanism potentially reduces the opportunity for other errors to be recognized 

and recovered before treatment is completed. Because the collimators are internal to the newer 

device and the operator no longer sees the calculations that generate coordinates, the operator 

has a limited ability to observe and notice anomalies. Although the latest version of the Gamma 

Knife is new enough that no adverse events have been reported at the time of this publication, 

one experienced operator reported a sense of uneasiness about the “black box” nature of the 

newest model. He was concerned that if something went wrong with the device, he would not be 

able to recognize it and diagnose the problem quickly.

Increasing automation is not necessarily a bad thing to do in many cases, it helps ease workload 

so operators can focus on more important tasks. However, when there is no clear mechanism 

through which the operator can gain an understanding of what the automation is doing, the 

situation may become more prone to errors that are difficult to detect and recover from before 

harm occurs. Strategies to keep the operator aware of what the automation is doing and engaged 

enough to recognize an anomaly are critical. Some have encouraged designers to consider the 

automation to be another team player to encourage information sharing and transparency [51].

Modern nuclear power plant control rooms include displays designed specifically to increase 

transparency and provide an authentic view of both the state of the power plant and the current 

state/actions of the automation.

4.2 Repetition and Automaticity

Radiation therapy devices are generally used as frequently as is practical, and for good reason.

Radiation therapy technology requires a large investment and is in limited supply. Their frequent 

use results in benefits for patients in that more patients receive treatment in a timely manner and 

costs for the device are spread over many treatments. However, administering multiple treatments 

per day involves a significant amount of repetitive behavior. This repetition can lead to finely 

honed skills and automaticity that allows for efficiency in completing repetitive tasks. In other 

words, experts are able to complete routine tasks very smoothly and efficiently, often with very 

little conscious effort. For the most part, this type of expertise is generally adaptive and allows 

people to work efficiently and effectively. However, this type of skill-based automaticity is also 

vulnerable to certain types of error [33].

Radiation therapy is complicated. The automaticity that comes with repetition makes it possible to 

carry out complicated activity with high reliability. Generally speaking, errors in skill-based 

behavior occur when deviations from routine and distraction create competing demands for 

cognitive resources [33]. Unfortunately, both of these circumstances frequently occur in the 

medical context. The nature of radiation therapy facilities is such that technicians are subject to 

interruptions and often need to do two (or more) things at once. Under these circumstances, 

errors in skill-based behavior are infrequent, but inevitable.

Reported events indicate that vulnerabilities associated with departures from routine and 

distractions are not adequately guarded against, even though (in light of the discussion above) 

errors are predicted in these circumstances. For example, in one vascular brachytherapy 

misadministration, incident AS 02-4 in Appendix E, an error was made in the treatment plan 

because the physician used a device that he or she was unfamiliar with but that was similar to a 

device he or she had used before. The physician had extensive experience with one device that 

uses the diameter of the artery in treatment planning calculations; on this day, however, the 

physician a different device that used the artery radius in calculations.
25

Following the normal routine, the diameter was entered, resulting in an incorrect dosimetry 

calculation, with an actual dose of 14.6 Gray (Gy) being administered (1460 rad) rather than the 

prescribed 8 Gray (800 rad).

Departure from routine also played a role in the highly publicized Beatson event in which a patient 

treated for a brain tumor at the Beatson Oncology Centre in Glasgow, Scotland received an 

overdose. In this case, new software required that units be normalized to “per treatment fraction” 

rather than “per 100 centigray” (cGy) as the treatment planner was accustomed to [52]. In this case, 

a range of other factors made it difficult to detect the problem, such as not enough staff, inadequate 

training on the new software and pressure to accommodate a backlog of patients waiting for cancer 

treatment. Such complicating factors increase the likelihood of automaticity errors.

Strategies to reduce vulnerability to departures from routine and distraction include interventions 

that highlight anomalies. Here the intent is to engage the operator in mindful behavior (rather than 

relying on routine, automatized actions) when it is most needed. This sort of “hypervigilance for 

the presence of latent problems and loopholes that may develop into unsafe events” [53] has 

been discussed as part of a work culture in the context of high reliability organizations [54, 55].

This should not be confused with superficial attempts to encourage operators to pay more 

attention. Rather, mindfulness is about the quality of attention. It includes noticing anomalies, even 

ones that are subtle (i.e., “weak signals”), and a willingness to act on potential hazards [54].

4.3 User Interfaces: Mode Errors

Mode errors occur when the operator performs a task appropriate to one mode when the device is 

in another mode. For example, some wrong site Gamma Knife procedures have been attributed to 

the use of the magnetic resonance imager (MRI) in caudal mode orientation rather than cranial 

mode orientation (see NRC 08-03 in Appendix E)—a change that is not obvious to the physician 

reading the images. In these cases, the physicians assumed the images were taken in the more 

common mode (cranial mode) and interpreted the image as a mirror of the actual anatomy, 

resulting in procedures being done at the wrong site. Situations in which the user’s understanding 

of a device’s actions is based on an incorrect understanding of the current mode have been well 

documented in a range of devices [56]. Strategies for decreasing mode errors include:

• Eliminating Modes—Mode errors cannot occur if there is only one mode. However, while 

adding dedicated control and display devices normally eliminates multiple modes, this is 

not always possible for equipment where there may be insufficient space. Also, adding 

more devices may increase the likelihood of choosing the wrong one.

• Making Modes Distinct—The goal of the second strategy is to ensure that the user is aware of 

the currently active mode by providing distinct, salient indications of the mode state.

• Coordinating Inputs across Modes—The consequences of mode errors can be reduced by 

ensuring that a command does not have very different meanings in different modes.

A special mode error consideration relates to systems that change modes automatically.

Automated systems should be designed to inform the operator of their current operating mode, 

mode transition points, limits on operator actions, and circumstances under which the operators 

need to assume control.

In addition, the operator must be aware of indications from the automated system or other sources 

about how to assume control without “fighting” the system or causing unnecessary disturbances.
26

4.4 User Interfaces: Data Entry

Errors associated with data entry, including digit transpositions and misordering of parameters, 

are particularly worrisome in the context of developing a treatment plan. These types of mistakes 

are easy to make, difficult to detect, and can have serious consequences for the patient.

Appendix E contains several examples of events resulting from such errors (see AS 07-05, 

AS 07-06, AS 09-03, AS 09-04, AS 10-02, and AS 11-06).

The use of default values can increase vulnerability to data entry errors. In an event involving 

HDR for breast treatment (see NRC 11-04 in Appendix E), the operator failed to change the 

default from “start at connector end” to “start at tip end,” resulting in the mispositioning of the 

source in the patient’s body. However, it is important to note that at this facility, all HDR 

procedures started at the connector end except breast treatments. Thus, one had to remember to 

change the default only for breast treatments, leaving the default untouched in all other cases.

While intended to decrease data entry burden, the use of a default value makes it possible for the 

software to proceed without operator interaction with the default field. Although operators intend to 

inspect each field when creating a treatment plan (and generally do), in a busy clinic with many 

distractions, errors of this type are likely.

Fortunately, data entry errors appear to be a diminishing problem as electronic transfer of data 

from treatment planning systems to therapy devices becomes more common. However, it is 

unlikely that data entry errors will go away completely. Therefore, it is important to consider 

strategies for noticing and recovering from errors as quickly as possible. One strategy that is 

widely used is to include a warning if a value entered is outside a normal range. The operator still 

has the opportunity to enter an out of range value if desired, but the warning requires confirmation 

that this was not a data entry error. Another strategy is to identify common errors (i.e., reversed 

MRI images) and infrequent but potentially high consequence errors (i.e., treatment plan entered 

for wrong patient) based on error reports and trend data. Checks and warnings can then be built 

into the system to reduce the likelihood that these specific types of errors will go undetected.

4.5 Independent Verification for Noticing Errors

Because there is no way to eliminate errors altogether, checking of one kind or another is relied 

on to intercept the inevitable errors. As a first level of checking, software programs often present a 

warning if an entry is out of a typical range as described in Section 4.4 . While very useful at 

catching potential errors of magnitude, other types of errors will go unnoticed by this type of 

algorithm (e.g., entries can be incorrect while still being within the normal range, thus not signaling 

a warning). As a result, most facilities require multiple signoffs on a treatment plan before 

treatment can be administered. Software can be designed to support these types of checks, 

preventing treatment until the proper signatures have been entered electronically.

In spite of these precautions, there is no shortage of examples of events in which an error was 

made and the double-check or “signoff” policy failed, allowing a misadministration to occur. In one 

event involving an HDR device (NRC 10-07 in Appendix E), catheter distances were recorded and 

confirmed with two manufacturer representatives present at the time of treatment. In spite of this 

“independent verification,” the measurements were incorrect and the radioactive source was 

placed 10 cm proximal to the intended position. 

In another example (NRC 10-02 in Appendix E), the written directive stated the correct site (the 

nerve on the right) on the top half of the form, but an error was entered in the daily treatment log 

on the lower half of the form indicating the nerve on the left. Even with multiple signoffs, no one 
27

noted the discrepancy from the top of the form to the bottom of the form, and the procedure was 

conducted on the wrong site (the left side).

There are several reasons why errors might not be detected in spite of multiple signoffs. Checks 

mandated by good practice or administrative procedures might not actually be performed. In some 

cases, personnel might “sign off” without having inspected the work. When checks are performed, 

errors might still go undetected, especially in settings in which experienced personnel repeatedly 

carry out routine actions. In such cases, personnel doing the checking, knowing that the work is 

being done reliably will tend to “see” what they expect to see even when it is a simple case of 

matching and no interpretation is required. In other words, expectancy effectively nullifies the 

desired human redundancy. This effect is known as the fallacy of social redundancy4 and was a 

major contributor to the “friendly fire” event involving the shooting down of Black Hawk helicopters 

by U.S. fighter aircraft in 1994 [57]. In that case, two U.S. Air Force F-15 pilots misidentified two 

U.S. Army Black Hawk helicopters as hostile in spite of multiple levels of “checks,” including the 

publication of a daily list of all friendly aircraft in the airspace and a weapons director tasked with 

providing real-time verification of the location of friendly aircraft. The Black Hawk helicopters were 

on a non-routine mission; given the actions of the Black Hawk helicopters, the F-15 pilots 

“expected” to see hostile aircraft.

Limiting or eliminating the effect of expectancy can improve the effectiveness of checking. For 

example, consider the task of setting device coordinates based on a written prescription. A typical 

double-check might involve one person looking over the shoulder of another to see that the 

settings were as specified (i.e., as expected). If the person setting the device is perceived to be 

reliable, the subsequent check will not be very effective in detecting the rare error, owing to 

expectancy. Compare this to a different type of double-check described in NUREG/CR-6323 [58], 

in which the checkers are required to independently determine the device settings, so that their 

reading of the settings is not influenced by knowledge of what the setting “ought” to be:

An impressive double-blind checking routine would consist of one person setting 

the shot coordinates from the prescriptions, which are left unknown to the 

checkers. Each of two checkers separately records their inspection of the set 

coordinates. Then both checks are compared to each other and the prescription. If 

there is any discrepancy among the records, the coordinates are reset and the 

checking procedure is repeated [58, p. 42].

Other elements that increase the effectiveness of cross checking for error detection include: 

• Different perspectives - The cross-checker(s) should have a different perspective from the 

developer of the treatment plan.

A different perspective might include different goals, responsibilities, functions, authority, 

stance, expertise, resources, methods, or knowledge/information.

• Knowledge of “typical mistakes”— An experienced cross-checker who has knowledge of 

typical mistakes or vulnerable aspects of the process is more likely to detect errors.

• Observable process—Insight into the plan development process may increase the 

cross-checker’s engagement and ability to mentally simulate potential errors.

 4 Redundancy in systems can lead to a diffusion of responsibility and a tendency for inaction; that is, a decrease in 

individual effort because of the social presence of other persons. “Sometimes, when everyone is responsible, no one is.”
28

• Visible rationale/intent—Insight into the planner’s rationale or intent behind a plan provides 

important background for the cross-checker and may expose inconsistencies or gaps.

• Focused review—Distractions or responsibilities competing for the cross-checker’s 

attention can reduce the effectiveness of cross-checking. The cross-checker’s time and 

attention should not be consumed by production pressures or competing cognitively 

challenging tasks.

• Big picture view—Cross-checkers must be able to detect anomalies. In order to do this, 

the cross-checker must have knowledge of the procedure to be accomplished and the 

context in which it occurs. The use of inexperienced staff or automated software that 

cannot take into account contextual variability (i.e., overloaded staff or a potentially 

disruptive patient) is unlikely to be effective [59].

Cross-checking is least effective when it is conducted routinely for a process with a low base rate 

for errors. This is characteristic of many healthcare procedures requiring cross-checks; most of 

the time the original act is correct. The hope is that the cross-checker will notice the anomaly or 

error that happens very infrequently. The difficulty of staying alert to potential errors in such 

circumstances is exacerbated by noise, uncomfortable environmental temperature, fatigue, 

circadian changes/shift work, and boredom [59].

4.6 Prospective Memory

Prospective memory involves remembering to perform an action at the appropriate time [60].

Because the healthcare setting requires multi-tasking and is characterized by frequent 

interruptions and distractions, healthcare workers carry a particularly high load for prospective 

memory [61, 62]. However, even more mundane work settings leave us vulnerable to prospective 

memory errors. Reason [44] highlights the common error of leaving the last page of the original in 

the photocopier. When manually feeding a document, the user must remove each page to place 

the next one. For the last page, however, there is no cue to remind you to remove it. As

Dismukes [63] further explains:

Prospective memory is distinguished by three features: (1) an intention to perform 

an action at some later time when circumstances permit, (2) a delay between 

forming and executing the intention, typically filled with activities not directly related 

to the deferred action, and (3) the absence of an explicit prompt indicating that it is 

time to retrieve the intention from memory—the individual must “remember to 

remember.” Typically, if queried after forgetting to perform an action, individuals 

can recall what they intended to do. Thus the critical issue in prospective memory 

is not retention of the content of intentions but retrieval of those intentions at the 

appropriate moment, which is quite vulnerable to failure [63, p. 909].

An event from the healthcare domain that is analogous to forgetting to remove the last page from 

the copier is remembering to change the collimator helmet during gamma knife treatment.

This is one example of a prospective memory challenge that has occurred in multiple events 

(e.g., AS 04-06 and NRC 09-02 in Appendix E). Because all shots for a given helmet size are run 

consecutively, personnel usually set the stereotactic coordinates, leave the room, and administer 

the shot. The operator has every intention of changing the helmet at the appropriate time, but 

there is no cue telling the operator at the right moment that this time the collimator helmet should 

be changed after setting the coordinates but before leaving the room.
29

In another example of prospective memory challenges (see AS 02-5 in Appendix E), a patient was 

prescribed radiation treatment to the left eye and received a larger than intended dose because 

the nurse and authorized user became distracted with other tasks and forgot to stop the 

treatment. A routine administration of the eye applicator requires one person to time the event 

with a stopwatch while the authorized user administers the dose. The nurse and the authorized 

user became distracted in conversing with the patient and lost track of the time. The stopwatch 

used was the old style that simply counted time up and the nurse lost focus in trying to make the 

patient more comfortable and at ease. At the same time, the authorized user was reminding the 

patient to gaze in a certain direction to treat the affected area. The treatment time administered 

was 109 seconds instead of the prescribed 44 seconds, resulting in a larger than intended dose.

Strategies for reducing prospective memory load generally involve the introduction of deliberate 

cues based on an event or on time [63]. Often they seem obvious after the prospective memory 

challenge has been identified. For example, newer versions of the Gamma Knife run a check 

before the treatment is administered and will not proceed with an incorrect collimator helmet. The 

stopwatch used for radiation treatments to the eye now counts down and includes an auditory 

alarm when the treatment time has elapsed. The first step in ameliorating prospective memory 

problems is to identify instances in which the operator must remember to do something in the 

future with no obvious prompt. When these prospective memory challenges are identified, often 

simple solutions such as appropriately timed or cued prompts can be implemented.

4.7 Summary

HRA, as an integrator of multiple disciplines and with a risk focus, offers a valuable 

perspective for understanding human error. Study of human cognition reveals predictable 

aspects of human behavior. Examination of error events reveals tasks and circumstances that 

represent system vulnerabilities. Furthermore, generalizable elements that introduce system 

vulnerability provide valuable leverage points for improving system design to better support 

humans in managing complexity.

31

5 SUCCESS STORIES IN USING A RISK

PERSPECTIVE ON HUMAN ERROR: ARE THE

CORRECTIVE ACTIONS WORKING?

Using a risk perspective to understand human error and to guide improvements in systems to 

reduce the risks has proved successful in many industries. The most notable of these industries 

are civil aviation and commercial nuclear power. This section examines efforts in these complex, 

high-profile industries to reduce errors through implementation of a risk perspective (i.e., HRA).

Because of the potential for catastrophic consequences to occur, in both civil aviation and 

commercial nuclear power, there has been a long-standing realization that the consequences of 

accidents are unacceptable. Both industries continue to work to reduce accident risks regardless 

of the current levels.

As with healthcare, the commercial aviation and nuclear power industries have found from 

experience that most accidents and significant events are the result of human performance 

issues. For example, Boeing reports that:

Human error has been documented as a primary contributor to more than 

70 percent of commercial airplane hull-loss accidents [64].

This estimate is consistent with other evaluations of the contributions of human performance 

problems for aviation; the estimate provided by the Federal Aviation Administration, for example, 

is 70 percent to 80 percent in civil and military aviation [15]. In commercial aviation, work has been 

performed by all parts of the industry to improve human performance: 

• The aviation manufacturing sector (see, for example, the description of the human 

performance improvement programs at Boeing) [64]

• The regulator, FAA [15] 

• The International Civil Aviation Organization (ICAO) [65].

Interestingly, Dougherty and Fragola [16] report similar rates of human performance issues in the 

nuclear power industry. More recently (in NUREG/CR-6751 [66]), an evaluation of the contribution 

of human errors to events involving a substantial increase in the risk of core damage in 

commercial nuclear power plants found the average human error contribution to the change in risk 

was 62% . This is consistent with the estimate of the worldwide nuclear industry as reported by 

the International Atomic Energy Agency (2010) [67], where human and organizational factors 

contributed up to 75 percent of the operational problems reported.

5.1 Improvements in Managing the Contributions of Human Error to Risk

In spite of the relatively high rates of human error, the risk associated with flying in commercial 

aviation has never been lower for flight operations in the developed world. The Aviation Safety 

Network, which maintains the database of worldwide aviation accidents for the Flight Safety 

Foundation, described the year 2012 as the safest year since 1945 in terms of absolute numbers 

of fatalities and flight accidents [68]:

The year 2012 was an extremely safe year for civil aviation, Aviation Safety

Network data show. The Aviation Safety Network recorded a total of 23 fatal 
32

airliner accidents, resulting in 475 fatalities and 36 ground fatalities. Both figures 

are extremely lower than the ten-year average of 34 accidents and 773 fatalities.

In fact, 2012 was the safest year since 1945 by number of accidents. The number 

of accidents involving passenger flights was the lowest since 1945: eleven 

accidents, as compared to the ten-year average of 16 accidents.

The low number of accidents comes as no surprise, according to Aviation Safety 

Network President Harro Ranter: “Since 1997 the average number of airliner 

accidents has shown a steady and persistent decline, probably for a great deal 

thanks to the continuing safety-driven efforts by international aviation organizations 

such as ICAO, International Air Transport Association (IATA), Flight Safety 

Foundation and the aviation industry.”

Not only is the absolute number of accidents continuing to decline, but the rates of accidents per 

departure have also decreased significantly as shown in Figure 5-1 [69].

Figure 5-1 Safety performance in the aviation industry

Similarly, there has been a steady improvement in the safety performance of U.S. nuclear power 

plants as measured by the numbers of “significant events”; this is shown in Figure 5-2. Significant 

events are events that have a significant adverse implication for safety. Examples include events 

that add to the likelihood of core damage, an event rated 2 or more on the International Atomic 

Energy Agency’s International Nuclear Event Scale [70], or a finding rated yellow or red in the 

NRC’s Reactor Oversight Process [1].

The reductions in the occurrences of events through time shown in Figure 5-1 and Figure 5-2 

have been accomplished in large part by implementing programs to improve human performance.
33

In both industries, these improvements have been accomplished through systematic approaches 

to the understanding of the causes of human errors, identifying the factors influencing them, and 

making changes based on this knowledge.

One significant example for aviation is the work of the Commercial Aviation Safety Team (CAST), 

which includes representatives from international safety authorities, employee groups (pilots and 

air traffic controllers), and the aviation industry (airplane and engine manufacturers and airlines).

Focusing on U.S. experience, CAST, along with the new aircraft regulations and other activities, 

successfully reduced the fatality risk for commercial aviation in the United States by 83 percent 

from 1998 to 2008. CAST aims to reduce the U.S. commercial fatality risk by another 50 percent 

between 2010 and 2025.

The Procedural Event Analysis Tool (PEAT) process developed by Boeing [64] is an example of 

the kinds of tools developed by the aviation industry specifically to identify the causes of and 

effective corrective actions for human errors. The PEAT process is focused specifically on errors 

involved in using procedures. More information about PEAT is provided in Appendix F. 

In the U.S. commercial nuclear power arena, both the NRC and the industry undertook significant 

efforts to improve performance—particularly human performance in the years following the 

accident at Three Mile Island Unit 2 (TMI-2). As part of its TMI Action Plan [71], the NRC required 

licensees to carry out detailed control room design reviews and to correct human factors 

deficiencies [72]. The NRC also developed human factors guidance [73, 74] to be used by staff 

reviewing these activities. Overall, NUREG-1355 [75] summarizes the variety and breadth of 

changes resulting from TMI-2 lessons learned in a 10-year review. A participant in the detailed 

control room design review process observed that its greatest influence “was a change in attitude 

and perspective about the viability of the input Human Factors makes to the design process” [76].

The NRC continues to update its guidance and technical basis [28] in order to keep pace with 

changing technology and support its use in designing and evaluating control room modernizations 

Figure 5-2 Rates of significant events at U.S. commercial nuclear power plants
34

such as digital control and computer based procedures [77, 78]. The NRC also developed a 

model for evaluating the human factors programs supporting new plant designs being submitted 

for licensing [79]. As noted in NUREG/KM-0001 [80], TMI-2 investigations and lessons learned 

brought about sweeping changes in the U.S. nuclear industry, including improvements in 

emergency response planning, reactor operator training, human factors engineering, radiation 

protection, and many other areas of nuclear power plant operations. The NRC and industry 

continue to respond to TMI-2 and other events worldwide to improve safety.

Improvements also came as a result of efforts by the Institute of Nuclear Power Operations 

(INPO), an industry group formed in response to recommendations by the Kemeny Commission 

after TMI to improve the standards of safety performance. One of the key recommendations was 

improvement in human performance. INPO has developed a variety of human performance 

improvement resources for use by the industry [81, 82].

The NRC has established a long history of using risk as one of its main guiding principles for 

regulation to protect the stakeholders of the domains it regulates. The agency developed the 

Probabilistic Risk Assessment Implementation Plan in 1994 to focus the agency’s efforts on 

PRA-related activities. The Risk-Informed Regulation Implementation Plan (RIRIP) superseded this 

plan in 2000. Then, in April 2007, the NRC replaced the RIRIP with the Risk-Informed, 

Performance-Based Plan (RPP). Each of these plans has guided the NRC in developing 

risk-informed, performance-based regulations. Within this umbrella of programs, the agency has 

developed methods particularly aimed at improving the modeling and management of human 

reliability, a key contributor to risks as quoted above. These include the Technique for Human Error 

Rate Prediction (THERP) found in NUREG/CR-1278 [83], the Accident Sequence valuation Program 

(ASEP) HRA Procedure found in NUREG/CR-4772 [84], and more recently A Technique for Human 

Event Analysis (ATHEANA) in NUREG-1624 [85]. Using these and similar approaches, the NRC has 

implemented a formalized risk perspective to: (1) understand the risk significance and the causal 

contributions to events that have occurred and (2) evaluate the adequacy of remedial actions.

5.2 Improving the Effectiveness of Error and Risk Reduction

One common theme to these improvements has been a focus on both the causes of human 

errors and a process to ensure that the results are leading to an improvement in overall safety.

Often, organizations put in place some kind of event analysis and/or corrective action process as 

a matter of course, but rarely check that the process is having any substantive effect. Figure 5-3

shows a typical performance improvement process. Often the cycle is triggered by some event 

that indicates a problem in performance that can be associated with safety or whatever metric is 

of concern to the system owner or regulator.

Figure 5-3 Performance evaluation
35

The event is then analyzed using tools to identify its causes and potential changes or 

improvements are then evaluated based on the causes identified. Following the evaluation, 

changes are made to the process in the belief that the performance will improve.

The problem with this simple process is that the changes made may or may not be effective in the 

“big scheme of things”. This limitation has been addressed in work by Argyris, who has articulated 

what has become known as “double-loop” learning [86, 87].

A second example might include considering whether specific corrective actions will contribute to 

higher-level goals such as reducing the risks to the public from nuclear radiation. For example, the 

NRC is considering a broader consideration of risk, including in the area of medical uses of 

byproduct materials. NUREG-2150, titled “A Proposed Risk Management Regulatory Framework,” 

recommends more explicit implementation of risk management and defense in depth, as well as 

environmental reviews [1]. Thus, if this framework were to be adopted, a relevant response to the 

question “What is my real goal?” could be “Am I accomplishing a reduction in risks consistent with 

this framework? Has there been a reduction in reportable events?” The intent of the second loop (as 

shown in Figure 5-4) is to evaluate whether the first loop process is accomplishing the real goal.

Thus, if there is no effective reduction of the risks of medical events, what must be done to the first 

loop to make it so?

Figure 5-4 Double loop learning
36

5.3 Significance for Reducing the Risks from Human Error in Radiation Therapy

The experience indicated in this section suggests that substantial reductions in the risks of 

technological activities, especially those dominated by human performance, have been 

accomplished by systematic approaches to understanding the causes of, and contributing factors 

to, human errors. In the case of aviation, which features multiple aspects of human performance 

(e.g., design, selection and qualification, training, and operational practices), this has been 

accomplished in part through a collaborative program (i.e., the CAST program) and in part through 

the leadership of the international regulatory body (ICAO). As a result of their and other related 

activities, human errors and the resulting risks of commercial aviation accidents have fallen 

substantially. In the case of the U.S. commercial nuclear power industry, the joint efforts of the 

NRC and the industry group, INPO, have led to substantial reductions in the incidence of 

significant events by focusing on the area of human performance.

Both of these efforts have been effective in large part because of the use of accident and incident 

investigation tools that not only concentrate on the immediate causes of events but also look to 

make sure the framework of analysis addresses the deeper concerns of accomplishing risk 

reduction and performance improvements.

5.4 Safety Developments from the Nuclear Safety Authority in France

Since 2007, Autorité de sûreté nucléaire (ASN), the French Nuclear Safety Authority, has 

received 1614 radiotherapy event notifications. For some, no health consequences have as yet 

come to light, while others entail serious complications for the patients, even resulting in death in a 

few cases, as in the Epinal overexposures between 2004 and 2005 [88]. In this context, ASN has 

since 2007 been inspecting all of the radiotherapy centers, concentrating in particular on 

organizational and human factors, and has published on July 1, 2008, technical decision No.

2008-DC-103 setting out the obligations of quality assurance in radiotherapy. In addition to 

publishing criteria for the notification of significant radiation protection events, ASN in conjunction 

with Société Française de Radiothérapie Oncologie (SFRO, the French society for oncologic 

radiotherapy) developed a scale for rating the severity of events affecting patients undergoing a 

medical radiotherapy procedure [89].

5.4.1 Why Notifying the Regulator is Important

Following each radiation protection event (e.g., technical anomalies or deviations from 

procedures), it is necessary to understand which measures will prevent its repetition. For patients 

or workers, only an event notification from the radiation therapy center to the regulator will allow 

an independent analysis of radiation protection events. Results of such independent analyses 

serve a key role in defense in depth by:

• Supporting the development of lessons learned in order to prevent event recurrence.

• Limiting the occurrence of worse events.

• Serving as a source of operating experience.

• Supporting the improvement of medical practices in hospitals and radiation therapy 

centers.

5.4.2 The Notification Approach in France

The reporting system defined in the ASN guide is not intended to identify or punish a person; it 

aims to analyze significant events, provide a source of knowledge, facilitate the subsequent 
37

evaluation of an incident or accident, and improve the practices of an institution and/or the 

radiation therapy sector.

As a consequence, with the objective to make permanent progress for patients’ and workers’ 

radiation protection, the ASN defined an approach and issued guidance and newsletters for 

radiation therapy professionals [90] in order to help radiation therapy centers to declare their 

radiation protection events more easily.

This approach rests on: 

• the definition, by both ASN and SFRO, of understandable criteria for radiation protection 

events,

The ability of ASN:

o to check radiation therapy centers analyses as well as the relevance of their 

corrective actions and implementation of operating experience, and

o to contribute to the public information.

5.4.3 The Notification Criteria

In order to better assess consequences (to patients, workers, and/or the public) and causes 

(sources, waste, security issues, etc.) of adverse events, different criteria were established in 

France. Criteria defined in guide n°16 [89] and published on ASN’s Web site aim to:

• Deal with in-depth event analysis in order to take into account events’ real or potential 

consequences, through which ASN would like to:

o Ensure the accuracy of individual analyses.

o Identify corrective actions and provide a source of operating experience from the 

radiation therapy center.

• Develop a justification for regulatory decisions.

• Allow for a consistent treatment of events.

Thus, different criteria were established to better assess consequences (to patients, workers, and/or 

the public) and causes (e.g., sources, waste, security issues). These criteria can be found on the 

French Nuclear Safety Authority website.

Each declared event provides feedback that should help improve the quality and safety of radiation 

therapy. The radiotherapists who agree to join in this move towards transparency (by declaring 

events that have occurred) contribute to strengthening of radiation protection. Between 

205 and 2013, annual radiation protection event notifications to the French regulator increased by 

more than 60 percent, rising from 158 events declared in 2005 to 263 in 2013 [91]. The declaration 

of incidents/accidents by these professionals indicates a positive change in cultural attitudes to 

radiation protection in France.

5.5 Summary

The aviation and commercial nuclear power industries are powerful examples of success stories with 

regard to improving safety. Success has been achieved, in part, because of the implementation of a 

two-loop process in which corrective actions are monitored and evaluated with regard to whether they 

adequately achieve safety-related goals. This effort to move beyond a process and toward an active 

evaluation of the real-world impact of interventions is key to safe operations in complex environments.

39

6 HUMAN RELIABILITY ANALYSIS APPROACH

TO RISK MANAGEMENT IN THE MEDICAL

USES OF BYPRODUCT MATERIALS

In order to effectively apply HRA principles at the level of inspection and corrective action, there 

must be a higher level regulatory framework in place to support these activities. The NRC, in its 

relatively recently published NUREG-2150, A Proposed Risk Management Regulatory 

Framework [1], stated that:

The materials program has successfully developed and incorporated risk insights 

and performance considerations into its rulemaking, policy development, and 

routine licensing and inspection activities. (Finding M-F-1)

Furthermore, it recommended:

The NRC materials program should continue to apply risk insights and 

performance-based considerations, as appropriate, in rulemaking, guidance and 

policy development, and implementation in accordance with the proposed risk 

management framework. (Recommendation M-R-1)

The present work is consistent with this recommendation, in that it recognizes that the dominant 

source of risk in the medical uses of materials is from human performance issues, as discussed in 

earlier sections.

6.1 Overall Approach to Risk Management

There are several levels at which risk-management activities can be applied in this area. Riskmanagement can be seen as applying to the entire process of delivering radiation therapy, 

including the design and use of therapy devices. It also can be seen as applying to just the area of 

the application and use of devices, and it can be seen as applying to just the human performance 

issues related to their use.

In an ideal world, risk-management is best accomplished when all aspects of the relevant system 

are controllable within a common framework. The responsibility for managing the medical uses of 

radioactive materials is, however, divided among different agencies. Most importantly, the 

evaluation and approval of devices is the responsibility of the Food and Drug Administration 

(FDA), part of the U.S. Department of Health and Human Services. The NRC regulates the use of 

such devices. Given this division of roles, the NRC can provide design review input to the FDA 

(including the review of human-device interactions errors) if a significant design flaw is found to 

have led to a medical event (i.e., after an event has occurred). However, the NRC does not 

evaluate potential causes of human error related to equipment design before a medical event 

occurs (e.g., during the FDA licensing review). While there currently is coordination and 

cooperation between the NRC and FDA, more coordination within their respective frameworks for

managing risk could contribute to making effective reductions in the current levels of human error.

There is a history of collaboration and communication between the NRC and FDA. However, 

given the improved understanding of how the interaction between design issues and user issues 

can lead to sources of risk to patients, workers, and the public, an increased level of coordination 

and collaboration could be important to making significant improvements in the levels of safety.

This separation of responsibilities is not unique to this area. In the aviation world, for example, 
40

manufacturers are responsible for the design of the aircraft, including its cockpit design, and the 

airlines are responsible for the training and qualification for the pilots. Different parts of the 

U.S. Federal Aviation Authority (FAA) are responsible for regulating these. In addition, the 

U.S. National Transportation Safety Board investigates all aspects of aviation accidents, thus 

providing the capability to identify and integrate contributions from the interactions of displays and 

human errors, for example.

In order to provide systematic strategies for identifying and implementing improvements, the 

aviation community has used a team approach to identify and implement improvements. The 

team-based CAST approach is credited, for example, with contributing to the continuous 

improvements in risk reduction in commercial aviation.

Given the distributed regulation of medical devices, the NRC might wish to consider strengthening 

the existing team approach with FDA to lead to a broader scope of the NRC’s regulatory 

influence, to allow failures in the operation of equipment to be traced back to causes associated 

with the design.

6.2 Framework for Risk Management for Patient Safety

The NRC and similar regulatory bodies have evolved risk-management processes to provide a 

structured basis for ensuring that acceptable standards of safety are met through the phases of 

licensed operations. Such an approach has been used widely by NRC in its licensing of nuclear 

plants as discussed in several appendices of NUREG-2150 [1].

The NRC’s PRA Policy Statement [92] suggests that medical devices (specifically identified as 

devices used in radiation oncology in the Policy Statement) do not require PRA studies:

Events associated with industrial and medical uses of nuclear materials generally 

involve a simple system, involve radiation overexposures, and result from human 

error, not equipment failure. Because of the characteristics of medical and 

industrial events, as discussed above, analysis of these events using relatively 

simple techniques can yield meaningful results.

These “relatively simple techniques” are not identified. This report presents a range of techniques 

for addressing such a need. As noted earlier, this report has adopted an HRA-informed approach 

which, while not specifically supporting a PRA study, still provides a systematic way to structure 

the analysis of events, to prioritize types of events and human actions for performance 

improvements (i.e., establish a risk focus), and to identify successful approaches for making these 

improvements by leveraging a variety of multidisciplinary tools and knowledge.

In 2000, the NRC sponsored NUREG/CR-6642 [93], which developed a risk-informed framework 

specifically for byproduct materials, including their use in medical applications. The risks considered 

in this framework were to the workers and the public but specifically not to patients.
41

This framework describes a generic risk concept based on the simple view that answering three 

simple questions provides a measure of risk: 

1. What can go wrong?

2. What is the likelihood?

3. What are the consequences? 

The framework identifies how risk is managed by identifying:

• What kinds of hazards exist 

• What physical barriers and controls are present to reduce the chances of or prevent 

people from receiving radiation doses from the hazard inappropriately

The report goes on to consider how this concept applies to different kinds of uses of byproduct 

materials, including HDR and LDR brachytherapy, gamma knife, and manual afterloading 

brachytherapy.

Later, in 2008, the NRC developed a document called Risk-Informed Decision making for Nuclear 

Material and Waste Applications [94], which described a risk-informed decision making (RIDM) 

framework with the intention that the staff could use this framework to supplement its approach to 

decision making on regulatory relief, safety considerations, and additional requirements. Figure 6-1

shows the generic form of risk-informed decision making presented in that document. Again the 

emphasis is on risks to workers and the public, but not those to patients in medical applications.

Human error is identified in both reports as a major contributor to risk significant events.

In principle, the issue of risks to patients can be viewed in the same light as the risks to workers and 

the public. The steps involved in the framework analysis (identifying the potential hazards and the 

barriers and controls against them) can apply equally to the patient-related hazards. In 1995, the 

NRC published studies of human factors issues relating to patient misadministration events of some 

medical uses [10, 11] that indicated that an HRA approach could be used to identify potential 

reductions in risk to patients during treatments. The work described in this current report is, in some 

ways, an extension to, and updating of, those studies.
42

Figure 6-1 Generic form of risk-informed decision making

The recent NRC discussion of the Proposed Risk Management Regulatory Framework presented 

in NUREG-2150 [1] highlights the robust requirements for medical uses of radioactive materials:

For example, the requirements for therapeutic applications of byproduct material, 

particularly those involving high activity sources, such as high dose rate 

afterloaders or gamma stereotactic radiosurgery units, are more robust than those 

for diagnostic nuclear medicine and may include multiple physical barriers and 

administrative controls to protect workers, patients, and members of the public.

However, as seen from the events associated with such devices described earlier in this report, 

there are opportunities for significant human errors during the use of high activity sources. The 

activity levels require treatment teams to operate devices remotely, requiring increased use of 

automation to operate the barriers and controls. These levels of automation create, for example, 

new opportunities for human errors to result in increased risks of erroneous treatments.

The NRC might wish to consider the increased use of human error analyses and HRA 

improvement techniques within a risk management framework to identify both significant causes 

of medical events and appropriate corrective actions to reduce the risk to patients. One simple 

approach could be to provide systematic feedback to all licensees about the occurrence of events, 
43

the causes of those events, and relevant corrective actions. This should parallel the approach 

taken for feedback about reactor operating experiences.

6.3 Recommendations Summarized

Human performance issues are a dominant source of risk in the medical use of byproduct 

materials. Given the complexity of human performance in healthcare and the division of regulatory 

responsibilities, the NRC might wish to consider increasing its teaming efforts with the FDA to 

provide more comprehensive and effective oversight and guidance. An increased emphasis on an 

HRA-informed perspective in the context of the medical use of byproduct materials is likely to lead 

to a better understanding of error, the role humans play in managing error, and effective corrective 

actions. The NRC might also wish to consider increased use of human error analyses and HRA 

improvement techniques within a risk-management framework.

45

7 SUMMARY AND CONCLUSIONS

Error will occur despite the best intentions, careful design, and skilled personnel. From a human 

reliability point of view there is a dual aim: 1) to reduce the likelihood of error and 2) to increase the 

likelihood of recovery from error before negative consequences occur. HRA offers a perspective for 

understanding human capabilities, strategies for identifying system vulnerabilities or “error traps,” 

methods for analyzing errors when they do occur, and recommendations for developing effective 

corrective actions. A comprehensive HRA-informed approach would also include two-loop learning 

(Figure 5-4), in which an organization deliberately articulates safety-related goals and evaluates 

corrective actions to determine whether they are, in fact, contributing to increased safety.

The NRC has been conducting human reliability studies about the use of byproduct materials in 

radiation therapy since the early 1990s. An important next step is to increase the emphasis on 

human reliability in operations. Although adapting HRA-informed approaches for use in healthcare 

is not a trivial undertaking, the authors of this report anticipate that application of HRA-informed 

approaches will lead to a better understanding of individual incidents, as well as error trends 

across sites. An improved understanding of error will lead to more effective corrective actions that 

reduce the likelihood of error and increase the likelihood of recovery from error. The aviation and 

commercial nuclear power industries stand as powerful examples of the value of an 

HRA-informed approach for increasing safety.

Of course, the scope of NRC’s regulatory responsibilities is currently limited. The agency is not 

directly involved in assessing the design of medical equipment, for example. However, if there are 

to be improvements made in the safety to patients, workers, and the general public, a start can be 

made on addressing the human reliability issues within NRC’s sphere of regulation, while setting 

the stage to address the potential for influencing the areas currently beyond that sphere.

Enhanced teaming efforts with the FDA to coordinate efforts may be an effective strategy for 

additional improvement.

In conclusion, it is recognized that the NRC has a strong track record in developing and 

implementing HRA methods, particularly in the nuclear power domain. Although healthcare 

provides new and different challenges, the authors believe an HRA framework will have value for 

increasing safety for the medical use of byproduct materials, as well.

Two recommendations are offered to follow up on these conclusions:

1. Human errors should be recognized as important contributors to radiation therapy events. 

Strategies for reducing such errors are best addressed by a combination of disciplines, 

including the medical and health physics fields, HRA, human factors, and cognitive 

science. In particular, HRA can be used to integrate inputs from multiple disciplines and 

provide a risk-informed perspective, allowing prioritization of potential fixes and balancing 

of competing needs.

2. Historical experience in addressing human errors in technologies such as commercial 

nuclear power and aviation has shown that, while there are multiple approaches to 

incorporating “human factors” into an error reduction effort, not all approaches are equally 

effective. The NRC has in-house experience on these “more effective approaches” 

(predominantly in the Offices of Nuclear Regulatory Research and Nuclear Reactor 

Regulation) that could assist or guide human error reduction efforts for radiation therapy. 

47

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A-1

APPENDIX A

ROOT CAUSE ANALYSIS

Many strategies for analyzing human error have been proposed. Root cause analysis is one of the 

most common, has been used extensively in nuclear power industry, and in more recent years 

has been adapted for use in healthcare. This section takes a closer look at the issue of root cause 

analysis, as defined in NUREG/CR-6751 [66] regarding human performance evaluation:

Root cause analysis generally is described as being a systematic method for analyzing the 

evidence collected about a hardware failure or human performance problem. The purpose of 

root cause analysis is to identify the basic set of conditions that, if eliminated or modified, 

would minimize the likelihood of the same and similar problems from happening again.

Because root cause analysis techniques have evolved to support specific technologies and 

applications, there is no definitive reference for these techniques. However, many approaches are 

likely to be based on early work in this area performed by the U.S. Department of Defense [95].

A.1 Root Cause Analysis in the Power Industry

In the U.S. nuclear power industry, the NRC requires that:

...In the case of significant conditions adverse to quality, the measures shall 

assure that the cause of the condition is determined and corrective action taken to 

preclude repetition.” (10CFR50, Appendix B, Criterion XVI) [95]

Similarly, the U.S. Department of Energy [96] defines root causes as “the causal factors that, if 

corrected, would prevent recurrence of the same or similar accidents. Root causes may be 

derived from or encompass several contributing causes.” With respect to corrective action, the 

DOE guidance notes that root causes “are higher order, fundamental causal factors that address 

classes of deficiencies, rather than single problems or faults” and that “correcting root causes 

would not only prevent the same accident from recurring, but would also solve line management, 

oversight, and management system deficiencies that could cause or contribute to other 

accidents.” Performing a systematic root cause analysis and identifying the direct, contributing 

and root causes of human performance problems, aids in ensuring that the problems are 

understood with sufficient depth to support the development of effective corrective actions.

A.2 Root Cause Analysis in Healthcare

The recurrent theme of these definitions is that the depth of investigation should be sufficient to 

lead to corrective actions that eliminate or reduce significantly the likelihood of the event being 

investigated and similar failure events. The healthcare community has developed a similar 

approach to defining root causes and corrective actions. Specifically, The Joint Commission on 

the Accreditation of Healthcare Organizations (JCAHO, also commonly referred to simply as “The 

Joint Commission”)5 requires hospitals and other health providers to perform root cause 

investigations on “sentinel events.”

 5 An independent, not-for-profit organization, The Joint Commission accredits and certifies healthcare organizations and 

programs in the United States.
A-2

A.2.1 The Joint Commission’s Sentinel Events

The Joint Commission defines a sentinel event as:

... any unexpected occurrence involving death or serious physical or psychological 

injury, or the risk thereof. Serious injuries specifically include a loss of limb or 

function. The phrase “or the risk thereof” includes any process variation for which a 

recurrence would carry a significant chance of a serious adverse outcome [97]. 

The list of specific events to be included as sentinel events only partly overlaps the NRC’s definition 

of a medical event. Specifically for radiation therapy, doses in excess of 1500 rad (15 Gy), any dose 

to the wrong site, and doses in excess of 25 percent above the planned dose are considered 

sentinel events by the Joint Commission. The complete list of representative events provided by the 

Joint Commission is shown in Table A-1. The Joint Commission’s representative categories and 

subcategories of root causes of sentinel events [98], listed in Table A-2, encompass:

• “Fundamental reason(s) for the failure or inefficiency of one or more processes.”

• “Point(s) in the process where an intervention could reasonably be implemented to change 

performance and prevent an undesirable outcome.”

It goes on to state that:

• “The majority of events have multiple root causes.”

Table A-1 The Joint Commission’s definition of a sentinel event

• The event has resulted in an unanticipated death or major permanent loss of function, not 

related to the natural course of the patient’s illness or underlying condition, or 

• The event is one of the following (even if the outcome was not death or major permanent 

loss of function):

o suicide of any individual receiving care, treatment or services in a staffed around the 

clock care setting, or within 72 hours of discharge

o unanticipated death of a full-term infant

o abduction of any individual receiving care, treatment or services

o discharge of an infant to the wrong family

o rape

o hemolytic transfusion reaction involving administration of blood or blood products having 

major blood group incompatibilities

o surgical and nonsurgical invasive procedure on the wrong patient, wrong site or wrong 

procedure

o unintended retention of a foreign object in an individual after surgery or other procedure

o severe neonatal hyperbilirubinemia (bilirubin > 30 milligrams/deciliter)

o prolonged fluoroscopy with cumulative dose > 1500 rads to a single field, or any delivery 

of radiation therapy to the wrong body region or 25 percent above the planned radiation 

therapy dose
A-3

Anesthesia Care—Planning, monitoring and/or discharge

Assessment—Adequacy, timing, or scope of assessment; pediatric, psychiatric, alcohol/drug, 

and/or abuse/neglect assessments; patient observation; clinical laboratory testing; care 

decisions

Care planning—Planning and/or collaboration

Communication—Oral, written, electronic, among staff, with/among physicians, with 

administration, with patient or family

Continuum of Care—Access to care, setting of care, continuity of care, transfer of patient, 

and/or discharge of patient

Human Factors—Staffing levels, staffing skill mix, staff orientation, in service education, 

competency assessment, staff supervision, resident supervision, medical staff 

credentialing/privileging, medical staff peer review, other (e.g., rushing, fatigue, distraction, 

complacency, bias)

Information Management—Information management needs assessment, confidentiality, 

security of information, data definitions, availability of information, technical systems, patient 

identification, medical records, aggregation of data

Leadership—Organizational planning, organizational culture, community relations, service 

availability, priority setting, resource allocation, complaint resolution, leadership collaboration, 

standardization (e.g., clinical practice guidelines), directing department/services, integration of 

services, inadequate policies and procedures, non-compliance with policies and procedures, 

performance improvement, medical staff organization, nursing leadership

Medication Use—Formulary, storage/control, labeling, ordering, preparing/distributing, 

administering, and/or patient monitoring

Nutrition Care—Nutrition care planning, timing, storage, and/or patient monitoring

Operative Care—Operative care planning, blood use, and/or patient monitoring

Patient Education—Planning education, providing education, effectiveness of education

Patient Rights—Informed consent, participation in care, end-of-life care, pain management, 

privacy

Performance Improvement—Improvement planning, design/redesign testing, 

design/redesign measurement, data collection, data analysis, improvement actions

Physical Environment—General safety, fire safety, security systems, hazardous materials, 

emergency management, smoking management, equipment management, utilities 

management

Rehabilitation—Rehabilitation care planning, patient monitoring

Special Interventions—Special intervention planning, assessment, restraint equipment, 

patient monitoring

Surveillance, Prevention, and Control of Infection—Sterilization/contamination, universal 

precautions

Table A-2 The Joint Commission’s categories of root causes and sentinel events
A-4

Figure A-1 shows the relative contributions of The Joint Commission’s categories for all sentinel 

events (roughly 2500) reported for 2010 through the second quarter of 2012 (the last time period 

available at the time of writing this report), and Figure A-2 shows the breakdown of root causes; 

there were 29 such events in the category of radiation overdoses alone. The proportions of events 

attributed to human error are virtually the same.

Seen together, both The Joint Commission’s and the nuclear industry’s approaches to root cause 

analysis and corrective action planning are similar. Both require investigating safety-significant 

events beyond simply describing the surface actions of the people and any hardware faults, to 

identify underlying influences that can be modified. A good example of the levels of analysis to 

accomplish this level of understanding is discussed in “A Tale of Two Stories,” a report produced 

following a workshop on patient safety [99]. The report takes two approaches to the investigation 

of three actual medical events to illustrate that simply taking a look at the actions of people without 

digging down into the processes and context leads to ineffective changes.

The “comparison and contrast” approach used in the “A Tale of Two Stories” illustrates a difference 

between getting at the superficial causes of an event and the more in-depth understanding of how 

complexity and attention to the whole story can identify causes that are not otherwise understood.

For example, in a case that involved using a computer managed infusion device, a patient was 

inadvertently administered a free flowing infusion of a vasoactive drug instead of a measured/timed 

dose. The event was originally classified as a user error in the setting up of the infusion device, with 

phrases like “You can’t make devices idiot-proof” and “You have to be careful, it can burn you” being 

used. However, when the “second story” investigation took place, it was found that similar events 

had occurred in the past and that, when the setup process was videotaped and analyzed, there 

were many opportunities for failure. As a result, use of the particular device was reduced and alerts 

were published in the professional press about the risk.
A-5

Figure A-1 Root causes for all of The Joint Commission sentinel events, 2010 through 

2012, 2nd quarter

Figure A-2 Root causes for The Joint Commission sentinel events involving radiation overdoses

Human Factors 

Leadership 

Communication 

Information 

Management 

Physical 

Environment 

Operative Care 

Care Planning 

Continuum of Care Medication Use 

Human Factors 

Leadership 

Communication 

Information 

Management 

Physical 

Environment 

Operative Care 

Care Planning Medication Use 
A-6

A.2.2 NRC Medical Events

NRC regulations (in 10 CFR 35.3045 Part M, , “Report and Notification of a Medical Event”)

require the reporting of events associated with the treatment of patients with radiation therapy 

using byproduct materials when a dose differs significantly from the prescribed dose; exceeds a 

specified dose to organ, tissue, or skin; or is administered to the wrong site. See Table A-3 for 

specific criteria.

Events caused by patient interventions are not reportable for the types of events described above.

However, a licensee is required to report any event resulting from intervention of a patient or 

human research subject in which the administration of byproduct material or radiation from 

byproduct material results or will result in unintended permanent functional damage to an organ or 

a physiological system, as determined by a physician.

The NRC’s NMED program provides a single repository for medical event reports for both the 

NRC and the Agreement States. As part of this program, events are classified according to cause.

Figure A-3 shows the breakdown of causes of medical events for Fiscal Year (FY) 2011 through 

the 3rd quarter.

Table A-3 NRC criteria for a reportable event

(1) A dose that differs from the prescribed dose or dose that would have resulted from the 

prescribed dosage by more than 0.05 Sv [Sieverts] (5 rem) effective dose equivalent, 

0.5 Sv (50 rem) to an organ or tissue, or 0.5 Sv (50 rem) shallow dose equivalent to the 

skin; and

(i) The total dose delivered differs from the prescribed dose by 20 percent or more;

(ii) The total dosage delivered differs from the prescribed dosage by 20 percent or more or falls 

outside the prescribed dosage range; or

(iii) The fractionated dose delivered differs from the prescribed dose, for a single fraction, by 

50 percent or more.

(2) A dose that exceeds 0.05 Sv (5 rem) effective dose equivalent, 0.5 Sv (50 rem) to an organ 

or tissue, or 0.5 Sv (50 rem) shallow dose equivalent to the skin from any of the following--

(i) An administration of a wrong radioactive drug containing byproduct material;

(ii) An administration of a radioactive drug containing byproduct material by the wrong route of 

administration;

(iii) An administration of a dose or dosage to the wrong individual or human research subject;

(iv) An administration of a dose or dosage delivered by the wrong mode of treatment; or

(v) A leaking sealed source.

(3) A dose to the skin or an organ or tissue other than the treatment site that exceeds by 0.5 Sv 

(50 rem) to an organ or tissue and 50 percent or more of the dose expected from the 

administration defined in the written directive (excluding, for permanent implants, seeds 

that were implanted in the correct site but migrated outside the treatment site).
A-7

While not a one-to-one match, there is considerable overlap with The Joint Commission 

categories of leadership, human error, and communications. In The Joint Commission’s case, 

therefore, these human factors categories amount to 57 percent of identified causes, and in the 

case of NMED, human factors amounts to 71 percent; that is, in both cases, human factors are 

the dominant contributing causes.

Figure A-3 Causes assigned to Abnormal Events from FY2008 through FY2011

The NMED database uses “cause” breakdowns of human error that are similar to those for 

radiation therapy events shown in Figure A-3. For example, within the category of human error, 

NMED includes the following possible causes:

• communication problems

• failure to follow procedure or wrong procedure used

• inadequate training, inattention to detail, and management deficiency

Human Error 

(71%)

Procedure 

Problem

(11%)

Equipment 

Failure

External

Other Not Reported

B-1

APPENDIX B

ERRORS IN REMOTE AFTERLOADING BRACHYTHERAPY

As discussed in Section 2.2.4, NUREG/CR-6125 [9, 10] describes a human factors evaluation of 

remote afterloading brachytherapy (RAB). In that study, a task analysis was used to decompose 

each of the functions comprising RAB into individual steps. For each critical step, the authors 

evaluated practices associated with the step and alternative techniques that might be employed to 

reduce the likelihood of misadministration.

The following summary table is based on those evaluations. The table shows the critical tasks, the 

contributing factors that were related to human performance vulnerabilities, and mitigating actions 

that could address the human performance vulnerabilities.

Table B-1 Contributing and mitigating factors for error in RAB from NUREG/CR 6125

Critical Task

(RAB)

Human Performance 

Vulnerability Contributing 

Factors”)

Potential Mitigating

Actions

(“Alternative Approaches”)

Patient Scheduling, 

Identification, Tracking

ID wrist bands not used on all RAB 

patients (inpatient treatments only); 

wrist band ID codes long and hard 

to remember; reliance on 

memorized information

Provide job performance aids; redesign ID and 

linkage procedures; staff training

Applicator Selection, 

Placement, and 

Stabilization

Must distinguish applicators from 

many similar ones; lack of 

feedback about applicator 

placement; mental calculations 

needed to translate 2D images into 

3D relationships

New methods/procedures to ID, preserve, and 

transfer applicator characteristics information; 

methods/procedures to detect and compensate 

for applicator movement; increased training

Target Volume 

Localization

Primitive interfaces with no 

feedback regarding performance or 

detection of translation errors; need 

for staff to do mental calculations to 

combine images of different 

magnifications and sources

Transfer images electronically without requiring 

staff to do mental calculations; visualization and 

calculation aids to help in translating different 

scales/magnifications into set of parameters; 

standardize records for magnification and view 

angles; train staff in mental imagery skills

Dwell Position 

Localization

Measurement of distances to 

treatment site and from machine to 

source guide tube; must identify 

end of applicator; must rely on 

dummy source measurements

Measurement of insertion distance after 

placement of dummy source; graphic feedback of 

orientation of entered dwell positions; make it 

easier to distinguish dummy sources in imaging; 

standardize dwell position measuring procedures

Dosimetry Manual entry of dwell positions and 

targets into planning system; no 

feedback on digitization accuracy; 

poor interface design; time 

pressure; distractions

Ability to scan entire simulation view into 

treatment planning computer manual numeric 

entry; increase computer feedback; improve 

document labeling; limit distractions and time 

pressure for dosimetrists; better training on 

software
B-2

Critical Task

(RAB)

Human Performance 

Vulnerability Contributing 

Factors”)

Potential Mitigating

Actions

(“Alternative Approaches”)

Treatment Setup Applicator labels usually small 

and low-contrast; difficult to 

assess whether applicators are 

connected correctly

Supply a map of connection specifications; 

improve labeling to make wrong connections 

more obvious; training in correct connections

Treatment Plan Entry Manual entry into treatment 

console; calculations not easily 

visible/verifiable by users; “swap” 

errors could occur with magnetic 

cards or disks; difficult to identify 

erroneous differences between 

initial directive and final 

parameters

Direct electronic transfer to treatment console; 

better feedback from system; erase old 

treatment plans from cards and disks; staff 

training in detecting errors

Quality Assurance 

and Maintenance

Inadequate checklists; hidden QA 

checks performed by equipment

Integrate equipment/software QA with manual 

QA procedures; use more checklists; clear 

feedback regarding which QA checks should 

be done, how often, and what passing/failing 

means for performance; training on 

QA/emergency procedures

Source Exchange Inadequate emergency 

procedures; emergency retraction 

devices difficult to operate; can’t 

inventory sources while in their 

safes

Means for detecting presence of a source (vs. 

absence); supply tools to cut stuck sources 

free from cables; test equipment after source 

exchanges to detect any changes; staff training 

(if exchanges aren’t done by manufacturer)

Source Calibration Inadequate feedback regarding 

calibration errors; lack of 

consensus on standard 

calibration procedures; calibration 

requires time and special 

equipment in some cases

Standard chamber to measure activity of each 

radionuclide; regular calibrations should be 

performed and compared to expected decay; 

staff training
C-1

APPENDIX C

ERRORS IN TELETHERAPY

As discussed in Section 2.2.4, NUREG/CR-6277 [11] describes a human factors evaluation of 

teletherapy. In that study, a task analysis was used to decompose each of the functions 

comprising teletherapy into individual steps. For each critical step, the authors evaluated practices 

associated with the step and alternative techniques that might be employed to reduce the 

likelihood of misadministration.

The following summary table is based on those evaluations. The table shows the critical tasks, the 

contributing factors that were related to human performance vulnerabilities, and mitigating actions 

that could address the human performance vulnerabilities.

Table C-1 Contributing and Mitigating Factors for Error in Teletherapy from NUREG/CR 6277

Critical Task 

(Teletherapy)

Human Performance 

Vulnerability (“Contributing 

Factors”)

Potential Mitigating Actions 

(“Alternative Approaches”)

Pre-Treatment 

Planning

Absent or informal because 

treatment is considered routine; 

time pressure; language 

difficulty

Stipulate plan conference for difficult 

cases; address workload issues; 

review communication processes and 

target improvements

Treatment 

Planning 

Computer Data 

Files

Opaque nature of systems; 

absence of procedures for 

performing “hidden” tasks; 

training isn’t thorough and relies 

on self-teaching

Render functions more visible; include 

interlocks; procedures to prevent 

out-of-date files from being used; more 

systematic training

Use of Patient 

Chart

Poor layout and design, 

including inconspicuous 

location of key information; 

illegible or ambiguous entries

Formal, iterative design process and 

consideration of best practices to 

improve chart layout; education 

regarding bad and good practices in 

chart entry

Patient Positioning 

and Immobilization

Reproducible patient 

positioning across different 

treatments; lack of critical 

analysis of existing aids; on 

skin tattoo markers, which can 

vary; high workload

Critically review and share info on 

different techniques; avoid sole 

reliance on skin markers; address 

workload issues

Treatment Setup 

Slips and Lapses

High workload and rapid pace 

of treatment schedules; 

understaffing; poor workstation 

layout, leading to distractions; 

competing tasks 

(i.e., answering phones)

Schedule patients in a more evenly 

distributed manner; address staffing 

issues; redesign workspace layout to 

reduce distractions; use computerized 

record and verify systems to check 

deviations from prescriptions; educate 

managers
C-2

Critical Task 

(Teletherapy)

Human Performance 

Vulnerability (“Contributing 

Factors”)

Potential Mitigating Actions 

(“Alternative Approaches”)

Data Transfer Treatment calculations being 

made while the patient waits; 

uncritical review of 

computer-generated treatment 

plans

Schedule patients in an evenly 

distributed manner and minimize 

calculations that are done while the 

patient waits for treatment; 

independent verification of all computer 

inputs; improve layout of interfaces to 

improve vigilance for errors

Equipment Checks 

and Calibration

Equipment failures; inadequate 

or nonexistent procedures; 

operator errors; lack of 

organizational equipment for 

conducting recommended 

procedures

Create panel of users to address 

organizational commitment issues and 

implement new ways of increasing 

vigilance; during transitions, employ a 

monitor or run old and new procedures 

in parallel until everyone shows 

proficiency in the new procedure; use 

systematic training

Radiation Field 

Alignment 

Accuracy

Day-to-day variations of 

treatment field alignment; 

discrepancies between 

simulated and actual treatment 

conditions; mechanical changes 

to equipment; changes in 

tumor/body contours/size

Take portal images about once a week; 

electronic imaging options available 

during treatment to better visualize 

radiation field; review films often as part 

of facility’s QA program

Radiation 

Treatment 

Delivery 

Therapists having to perform 

other tasks during treatment 

time (increases workload); 

habituation to warning cues; 

lack of training in emergency 

procedures

Address therapist workload issues; add 

an auditory alarm that is unique to an 

emergency situation (stuck source); 

scenario-based training for emergency 

procedures

Quality Assurance 

(QA) Procedures

Lack of resources for 

implementing a QA program; 

knowledge and skill base 

needed; sustaining QA 

programs is difficult because 

many of the tasks are mundane 

and of low visibility

Involve upper management; develop 

strong QA program based on written 

guidelines; develop structure to support 

QA activities within a department
D-1

APPENDIX D

ANALYSIS OF RADIATION THERAPY EVENTS FROM 2001 

THROUGH 2010: ERROR CATEGORIES AND SAMPLE EVENTS

In Section 2 of this report (in particular, Table 2-1), we refer to an analysis that was conducted to 

identify error trends across different types of treatment modalities. Below is a summary of the 

results of that analysis.

D.1 High Dose Rate (HDR) Brachytherapy

Event review. Of the 16 Abnormal Occurrences (AOs) reviewed, four events resulted in the wrong 

dose being administered and 12 events resulted in the wrong site being treated. There were a 

total of 21 errors, most of which involved human performance issues. Many errors involved 

mistakes in interacting with the treatment planning software (typographical errors or interface 

issues). There were several measurement errors in which the length of the catheter was not 

measured accurately, leading to the source treating the wrong site. There were a few errors in 

which the equipment was improperly set up, mainly because the connections between the 

catheters were not correct. It’s also interesting to note that some errors went unnoticed during 

verification checks of treatment plans. Each type of error is described below.

D.2 Data Entry and Planning-Software Errors

14 HDR brachytherapy errors fell into the category of data entry and planning software errors. In many 

of these events, the medical physicist simply committed a typographical error when entering a value 

(i.e., catheter length, starting location, or step distance) in the treatment planning software. Some 

event reports state that the software reset to default values after the other data had been entered.

An example event that fell into this category is event AS 07-06, which involved a patient who was 

prescribed to receive several HDR brachytherapy fractions. The wrong isodose line was chosen

and entered into the treatment planning system, which then normalized the calculations and 

produced a treatment plan around the incorrect isodose line. The oncologist signed and approved 

the plan and the radiation safety officer performed a second calculation check. An independent 

physicist identified the error before the second treatment fraction.

Another example event, uncovering a serious interface issue, is event AS 11-07. A therapist 

accidentally hit an “auto radiography” button instead of the “treatment” button while administering 

HDR treatment to a patient’s ear. This resulted in the patient receiving a dose that was nine times 

the intended dose. The facility had to disable the “auto radiography” function after this occurred.

D.3 Equipment Errors

There were 5 HDR brachytherapy equipment errors. Due to the nature of the HDR brachytherapy 

procedure, several equipment connections must be properly set up. The errors in this category 

involve the incorrect connections of catheters to the HDR unit.

An example event in this category is event AS 09-05. The aluminum connector to an HDR unit’s 

needle detached from the unit, which prevented the patient from receiving the full dose. The 

adhesive that attached the connector to the unit failed because hospital staff had sterilized the 

needle and the plastic extension adaptor that connects the aluminum connector to the unit had 
D-2

been used more than once. The manufacturer’s written product information warns that sterilization 

may cause adhesive failure, and that the plastic extension adaptors (to which the aluminum 

connector attaches to the unit) should only be used once. The corrective actions in response to 

this event included procedure modifications.

D.4 Verification Errors

There were two verification errors in HDR brachytherapy. Both events are categorized as one of 

the above error types (i.e., data entry) and as a verification error. Verification errors are usually 

secondary errors, in which review of the treatment plan did not identify the original error. These 

errors are troubling, because many corrective actions that are implemented after an event occurs 

involve adding additional checks and reviews.

One example of an event in this category is event AS 07-06, mentioned in the section on data 

entry and treatment planning software errors. An initial error (choosing the wrong isodose line) 

was used to create a valid treatment plan. The oncologist signed and approved the plan, and 

someone else did a calculations check. Neither of these verification checks picked up on the initial 

error because the initial error was in the original parameters of the calculations and not in the 

calculations themselves.

D.5 Gamma Knife

Event review. There were ten gamma knife AOs were reviewed, four of which resulted in a wrong 

dose, being administered and six of which resulted in the wrong site receiving treatment. A total of 

14 errors were associated with these 10 events, all involving human performance issues. Many 

older events (late 1990s to early 2000s) involved data entry and treatment planning system errors.

Because of changes (i.e., electronic transfer of information) in the gamma knife units and 

software, some of the errors in this category are becoming less prevalent. However, there have 

still been events in which the wrong site was treated because of imaging and laterality errors.

Errors have also resulted from the incorrect setup of the gamma knife equipment and verification 

errors. Each category of error is discussed in turn.

Data entry and planning software errors

Two events were associated with data entry and planning software errors for the gamma knife.

Many older events that occurred before an updated treatment planning software package 

happened because x, y, and z coordinates were transposed (e.g., the values x-y-z were entered 

as y-z-x instead). However, newer software does not involve manually typing in these coordinates, 

thereby reducing the likelihood of this type of error. Other types of data entry errors still occur, 

however, such as entering the incorrect treatment time. Some events resulted from issues with 

the planning software itself, such as the inadvertent use of default values, and a system that had 

an incorrect date, which affected source strength calculations.

Event AS 07-05 is an example of a data entry error in this category. A physician prescribed a 

gamma knife treatment of 18 Gy (1800 rad) for a patient. Instead of waiting for the medical 

physicist to enter the prescribed value in the computer treatment plan (as was the usual 

procedure), the physician erroneously entered it into the computer as 28 Gy (2800 rad). The event 

description implies that the physician was in a hurry to leave for the day, so decided to enter the 

information directly. The mistake was not identified until after the patient received the full incorrect 

dose (a 56% overdose).
D-3

Equipment errors

Three errors were categorized as resulting from the incorrect setup of the gamma knife 

equipment. With older models of the gamma knife, errors usually arose when the medical 

physicist failed to change the collimator helmet between treatment fractions. However, newer 

gamma knife models do not need separate collimator helmets; instead, the computer controls the 

size of the radiation beam. There are still several possibilities for error in setting up the gamma 

knife equipment, though.

For example, event NRC 10-05 describes an equipment malfunction that was also compounded by 

an ambiguous error message. Two patients were scheduled to receive gamma knife treatments on 

the same day. During the first patient’s treatment, the automatic positioning system reported an 

error. The operators called the manufacturer, who told them to reinitialize the positioning system, 

then complete the treatment. During the second patient’s treatment, the same error appeared. The 

manufacturer sent a service representative to inspect the unit. The representative noted that one of 

the axis indicators was off by 4.5 mm. An NRC investigation concluded that one of the causes of this 

event was inadequate procedures that did not require physical verification of the automatic position 

system’s coordinates against the electronic coordinates before treatment. This was probably a result 

of overconfidence in, or the opacity of, the automation.

Target errors

There were four target errors with the gamma knife. These errors were mainly laterality errors, in 

which one side of the head received treatment instead of the intended side of the head. There 

were also errors in which the fiducial marker was not aligned properly in the imaging process, 

which led to the incorrect site being designated for treatment.

Event NRC 10-02 is an example of this category of error. A laterality error occurred while a patient 

was being treated for trigeminal neuralgia with a gamma knife procedure. The prescribing 

physician had mismarked the treatment planning sheet with the wrong location of treatment (the 

planning sheet was marked as the left side of the face to receive treatment, when in reality the 

right side needed treatment). The error was not identified during review of the planning sheet 

document, and the patient received the incorrect treatment.

Another example of this type of error is event AS 09-04. A patient was prescribed to receive 

8000 centigrays (cGy) (8000 rad) to the 5th intracranial nerve during a gamma knife treatment for 

trigeminal neuralgia. All necessary people completed and signed the written directive; however, the 

wrong nerve was designated for treatment while preparing the gamma knife unit. The cause of this 

incident was a misidentification of the anatomical target site that was listed on the written directive.

Verification errors

Four verification errors were associated with the gamma knife. As with the other types of therapy, 

many gamma knife errors were able to slip past the verification and review processes that were in 

place at a facility, although one event fell under this category because the hospital staff failed to 

verify that the treatment plan matched the patient they were treating (02-2). Event NRC 10-02, 

listed above in the section about wrong site errors, is a typical example of verification procedures 

failing. In this event, the treatment planning sheet itself was mismarked as to the location to be 

treated and the error was not detected during review of the document.
D-4

D.6 Intravascular (IV) Brachytherapy

Event review. Six IV brachytherapy AOs were reviewed in connection with seven total errors. Two 

events resulted in a wrong dose being administered and the remaining four events resulted in the 

wrong site receiving treatment. Most errors involved equipment failures. All occurred before 2004, 

when IV brachytherapy was an emerging procedure and new technologies were being put to use 

for the first time in the field. The events that seemed to arise from human performance issues all 

dealt with dosage errors.

Data entry and planning-software errors

There was one data entry and planning-software error with IV brachytherapy. The authorized user 

was accustomed to operating a different treatment planning system that used the radius of the 

artery to be treated; however, the event resulted when the user operated a different system that 

used the diameter of the artery to create the treatment plan. This error resulted in a wrong dose to 

the artery that was being treated (AS 02-4).

Equipment errors

Four errors fell under the category of equipment errors for IV brachytherapy. One event involved a 

machine malfunction in which the source did not reach the intended site. When the physician 

noticed this error and tried to retract the source, it could not be retracted. The physician had to 

physically pull the catheter out of the patient with the source still inside the catheter (03-01). The 

rest of the events involved issues with the catheter that prevented the source from getting to the 

intended site either (1) kinks or (2) too much external pressure caused by the artery it was 

threaded through.

Target and visualization errors

One event in the IV brachytherapy review fell under both the target error and visualization error 

categories. In event 03-03, the source train failed to get to the intended target site because the 

licensee failed to properly visualize the source train’s placement. This was partly because there is 

an inherent inability to differentiate between the proximal and distal markers of the source train.

D.7 Low Dose Rate (LDR) Brachytherapy

The term “low dose rate brachytherapy” is used to describe several different modalities of 

treatment in which sources are typically are left in a patient for extended periods of time (hours to 

days). These modalities are manual brachytherapy, manual afterloading brachytherapy, and 

manual implant brachytherapy. While the details of each are quite different medically, they are 

grouped together here because they represent typically much lower risks to workers and the 

public than do the high dose rate treatments, and there are strong similarities the kinds of human 

errors that occur.

Event review. Twenty-two AOs associated with LDR brachytherapy events were reviewed in 

connection with 37 total errors. Only one event was the result of a dosage error; the rest involved 

the wrong site being treated. Many events occurred as a result of incorrect source placement.

Most of the events involved target errors in which the sources were incorrectly placed.
D-5

Data entry and planning-software errors

One error for LDR brachytherapy fell under this category. Event AS 07-03 resulted from the wrong 

Dose Rate Factor (DRF) being used in the treatment plan. This occurred because the licensee 

entered the DRF for air kerma, but the seed strength was entered in milligram radium equivalent 

in the treatment planning system. An additional facet of this event was that the treatment planning 

system that was used had undergone acceptance testing for treatments using Cs-137 and I-125, 

but not for Ir-192, which was the source called for in this particular patient’s treatment (along with 

Cs-137). These discrepancies resulted in the patient receiving the wrong dose.

Equipment errors

There were three equipment errors in the review of LDR brachytherapy AOs. Two events resulted 

from a piece of equipment not being the correct length, resulting in treatment to the wrong site.

The last event, event 05-03, was the result of improper source selection. The Cs-137 sources that 

were used to treat several patients were the wrong size for the applicator used. They migrated 

from the treatment site through a placement spring that was supposed to keep them in place 

when the patients moved as part of the treatment. This error was exacerbated by inadequate 

manufacturer instructions and inadequate management oversight and procedures.

Target errors

With 18 errors in this category, target errors were the most common errors relating to LDR 

brachytherapy. Most of these events involved the use of I-125 brachytherapy seeds to treat prostate 

cancer. For example, event AS 04-01 describes a typical occurrence for events in this category. The 

brachytherapy seeds were implanted under ultrasound guidance. However, when the patient 

returned for the post implant CT scan, it was discovered that the seeds were implanted 2 cm too 

low, and missed treating the upper portion of the prostate, making additional treatment necessary.

Many of the corrective actions that are implemented in response to this type of error, particularly 

with the use of I-125 brachytherapy seeds for prostate cancer treatment, involve the use of realtime imaging during the procedure. However, as this event demonstrates, modifying procedures 

to allow imaging during the actual treatment may not fully address the underlying issues.

Verification errors

One event was classified as a verification error as well as a data entry and treatment planning 

software error. Event AS 07-03, described in the data entry and treatment planning software error 

section, was also classified as a verification error because multiple checks should have caught the 

discrepancy in DRF but did not. For example, the treatment pre-plan should have been checked 

before the seeds arrived at the facility, but it was not. Also, the calculations should have been 

double-checked before implant or shortly after, but this did not occur either.

Visualization errors

Ten of the target errors identified above also fell under the category of visualization errors. All of 

these events involved the use of I-125 brachytherapy seeds to treat prostate cancer.

All of the event narratives in this category state that the misplacement of seeds was a result of 

failing to accurately identify the location and/or size of the prostate gland.
D-6

In event NRC 08-04, a prostate brachytherapy treatment was terminated after the placement of 

only 37 seeds. The procedure was terminated because it was determined that the first 37 seeds 

were placed 2 cm below the patient’s prostate. The official cause of the event was 

misidentification of the patient’s prostate because of inadequate procedures. Event NRC 10-03 

involved the placement of 93 seeds outside the target site, also because of a failure to adequately 

visualize and identify the prostate before the procedure. Interestingly, the event narrative states 

that several of the staff, who were present for the procedure, including the Authorized Medical 

Physicist, questioned the accuracy of the prostate visualization, but no action was taken to resolve 

the question. Event AS 09-06 describes an under dose because of a miscalculation of the size of 

the patient’s prostate gland.

Many corrective actions to address this type of error involve imaging during the procedure to ensure 

the proper placement of seeds. However, in event NRC 10-06, brachytherapy seeds were still 

placed incorrectly, even though ultrasound was being used during the procedure. This event was 

caused by inadequate ultrasound identification of the prostate during implant because of the 

patient’s unusual anatomy and obesity. Events such as this one indicate that just adding procedures 

to allow imaging during the procedure may not be enough to ameliorate errors in this category.

Errors caused by patient movement

Three events occurred because the patient moved, which affected the placement of the source. In 

all three events, the patient was receiving brachytherapy seed implants for prostate cancer 

treatment. The patient’s movement was not detected in any of the events. One event (NRC 10-04) 

occurred even though the seeds were being placed under ultrasound guidance.

Procedural errors

One event (NRC 08-02) involved over 90 patients receiving suboptimal brachytherapy seed 

treatments for prostate cancer. This is a very serious event because even though follow-up scans 

showed that seeds were not correctly placed, no corrective actions were taken to either retreat 

patients or modify the procedure to improve outcomes. Investigations showed that the procedures 

in place at the facility were inadequate to assure quality implants.
E-1

APPENDIX E

SUMMARIES OF EXAMPLE ABNORMAL EVENTS

The NRC issues (as serial volumes of NUREG-0090) [100-103] annual reports to Congress on 

“abnormal occurrences”—unscheduled incidents or events determined to be significant from the 

standpoint of public health or safety. The event reports originate with either the NRC or with 

Agreement States, which maintain their own compatible programs for the regulation of byproduct 

and source materials and of certain quantities of special nuclear materials.

This appendix contains selected sections of NUREG-0090 abnormal occurrence reports used in 

the present document to illustrate the various types of errors that are associated with medical 

events. The event summaries are ordered by fiscal year (i.e., according to the NUREG-0090 

volume in which they appeared). The identifier for all Agreement State licensee AO reports starts 

with “AS.” Similarly, the identification number for all U.S. Nuclear Regulatory Commission (NRC) 

AO reports starts with “NRC.” The remainder of each event identifier (in the format FY-N or 

FY-NN) indicates the fiscal year (FY) covered by the volume in which the event description 

appears and the serial number.

The following are the FYs for which events in NUREG-0090 volumes were reviewed:

FY 2002

FY 2004

FY 2007

FY 2008

FY 2009

FY 2010

FY 2011
E-2

FY 2002

AS 02-4 Intravascular (IV) Brachytherapy Misadministration at Rhode Island Hospital, 

Providence, Rhode Island

Nature and Probable Consequences—A patient was prescribed a dose of 8 Gray (800 rad) to the 

coronary artery during a Cordis CheckmateTM IV brachytherapy procedure using 10 Ir-192 (Ir-192) 

seeds emitting a dose of 8991 megabecquerels (MBq) (243 millicuries (mCi)). On 

January 31, 2002, during a review of dosimetry and physician records, the licensee discovered 

that the diameter of the artery was used in the treatment plan calculation instead of the radius.

This error resulted because the physicians (authorized users) using the Cordis device were more 

familiar with the procedures for a NovosteTM device also in use at this institution. The Novoste 

device uses the diameter of the artery in the dosimetry calculations whereas the Cordis device 

uses the radius. The authorized user provided the wrong dimension (diameter instead of radius) 

which led to an incorrect dose being calculated. As a result the patient received an actual dose of 

14.6 Gray (1460 rad) to the outer coronary artery site instead of the prescribed 8 Gray (800 rad).

The licensee indicated that there will probably be no adverse health effect to the patient.

Cause or Causes—As stated, the misadministration occurred because of human error in the use 

of the diameter of the artery instead of the radius of the vessel as required when using the Cordis 

system. The physicians’ (authorized users’) familiarity with the procedures for a Novoste device 

was a contributing factor.

Actions Taken to Prevent Recurrence

Licensee—The licensee informed the State of Rhode Island the next day by telephone of the 

potential misadministration and provided a written report of the incident on February 14, 2002. Inservice training has been conducted concerning the misadministration. In addition, the 

prescription form has been modified to indicate whether the radius or the diameter of the vessel is 

being used for the treatment plan.

State Agency—The Agency has been in contact with the licensee concerning this matter and the 

effectiveness of the corrective measures implemented. The licensee indicated that there will probably 

be no adverse health effects to the patient. To date there has been no recurrence of the problem.

AS 02-5 Strontium-90 (Sr-90) Eye Applicator Brachytherapy at South Broward Hospital 

District in Hollywood, Florida

Nature and Probable Consequences—A patient was prescribed radiation treatment for pterygium 

in his left eye. The patient was to receive a total dose of 30 Gy (3000 rad) in three 10-Gy 

(1000-rad) fractions spaced approximately a week apart. Because of human error, the third and 

final fraction, given on January 4, 2002, was 24.84 Gy (2484 rad) instead of the prescribed 10 Gy 

(1000 rad).

The prescribed dose was to be administered through a 3M® Company Model 6D1A eye 

applicator using a 973-MBq (26.3-mCi) strontium-90 (Sr-90) source. The written directive called 

for each fraction to consist of a treatment duration of 44 seconds to deliver a 10 Gy (1,000 rad) 

dose. The correct fractionated dose was administered as planned on December 20, 2001, and 

December 28, 2001. A routine administration of the eye applicator required one person to time the

event with a stopwatch while the authorized user administered the dose. The nurse and the 

authorized user became distracted in conversing with the patient and lost track of the time. The 
E-3

stopwatch used was the old style that simply counted time up and the nurse lost focus in trying to 

make the patient more comfortable and at ease. The authorized user had to remind the patient to 

gaze in a certain direction to treat the affected area. As a result, the third fractionated treatment 

time was 109 seconds instead of the prescribed 44 seconds resulting in a dose of 24.84 Gy 

(2484 rad). The patient was counseled about the slight increase in latent effects, including 

cataract formation and scleral scar tissue formation.

Cause or Causes—The State found and the licensee agreed that the misadministration occurred 

because of human error and the failure of staff to attend to details as required in licensee’s 

procedures.

Actions Taken to Prevent Recurrence

Licensee—The licensee has identified and made changes in their procedures for use of the Sr-90 

ophthalmic applicator. The facility purchased a digital stopwatch that has a large display, counts 

time down and not up, audibilizes the time in the last 10 seconds, and sounds an alarm at the end 

of treatment. In addition, the nurse has been counseled and all personnel have received training 

in the revised procedures using the new stopwatch.

State Agency—The Florida Bureau of Radiation Control performed an onsite investigation on 

February 7, 2002, to review the licensee’s corrective actions, which were found adequate by the State.

The State also determined that while the patient was informed verbally of the misadministration, 

the licensee did not inform the patient in writing as required. The licensee was cited for failure to 

notify the patient in writing within 15 days.
E-4

FY 2004

AS 04-01 Iodine-125 (I-125) Brachytherapy Seed Medical Event at Central Arkansas 

Radiation Therapy Institute in Conway, Arkansas

Nature and Probable Consequences—The licensee reported that a patient received a radiation 

dose to an unintended area during an I-125 prostate-seed implant procedure. The patient was 

prescribed treatment with 122 I-125 seeds, with each seed containing an activity of 13.3 MBq 

(0.36 mCi). During the patient’s post-implant CT scan on December 18, 2003, the licensee 

discovered that the seeds had been implanted 2 centimeters (cm) too low and missed treating the 

upper portion of the prostate gland. As a result, 68 cm3 of adjacent tissue received the prescribed 

dose of 144 Gy (14,400 rads). The licensee reported that the adjacent tissue should not be 

affected adversely by the dose delivered by the seeds. The licensee administered additional 

treatment to deliver the intended dose to the upper 2 cm of the prostate gland. The licensee 

notified the patient and the patient’s referring physician of the event.

Cause(s)—This event was attributed to human error in that the treatment site was not verified.

Actions Taken to Prevent Recurrence

Licensee—The licensee wrote a new procedure to implement the use of fluoroscopic guidance to 

ensure the correct placement of seeds.

State Agency—The State has reviewed and accepted the licensee’s corrective actions.

AS 04-06 Gamma Stereotactic Radiosurgery (Gamma Knife) Medical Event at 

Radiosurgical Center of Memphis in Memphis, Tennessee

Nature and Probable Consequences—The licensee reported that a patient received 27 Gy 

(2700 rads) to a brain metastasis instead of the intended 18 Gy (1800 rads) during gamma knife 

treatment. The physicist did not determine that an error had occurred until the treatment was 

complete. The RSO determined that one of the four brain metastases received greater than the 

prescribed dose. The other three brain metastases received the prescribed dose. The tumor that 

received the incorrect dose was at the periphery of the brain next to the skull in a non-critical area 

so that much of the extra dose was delivered to the space between the brain and the skull. The 

cause of the incident was that a 14-millimeter (mm) (0.55-inch) collimator helmet was used 

instead of the prescribed 8-mm (0.31-inch) collimator helmet. The personnel setting up the 

treatment neglected to change the helmet. The tumor that received the unintended dose was 

located at the periphery of the brain, adjacent to the skull. Because most of the unintended dose 

was delivered to a non-critical space, between the brain and skull, the additional radiation 

exposure should have no significant effect on the patient.

The referring physician was notified of the event and informed the patient’s family of the 

unintended dose.

Cause(s)—The cause was human error, in that the event resulted from use of the wrong 

collimator helmet.
E-5

Actions Taken to Prevent Recurrence

Licensee—The licensee established a new procedure to require the physician, physicist, and 

nurse to sign off on the treatment time, helmet size, and position before each shot. Also, new 

labels identifying the size of the helmet were attached to each of the four helmets.

These labels can be seen by personnel on the TV monitor located at the control panel outside the 

treatment room. The physician will verify the correct size before the control panel button is pushed 

to start the treatment.

State Agency- The State reviewed and approved the licensee’s new procedures.

AS 04-09 IV Brachytherapy Medical Event at Ireland Cancer Center in 

Middleburg Heights, Ohio

Nature and Probable Consequences—The licensee reported that a patient received a radiation 

dose to an unintended site 3 cm proximal to the prescribed treatment site during an 

IV brachytherapy treatment procedure. The dose delivered to the unintended site was 

approximately 18.40 Gy (1840 rads). The event involved an IVB device that used a 3.5-mm 

catheter and a source train that contained Sr-90 with an activity of 2.0 gigabecquerels (GBq) 

(53.8 mCi). The source train traveled to a location approximately 3 cm proximal to the intended 

treatment site. It was determined that there was a kink in the delivery catheter which kept the 

source train from traveling to the correct site. The kink was not substantial enough to affect the 

flow of sterile water used to send and retrieve the source train. The kink was discovered the 

following day during medical physics quality checks. The referring physician and patient were 

notified of the event. According to the licensee, no adverse effects are expected.

Cause(s)—The cause of the event was determined to be a kink in the delivery catheter which kept 

the source train from traveling to the correct site.

Actions Taken to Prevent Recurrence

Licensee—Corrective actions incorporated by the licensee included additional films taken during 

procedures to verify the placement of the catheter. When there is any doubt of the placement of 

the catheter, the treatment will be aborted. The treatment team will then evaluate whether to 

attempt treatment with a different catheter.

State Agency—The Ohio Department of Health conducted an investigation, reviewed the 

licensee’s corrective actions, and found them adequate to prevent recurrence.
E-6

FY 2007

AS 07-03 Medical Event in New York

Nature and Probable Consequences—The licensee reported a brachytherapy medical event 

to the New York State Department of Health. The event involved a 31-year-old female 

patient with a history of vaginal cancer. The treatment involved the use of both cesium-137 

(Cs-137) and Ir-192 seeds. Each ribbon contained 8 seeds with an activity of 

1.855 milligram radium equivalent (118 MBq or 3.19 mCi). The patient was to be 

administered a total dose of 25 Gy (2500 rad) through interstitial brachytherapy, to be 

delivered to the 0.5-Gy (50-rad) isodose line for a total treatment time of 50 hours.

On March 6, 2007, the Ir-192 seeds and the Cs-137 seeds were placed into the patient. Late 

in the morning of March 7, 2007, the medical physicist performed a manual check of the 

treatment plan calculations and discovered that the hand calculations indicated a 

significantly higher dose rate than was generated using the treatment planning software.

The ensuing investigation revealed that the original treatment plan was in error. On 

March 7, 2007, after 27 hours of treatment, the seeds were removed from the patient.

The patient received an estimated dose of 45.9 Gy (4590 rad) to the treatment site rather 

than the intended 25 Gy (2500 rad). The rectal dose was 73 Gy (7300 rad). The radiation 

oncologist disclosed that the patient is at risk for radiation cystitis, rectal proctitis, and more 

importantly, fistula formation between the rectum and the vagina. The patient and the 

referring physician were informed of this event. The patient will be monitored closely over 

the next year by both her gynecologic oncologist and the radiation oncologist. The patient is 

being treated with broad-spectrum antibiotics along with daily treatments in a hyperbaric 

oxygen chamber.

Cause(s)—The primary cause was the use of an inappropriate Dose Rate Factor (DRF) in 

the treatment planning system. The value used corresponded to the DRF for air kerma; 

however, the seed strength entered was in milligram radium equivalent. Other causes and 

contributing factors included failure to check the treatment pre-plan before the seeds 

arrived, although there was time to do so; failure to double-check the calculations either 

before the implant or shortly thereafter; use of a treatment planning system that underwent 

acceptance testing for Cs-137 and I-125, but not Ir-192; and lack of recent experience 

preparing a treatment plan using Ir-192. Neither the physicist nor the radiation oncologist 

had prepared a treatment plan using Ir-192 in 6 years.

Actions Taken to Prevent Recurrence

Licensee—The licensee changed its policy and procedures to require a check of 

calculations for any single-fraction brachytherapy treatment.

State—The State plans to follow up on the licensee’s implementation of their new 

procedures during the next scheduled inspection.
E-7

AS 07-05 Medical Event at University of Washington Harborview Gamma Knife of 

Seattle, Washington

Nature and Probable Consequences—University of Washington Harborview Gamma Knife 

(the licensee) reported that a patient who was prescribed to receive 18 Gy (1800 rad) during 

a gamma knife treatment actually received 28 Gy (2800 rad).

The gamma knife contained 267.7 terabecquerels (TBq) (7236 curies (Ci)) of Co-60 (Co-60).

The patient and the referring physician were informed of this event. The licensee concluded 

that no significant adverse health effect to the patient is expected.

Cause(s)—The cause of the incident was determined to be human error. The prescribing 

physician prescribed 18 Gy (1800 rad) and erroneously entered 28 Gy (2800 rad). The 

physician entered the prescribed value into the computer treatment planning system rather 

than having the medical physicist enter the value as is the usual procedure, resulting in a 

failure to follow an established procedure.

Actions Taken to Prevent Recurrence

Licensee—Corrective actions taken by the licensee included a verification process to ensure 

that the prescribed treatment value is transferred from the treatment planning computer to 

the gamma knife computer before patient therapy. Also, a treatment plan signed by the 

treating oncologist, physicist, and neurosurgeon is now required. In addition, the treating 

oncologist and physicist will verify and initial the prescribed dose and isodose treatment 

parameters before patient therapy.

State—The State reviewed the licensee’s corrective actions and determined that the 

procedures were adequate to ensure that this type of event should not happen in the future.

AS 07-06 Medical Event at Physician Reliance of Fort Worth, Texas

Nature and Probable Consequences—Physician Reliance (the licensee, doing business as 

(dba) Texas Oncology at Klabzuba) reported that a patient who was being treated for lung 

cancer with an HDR afterloader and an Ir-192 source received 2500 cGy (2500 rad) during 

the first fraction instead of the prescribed dose of 500 cGy (500 rad). The patient was 

prescribed to receive five fractions with 500 cGy (500 rad) per fraction over five weeks. The 

incident was discovered following an independent physicist’s review of the treatment plan.

The patient and the referring physician were informed of this event. The patient’s 

pulmonologist concluded that no significant adverse health effect to the patient is expected.

Cause(s)—The incident occurred as a result of the incorrect isodose line being chosen and 

entered into the treatment planning system. The oncologist signed and approved the 

treatment plan and the radiation safety officer performed a second calculation to check the 

treatment plan. The treatment planning system then normalized the calculations to the 

incorrect isodose line and delivered the resulting treatment. The calculation error was 

identified by an independent physicist before administration of the second fraction.
E-8

Actions Taken to Prevent Recurrence

Licensee—The licensee’s corrective action was to change their procedure to include a 

second check by a licensed medical physicist of all treatment plans.

State—The State issued two violations related to this event: (1)the licensee was cited for a violation of 

25 Texas Administrative Code (TAC) §289.256(p)(4)(A) and (B) because the procedure as 

implemented was insufficient to ensure that a second check of the printed output of the treatment plan 

was performed to verify the accuracy of the planned treatment factors before treatment; and (2) the 

licensee was cited for a violation of 25 TAC §289.256(o)(1) and §289.256(p)(1) because the 

instructions of obtaining the authorized physician’s signed and dated written directive for each 

therapeutic administration were not followed.

In addition, the State reviewed the licensee’s corrective action of changing their procedures 

to include a second check by a licensed medical physicist of all treatment plans.
E-9

FY 2008

AS 08-02 Medical Event at University of Mississippi Medical Center in 

Jackson, Mississippi

Nature and Probable Consequences—University of Mississippi Medical Center (the licensee) reported 

that a medical event occurred during an HDR treatment for cervical cancer using an Ir-192 source with 

an activity of 185 GBq (5.0 Ci). The authorized user physician prescribed five fractionated doses of 

600 cGy (600 rad) each to be administered using tandem and ovoid applicators. The licensee 

calculated that during the first, second, and third fractionated treatments, the patient received a total 

dose of 470 cGy (470 rad) to the treatment area and 1300 cGy (1300 rad) to the vaginal region inferior 

to the treatment area. The patient and the referring physician were informed of this event. The 

licensee concluded that no significant adverse health effect to the patient is expected.

Cause(s)—The medical event was caused by human error; namely, the incorrect catheter length 

being entered into the treatment planning system. The incorrect value of 128 cm was entered as the 

length instead of 120 cm, resulting in an 86-mm displacement. An HDR service technician identified 

the error in the treatment planning system on March 25, 2008.

Actions Taken to Prevent Recurrence

Licensee—The licensee committed to taking several corrective actions as a result of the medical 

event, including (1) verification of the length of all disposal catheters and checking the integrity of the 

catheters before treatment, (2) placing an order for and use of a single set of reusable catheters for 

HDR cervical cancer treatments, (3) the treatment plan and catheter measurement will be 

independently checked before treatment, and (4) review and modification, if necessary, of the quality 

assurance plan to ensure accuracy.

State—The State cited the licensee with two violations for failing to verify the treatment plan.

NRC 08-03Medical Event at Karmanos Cancer Center in Detroit, Michigan

Nature and Probable Consequences—Karmanos Cancer Center reported that a medical event 

occurred associated with its gamma knife. A patient being treated for a metastatic brain tumor was 

scheduled to receive 18 Gy (1800 rad) to the lesion in the right cerebellum area of the brain but 

received 18 Gy (1800 rad) to an unintended area adjacent to the tumor. An error in the setup of the 

magnetic resonance imaging (MRI) unit caused the MRI scan to be reversed (i.e., the image of the 

right side of the head was on the left side and vice versa). The patient and the referring physician were 

informed of this event. Before the treatment, the medical physicist, authorized user physician, and 

neurosurgeon reviewed the MRI scan and treatment plan but failed to recognize the reversed MRI 

images. The reversed MRI images were scanned into the gamma knife treatment planning computer 

and a treatment plan was generated based on the reversed MRI images. The authorized user 

physician and neurosurgeon reviewed and approved the treatment plan generated from the reversed 

MRI images, and again the reversed MRI images were not recognized.

The NRC staff conducted a reactive onsite inspection on October 29, 2007. The NRC-contracted 

medical consultant reviewed the case and agreed with the licensee’s analysis, stating that no 

significant adverse health effect to the patient is expected.

Cause(s)—The medical event was caused by the MRI technologist, who inadvertently performed the 

MRI scans in the “caudal” mode (from the jaw to the top of the head) rather than the “cranial” mode 
E-10

(from the top of the head to the jaw). This change in device mode caused the MRI images to be 

reversed.

Actions Taken to Prevent Recurrence

Licensee—The licensee initiated several corrective actions to reduce the likelihood of 

recurrence of a similar event. Specifically, those corrective actions included (1) weekly 

meetings with the physics staff to discuss technical issues, focusing on the importance of 

good communication, and (2) new written procedures and policies for the MRI staff and 

gamma knife facility staff that require dual verification of the various steps in the process to 

ensure that the correct treatment plan is generated from the MRI images.

The NRC—On January 10, 2008, NRC issued a Notice of Violation related to this event.

NRC 08-04Medical Event at Reid Hospital and Health Care Services in Richmond, Indiana

Nature and Probable Consequences—Reid Hospital and Health Care Services reported that a 

medical event occurred during a brachytherapy seed implant procedure to treat prostate cancer.

The written directive prescribed a total dose of 110 Gy (11,000 rad) to the patient’s prostate using 

62 I-125 seeds as permanent implants. The licensee calculated that the patient received less than 

15 Gy (1500 rad) to the prostate and that the region of the patient’s perineum, where the seeds 

were placed, received a dose of 55 Gy (5500 rad). The patient and the referring physician were 

informed of this event.

According to the licensee, the base of the prostate was misidentified through ultrasound, causing 37 of 

the prescribed 62 seeds to be placed approximately 1 cm to 2 cm below the prostate in the perineum.

When it was recognized that the seeds were not in the prostate, the procedure was halted. The 

licensee physicians stated that the patient may develop possible complications, including fibrosis and 

necrosis of the tissue in the perineum, where the seeds were implanted.

The NRC-contracted medical consultant agreed with the licensee’s dose estimate and stated 

that it was unlikely that the patient would experience radiation-induced rectal wall necrosis or 

soft tissue necrosis below the prostate in the perineum area, but that it was possible to have 

delayed fibrosis of some areas of the genital tract. The NRC-contracted medical consultant 

further stated that because no tissue necrosis had occurred one month after the medical event, 

tissue necrosis was very unlikely to occur.

Cause(s)—The licensee determined that the root cause of the medical event was the misidentification 

of the base of the prostate. Specifically, the prostate/bladder interface was not identified properly using 

the ultrasound because of poor image quality. As a result, the needle used to implant the seeds was 

not located in the prostate during the implantation.

Actions Taken to Prevent Recurrence

Licensee—The licensee’s corrective actions to prevent recurrence included revising its procedure for 

prostate seed implants to require that the needle location in the prostate be verified by x-ray imaging at 

the beginning of the procedure, before any seeds are implanted, and halting the procedure if the 

location of the needle in the prostate cannot be verified with certainty.

The NRC—On July 11, 2008, NRC issued a Notice of Violation related to this event.
E-11

FY 2009

AS 09-03 Medical Event at St. Vincent’s Medical Center, Inc., in Jacksonville, Florida

Nature and Probable Consequences—St. Vincent’s Medical Center, Inc. (the licensee), reported that 

a medical event occurred associated with an HDR mammosite treatment for breast cancer containing 

199.8 GBq (5.4 Ci) of Ir-192. A patient was prescribed to receive 34 Gy (3400 rad) to the right breast 

but received 34 Gy (3400 rad) to the skin of the left breast.

On October 16, 2008, the patient notified her physician of erytherma on her left breast. During a 

records review, the medical physicist determined that an error in programming the catheter length in 

the HDR device caused the source to stop 10 cm short of the intended tumor site in the right breast.

Because of this programming error, the dose intended for the right breast was delivered to the skin of 

the left breast. The authorized user concluded that no chronic health effect to the patient is expected.

Cause(s)—The medical event was caused by human error in failing to verify that the correct catheter 

length was entered into the treatment planning system.

Actions Taken To Prevent Recurrence

Licensee—The licensee committed to taking several corrective actions as a result of the medical event 

that include (1) using a catheter length worksheet to determine and verify the mammosite catheter 

length; (2) documenting the mammosite catheter length by two individuals—one a physicist and one a 

dosimetrist, physicist, or radiation therapist—during simulation treatment setup; (3) providing 

procedures for the medical physicist and authorized user on documenting the catheter length on the 

catheter worksheet during the review of the treatment control unit and treatment plan; and 

(4) conducting a second measurement of the catheter length to verify that the length agrees with the 

data in the treatment control unit.

State—The Florida Bureau of Radiation Control conducted an investigation and reviewed the 

licensee’s corrective actions and found the corrective actions to be adequate.

AS 09-04 Medical Event at Presbyterian Hospital of Dallas in Dallas, Texas

Nature and Probable Consequences—Presbyterian Hospital of Dallas (the licensee) reported that a 

medical event occurred associated with its gamma knife containing 125.8 TBq (3400 Ci) of Co-60.

A patient being treated for trigeminal neuralgia was prescribed to receive 80 Gy (8000 rad) to the fifth 

intracranial nerve but received 14.95 Gy (1495 rad) to the seventh intracranial nerve. The patient and 

the referring physician were informed of this event.

An error in entry of information into the treatment planning system caused the wrong nerve to receive 

treatment. The error was identified by the neurosurgeon 9 minutes into the 45-minute treatment. The 

licensee concluded that no significant adverse health effect to the patient is expected.

Cause(s)—The medical event was caused by the misidentification of the anatomical target site listed 

on the written directive.
E-12

Actions Taken to Prevent Recurrence

Licensee—The licensee modified its written procedure to include verification of the target sit, by the 

neuroradiologist for each treatment. In addition, an updated written directive will document the new 

procedure to ensure that the correct treatment site is targeted and treated in each procedure.

State—The State will conduct a review of at least 20 percent of the past treatment cases to ensure 

that this error has not previously occurred.

AS 09-05 Medical Event at Cancer Care Northwest PET Center in Spokane, Washington

Nature and Probable Consequences—Cancer Care Northwest PET Center (the licensee) reported 

that a medical event occurred associated with an HDR brachytherapy treatment for prostate cancer 

containing 185 GBq (5 Ci) of Ir-192. During patient treatment, the aluminum connector to needle 13 

became detached from the plastic guide tube and a dose of 12.5 Gy (1,250 rad) was delivered to a 

small area of the patient’s inner thigh (wrong treatment site). The patient and the referring physician 

were informed of this event.

The source wire for needle 13 hung about 6 inches past the disconnected guide tube, which resulted in the 

skin dose. The licensee conducted several follow-up examinations of the patient’s inner thigh and noted 

that no skin reddening or injury has occurred and the patient is not experiencing any pain in this area.

Therefore, the licensee concluded that no significant adverse health effect to the patient is expected.

Cause(s)—The cause of the medical event was that the source wire for needle 13 snagged on the 

seam between the aluminum connector and the plastic guide tube during retraction.

Actions Taken to Prevent Recurrence

Licensee—The licensee committed to taking several actions as a result of the medical event that 

include (1) requiring the staff to sign the patient quality assurance list when they check the applicators, 

transfer guide tubes, and aluminum connectors; (2) inspecting the guide tube catheters daily and 

examining the aluminum connectors before patient use; and (3) revising the refresher training to 

include new procedures for staff before patient treatment.

State—The State conducted follow-up inspection activities during April and May 2009 and reviewed 

the licensee’s corrective actions. The State found the licensee’s corrective actions adequate and did 

not take any enforcement action regarding this event.

AS 09-06 Medical Event at The Urology Center in Cincinnati, Ohio

Nature and Probable Consequences—The Urology Center (the licensee) reported that a medical 

event occurred associated with a brachytherapy seed implant procedure to treat prostate cancer. The 

patient was prescribed to receive a total dose of 144 Gy (14,400 rad) to the prostate using 64 I-125 

seeds as permanent implants. Instead, the patient received an approximate dose of 76 Gy (7600 rad) 

to the urethra and bulb of the penis (unintended sites). The patient and the referring physician were 

informed of this event.

According to the licensee, an interpretation of the ultrasound image of the patient’s prostate resulted in 

30 of the 64 seeds being delivered to the prostate while the other 34 seeds were delivered outside the 

prostate. Because the patient’s prostate was smaller than normal, the prostate received 68 Gy 

(6800 rad) of the prescribed dose and the urethra and bulb of the penis (unintended sites) received 
E-13

approximately 76 Gy (7600 rad). Before the seeds were implanted, the urologist and radiation 

oncologist should have consulted on the ultrasound image of the patient’s prostate to determine the 

correct seed placement. The licensee concluded that no significant adverse health effect on the 

patient is expected. On May 19, 2009, the patient returned for a second treatment to compensate for 

the original underdosing to the prostate.

Cause(s)—The cause of the medical event was the misinterpretation of the correct size of the patient’s 

small prostate gland by ultrasound.

Actions Taken To Prevent Recurrence

Licensee—Corrective actions taken by the licensee included instituting a new policy requiring 

agreement by both the urologist and radiation oncologist on seed placement for all prostate glands 

measuring 20 cubic centimeters or less. On May 26, 2009, the licensee submitted a written report of 

this event to the Ohio Department of Health’s Bureau of Radiation Protection (ODH BRP).

State—On June 12, 2009, ODH BRP conducted an inspection of this event and determined that the 

licensee had followed the correct procedures for administrations requiring a written directive. ODH BRP 

reviewed the licensee’s corrective actions for this event and found the corrective actions to be adequate.

NRC 09-02Medical Event at Gamma Knife Center of the Pacific in Honolulu, Hawaii

Nature and Probable Consequences—Gamma Knife Center of the Pacific (the licensee) reported that 

a medical event occurred associated with its gamma knife containing 104.86 TBq (2834 Ci) of Co-60.

A patient being treated for multiple brain metastatic sites was prescribed to receive 24 Gy (2400 rad) 

to seven discrete brain sites using an 8-mm collimator. However, an 18-mm collimator was used to 

treat two of the discrete brain sites, resulting in a dose of 24 Gy (2400 rad) to additional brain tissue.

The patient and the referring physician were informed of this event.

The patient received treatment to the first and second discrete brain sites; after receiving treatment to 

the second discrete site, it was discovered that an 18-mm collimator had been used to deliver 

treatment instead of the prescribed 8-mm collimator. The larger collimator caused the volume of each 

of the two discrete sites to increase by 2.45 cubic meters, resulting in a dose of 24 Gy (2,400 rad) to 

additional brain tissue. After the 18-mm collimator was discovered, it was replaced with the 8-mm 

collimator and the patient received treatment to the five remaining discrete sites as prescribed. The 

licensee concluded that no significant adverse health effect to the patient is expected.

Cause(s)—The cause of the medical event was human error in failing to check the collimator size 

before patient treatment.

Actions Taken To Prevent Recurrence

Licensee—Corrective actions taken by the licensee included (1) sending a notice to all authorized 

users, neurosurgeons, and medical physicists reiterating that they should each independently check 

the collimator size before patient treatment and (2) revising procedures to have a second independent 

verification of all treatment parameters, including the collimator size, by a treatment team member.

The NRC—The NRC conducted an onsite inspection and hired a medical consultant to review the 

event. The conclusions from the onsite inspection are pending and the medical consultant’s 

review is ongoing.
E-14

FY 2010

NRC 10-02 Medical Event at Chippenham & Johnston-Willis (CJW) Medical Center in 

Richmond, Virginia

Nature and Probable Consequences—Chippenham & Johnston-Willis (CJW) Medical Center 

(the licensee) reported a medical event with its gamma stereotactic radiosurgery (GSR) unit.

A patient being treated for trigeminal neuralgia (inflammation of the nerve) was prescribed a 

treatment of 40 Gy (4000 rad) to the right trigeminal nerve but received the treatment dose to 

the left trigeminal nerve (wrong treatment site). The patient and referring physician were 

informed of this event.

The licensee noted that on the day of the treatment, the top portion of the written directive 

correctly documented the prescribed treatment site; however, while the staff was preparing 

the daily patient treatment log, it was inadvertently annotated that the dose was to be 

delivered to the left trigeminal nerve. This error was carried through by the medical physicist 

during preparation of the patient’s treatment plan and completion of the bottom part of the 

written directive. On completion of the procedure and after reviewing the patient’s file, the 

treatment team identified the inadvertent treatment of the left trigeminal nerve. An 

NRC-contracted medical consultant concluded that although the patient suffered no 

immediate adverse consequences, 1) an unlikely injury to the brain stem could have resulted 

from a high radiation dose to a tiny volume of the brain-stem tissue and 2) the patient might 

have an increased risk of cataract formation.

Cause(s)—The cause of the medical event was the licensee’s failure to have adequate 

procedures that verify the location of treatment sites and to ensure that any inconsistencies in 

the written directives are resolved before administration.

Actions Taken to Prevent Recurrence

Licensee—The licensee revised their GSR treatment procedures to affirm that (1) a “Physician 

Order” will be the primary source of documentation of the treatment site and will accompany 

the patient through the entire course of the treatment, (2) the radiation oncologist and the 

neurosurgeon will independently verify and document the treatment site, (3) the nurse and the 

medical physicist will confirm that the treatment site identified by the radiation oncologist in the 

written directive and the neurosurgeon’s “physician order” both match, (4) the neurosurgeon 

will mark the treatment site with ink in the presence of a nurse, and (5) a “Time-Out” process 

involving independent verification of the final treatment plan by each of the four members of 

the clinical team (who are required to sign off on their presence and acceptance of time-out in 

the presence of the patient before moving ahead with the treatment) will be used with the 

patient or the patient’s authorized representative to confirm the treatment site.

The NRC—The NRC initiated an inspection on December 18, 2008; completed the inspection 

on November 30, 2009; and issued one Severity Level III violation to the licensee on 

January 21, 2010.

NRC 10-03 Medical Event at Virtua Health System in Marlton, New Jersey

Nature and Probable Consequences—Virtua Health System (the licensee) reported that a 

medical event occurred associated with a brachytherapy seed implant procedure to treat 

prostate cancer. The patient was prescribed to receive a total dose of 145 Gy (14,500 rad) to 
E-15

the prostate using 93 I-125 seeds. Instead, the patient received an approximate dose of 

12.2 Gy (1220 rad) to the rectum (wrong treatment site). The patient and referring physician 

were informed of this event.

On January 19, 2009, the urologist inserted needles in the patient’s prostate gland under 

transrectal ultrasound guidance while the radiation oncologist left the operating room to obtain 

the radioactive seeds. The licensee’s staff (including the authorized medical physicist (AMP)) 

questioned the accuracy of prostate visualization before implantation of the seeds but took no 

action to resolve the question. On February 23, 2009, following a post-implant computed 

tomography (CT) scan, it was noted that some mispositioning of the sources had occurred 

and the patient was notified that additional treatment might be necessary. On March 19, 2009, 

the AMP reviewed the case and determined that 100 percent of the seeds were implanted 

outside the prostate, which received about 10 Gy (1000 rad). The NRC contracted with a 

medical consultant who concluded that although the probability of long-lasting negative health 

effects to the patient was low, an increased risk of impotency and fibrosis was possible 

because of the high radiation dose.

Cause(s)—The cause of the medical event was failure of the medical implant team to 

adequately visualize and identify the prostate before the implant.

Actions Taken To Prevent Recurrence

Licensee—The licensee revised its policy and procedures to require that (1) all members of 

the implant team be present before the patient is brought to the operating room and placed 

under anesthesia, (2) the AMP be included in the pre-implantation ultrasound, (3) the 

authorized user consult with the urologist before needle insertion, (4) both the radiation 

oncologist and the urologist agree on the positioning and the visualizing of the target anatomy, 

(5) any objection or question by an implant team member is cause for stopping the implant 

and performing a review, and (6) the implant is to be stopped if there are any ultrasound 

image questions. The licensee’s staff was also trained on the revised procedures, the 

definition and reporting requirements of a medical event, and the communication of any 

CT-scan abnormalities or seed misplacement to the radiation safety officer (RSO).

The NRC—The NRC initiated an inspection on March 20, 2009; completed the inspection on 

August 26, 2009; and issued one Severity Level III violation to the licensee on 

October 21, 2009.

NRC 10-04 Medical Event at Nanticoke Memorial Hospital, in Seaford, Delaware

Nature and Probable Consequences—Nanticoke Memorial Hospital (the licensee) reported that a 

medical event occurred involving a brachytherapy seed implant procedure to treat prostate cancer.

The patient was prescribed a total dose of 145 Gy (14,500 rad) to the prostate using 61 I-125 seeds.

Instead, the patient received an approximate prostate dose of 26 Gy (2600 rad) (18 percent of the 

prescribed dose) and a dose of 139 Gy (13,900 rad) to unintended tissue (wrong treatment site). The 

patient and referring physician were informed of this event.

The seeds were implanted under ultrasound guidance using an axial view; however, following 

the implant, the urologist performed a cystoscopy to remove 22 of the seeds from the bladder.

When the patient returned to the hospital for a post-implant CT scan, the images revealed that 

32 seeds were displaced superiorly to the prostate and 7 seeds were implanted in the 
E-16

prostate. The NRC contracted with a medical consultant who concluded that no significant 

adverse health effects to the patient were expected.

Cause(s)—The cause of the medical event was a miscalculation of the prostate depth in 

relation to the skin surface because of possible patient movement during the procedure.

Actions Taken to Prevent Recurrence

Licensee—The licensee revised its prostate implant procedure to include the use of both the 

axial and sagittal views of an ultrasound probe to determine prostate depth. In addition, the 

licensee revised its medical event policy to ensure timely reporting of medical events and to 

clearly state the parameters under which a medical event must be reported. The licensee 

provided training on the revised policies and procedures to its staff.

The NRC—The NRC initiated an inspection on July 19, 2009; completed the inspection on 

January 6, 2010; and issued one Severity Level III violation to the licensee on 

February 2, 2010.

NRC 10-05 Medical Event at Yale New-Haven Hospital, in New Haven, Connecticut

Nature and Probable Consequences—Yale New-Haven Hospital (the licensee) reported that a 

medical event occurred associated with its GSR unit. A patient being treated for brain 

metastases was prescribed 18 Gy (1800 rad). However, while treating a patient earlier in the 

day, an equipment malfunction occurred with the GSR unit that resulted in a positioning shift 

of the x-axis by 4.5 mm. The positioning shift in the x-axis resulted in an underdose to the 

treatment site and an overdose to a wrong treatment site. The patient and physician were 

informed of this event.

The malfunction occurred following the treatment of the first patient on August 5, 2009. The 

automatic positioning system (APS) malfunctioned and, after discussion with the GSR 

manufacturer, the position error codes were cleared by the AMP. A second patient was 

treated for multiple brain metastases later that day. GSR service personnel noted on 

August 5, 2009, that the APS positioning was off by about 5 mm. After further evaluation, the 

manufacturer determined that a position shift (offset) occurred when licensee personnel 

accepted an error message concerning position deviation. The NRC contracted with a medical 

consultant who concluded that no clinically significant side effects from radiation damage to 

the wrong treatment sites would be expected.

Cause(s)—The cause of the medical event was failure of licensee personnel to verify that the 

APS coordinates were in accordance with the written directive.

Actions Taken to Prevent Recurrence

Licensee—The licensee issued a memorandum to all personnel involved in GSR treatments to 

require visual verification of the physical coordinates against the electronic coordinates before 

the start and at the end of each treatment run. The licensee also retrained all GSR personnel 

on the importance of fully understanding error conditions and reviewing unexpected errors 

with other staff involved in the treatment (e.g., radiation oncologist, AMP, etc.) before clearing 

any unexpected error.
E-17

The NRC—The NRC initiated an inspection on August 13, 2009; completed the inspection on 

April 7, 2010; and issued one Severity Level III violation to the licensee on May 21, 2010.

NRC 10-06 Medical Event at Valley Hospital in Paramus, New Jersey

Nature and Probable Consequences—Valley Hospital (the licensee) reported that a medical 

event occurred associated with a brachytherapy seed implant procedure to treat prostate 

cancer. The patient was prescribed a total dose of 65 Gy (6500 rad) to the prostate using 

46 cesium-131 seeds. Instead, the licensee determined that an unintended volume (30.1 ml) 

of soft tissue received 100 percent of the prescribed prostate dose. The patient and referring 

physician were informed of this event.

On August 6, 2009, the patient returned to the hospital for a post-implant CT scan. The 

images revealed that the seeds were implanted in soft tissue 4 to 5 cm from the prostate.

Post-implant dosimetry calculations indicated that none of the prostate received the 

prescribed dose of 6500 cGy (6500 rad). The NRC contracted with a medical consultant who 

concluded that the additional dose could increase the risk of soft tissue fibrosis or increase the 

risk of impotency.

Cause(s)—The cause of the medical event was the licensee’s failure to identify the position of 

the prostate because of the patient’s unusual anatomy and obesity.

Actions Taken To Prevent Recurrence

Licensee—The licensee revised their prostate implant procedures to include steps to ensure 

that the prostate and surrounding anatomy is adequately visualized before implant.

The NRC—The NRC initiated an inspection on August 13, 2009; completed the inspection on 

October 29, 2009; and determined that no violations of NRC requirements occurred.

NRC 10-07 Medical Event at Christiana Care Health Center in Wilmington, Delaware

Nature and Probable Consequences—Christiana Care Heath Center (the licensee) reported 

that a patient was prescribed an HDR mammosite (brachytherapy) multi-lumen catheter 

treatment of 34 Gy (3400 rad) over a 5-day period to the left breast. The patient received an 

average dose of 17 Gy (1700 rad) to 100 cm3 of unintended breast tissue; 68 Gy (6800 rad) 

to 7.5 cm3 of unintended skin and underlying tissue; and 3.4 Gy (340 rad) to 35 cm3 of 

intended breast tissue. The patient and referring physician were informed of this event.

On February 22, 2010, during a follow-up examination, the patient complained about skin 

reddening on the external breast. In reviewing the treatment plan, it was discovered that the 

AMP performed measurements using a source position simulator (SPS) measurement tool 

following a CT scan to determine the treatment distance for each catheter. The catheter 

distances were recorded and confirmed with two manufacturer representatives that were 

present at the time of the treatment. However, it was noted that an incorrect measurement 

caused the placement of the radioactive source 10 cm proximal to the intended position. The 

NRC-contracted medical consultant concluded that the dose that was administered to the 

unintended left breast tissue is unlikely to result in any significant or unusual adverse effect.

However, a significant risk exists that local tumor recurrence could occur if additional 

intervention is not performed.
E-18

Cause(s)—The cause of the medical event was human error in the failure to identify that the 

measurement tool was functioning improperly and to identify an incorrect measurement 

distance.

Actions Taken To Prevent Recurrence

Licensee—The licensee revised its procedures for HDR brachytherapy to require a 

double-check of all patient measurements, a daily and monthly quality assurance requirement 

to confirm that the SPS tool is functioning properly, and a process to ensure that all members 

of the treatment team agree on the specifics of the treatment. In addition, the licensee 

acquired a new SPS tool, developed and posted a reference table at the HDR control console, 

provided training on revised procedures to staff involved in the HDR program (to be repeated 

annually), and implemented a “New Product” committee to review all new product plans.

The NRC—The NRC conducted an inspection on July 12, 2010, and issued one Severity 

Level III violation to the licensee on August 24, 2010.
E-19

FY 2011

AS 11-06 Medical Event at University Community Hospital in Tampa, Florida

Nature and Probable Consequences—University Community Hospital (the licensee) reported that 

two patients were prescribed single-channel HDR brachytherapy treatments of 34 Gy (3400 rad).

An actual average dose of 17 Gy (1700 rad) to the first patient, and 26 Gy (2600 rad) to the 

second patient, were delivered to the target area of the breast, and some parts of the planned 

volume received greater than 700 percent (first patient) and 220 percent (second patient) of the 

prescribed dose. In addition, other areas of the breast not in the target region received up to 

136 Gy (13,600 rad) in the first patient and 75 Gy (7500 rad) in the second patient. The maximum 

skin dose was calculated to be 42.5 Gy (4250 rad) to the first patient and 75 Gy (7500 rad) to the 

second patient. The patients and their referring physicians were informed of the events.

On February 14, 2010, the licensee noted that the source within the mammosite catheter was 

erroneously positioned approximately 2 to 2.5 cm away from the tumor. This was the result of the 

operator entering the wrong dwell position into the planning system. The licensee concluded that 

no significant adverse health effects to the patients are expected.

Cause(s)—The cause of the medical events was human error involving entering the wrong 

position of the reference end of the catheter into the planning system.

Actions Taken to Prevent Recurrence

Licensee—Corrective actions included implementing various quality assurance steps to ensure 

that the correct treatment calculations and data are used for future treatments. Additional 

procedural guidance will be created with detailed instructions.

State—The Florida Bureau of Radiation Control initiated an inspection on February 18, 2010; 

completed the inspection on March 1, 2010; and determined that the licensee’s corrective actions 

were adequate. No enforcement action was taken and the State forwarded the final update of the 

event to the NRC on February 1, 2011.

NRC11-04 Medical Event at Community Hospitals of Indiana in Indianapolis, Indiana

Nature and Probable Consequences—Community Hospitals of Indiana (the licensee) reported 

that a medical event occurred associated with an HDR brachytherapy treatment for breast cancer; 

the treatment consisted of 340.4 GBq (9.2 Ci) of Ir-192. The patient was prescribed to receive a 

total dose of 34 Gy (3400 rad) in 10 fractionated doses to the postsurgical cavity in the left breast 

following excision of a cancerous tumor (treatment site). It was determined that the first eight 

treatment fractions resulted in a portion of the treatment site receiving a dose of 266 Gy 

(26,600 rad). In addition, a portion of the patient’s skin on the left breast and the chest muscle 

tissue (tissue other than the treatment site) received doses of 105 Gy (10,500 rad) and 1,002 Gy 

(100,200 rad), respectively. The patient and referring physician were informed of this event.

On October 6, 2010, following the eighth fractionated treatment dose, an error was discovered in 

the treatment plan by the medical physicist who remembered that he had not changed a default 

entry in the treatment planning system. This error caused the source placement to be flipped 

180 degrees along the applicator’s long axis which resulted in a portion of the treatment site at the 

tip end of the applicator receiving less than the prescribed dose and a portion of the treatment site 

at the connector end of thentrye applicator receiving more than the prescribed dose.
E-20

The licensee concluded that no long-term medical effects are expected for the patient. The NRC 

contracted with a medical consultant who determined that the overall impact to the patient is 

minimal.

Cause(s)—The medical event was caused by human error in that the medical physicist failed to 

change a default entry in the treatment planning system as required by the licensee’s procedure.

Actions Taken to Prevent Recurrence

Licensee—The licensee revised its written directive form to remind staff to change the default 

entry in the treatment planning system as applicable, added a step to its procedure for 

multicatheter HDR breast treatments to verify that the default was changed as applicable, and 

trained its staff on the revised written directive form. In addition, the licensee evaluated all of the 

other HDR breast treatments that were conducted in 2010 to verify that the applicators were 

accurately reconstructed in the treatment planning computer.

The NRC—The NRC conducted a reactive inspection on October 18 through 20, 2010, with 

continued in office review through January 18, 2011, and issued two Notices of Violation to the 

licensee on March 1, 2011, and April 20, 2011, respectively.
F-1

APPENDIX F

PROCEDURAL EVENT ANALYSIS TOOL (PEAT) FOR 

UNDERSTANDING HUMAN ERROR AND IMPROVING PERFORMANCE

The field of aviation has had a long history of recognizing the importance of human factors in 

ensuring safety. For example, Boeing reports that more than 70% of hull-loss accidents6 result 

from human error [64]. As a result the industry has collectively invested significant resources in 

understanding the causes of human errors and developing ways to improve safety performance 

by making changes to the underlying causes. An example of a process used in aviation is the 

Boeing Airplane Company’s Procedural Event Analysis Tool (PEAT). The following is quoted from 

Boeing’s description:

“PEAT originated from an extensive effort to identify the key underlying cognitive factors that 

contributed to procedural noncompliance in past accidents. In 1991 Boeing concluded a 10-year 

study that showed that flight crew deviation from established procedures contributed to nearly 

50 percent of all hull-loss accidents. The aviation industry still lacks sufficient knowledge about the 

reasons for these deviations, however, and had no formal investigation tool to help identify them.”

PEAT was developed to address this lack of understanding of the causes of such errors. Figure

F-1 depicts the overall PEAT process, which shows the essential nature of the feedback from 

investigations of operational events to enhancements in operational practices. Of interest is the 

model, shown in Figure F-2, used to identify the causes and contributing factors for the identified 

procedural errors. On this basis, information is gathered in the investigation for explaining the 

causes of errors and developing the improvements. This model is essentially the same as that 

used in this report to explain human performance issues in radiation therapy, though with an 

aviation context.

As discussed earlier, accident rates for commercial aviation showed an initial strong downward 

trend from 1959 that largely resulted from improvements in mechanical and structural changes in 

aircraft design. However, it later became clear to the International Civil Aviation Organization 

(ICAO) that human performance was becoming a significant cause of accidents; recall Boeing’s 

estimate that 70 percent of major accidents were caused by human error. In a speech given by 

the then president of ICAO [104], the organization realized that human errors required attention:

 6 An accident so severe that the damage to the aircraft means it must be written off, or in which the aircraft is totally 

destroyed.
F-2

Figure F-1 The Boeing PEAT Process for Aviation Errors in Using Procedures

Figure F-2 The Model of Human Performance used in Boeing’s PEAT Process

“Around the mid-seventies, it was becoming obvious that human error was capable of 

circumventing even the best designed technical safety defenses. The authors consequently 
F-3

shifted our thinking to the human perspective. Two points are worth noting from this era. First, 

our human factors endeavors at that time were focused on the individual; they were aimed at 

improving individual performance, mainly through training and regulations. Second, a substantial 

part of our human factors efforts attempted to eliminate error through design, to replace human 

functions with machine functions, thus seeking to minimize human intervention and, therefore, 

opportunities for error. Addressing human error and its negative effect on safety from an 

individual perspective was an important step. It had to be taken, and yet it was not enough.

This realization came in the nineties, when there was an understanding that human 

performance does not take place in a vacuum but rather within social contexts. Individual 

human behavior is modelled after behavior that organizations foster and expect from their 

members. The safety concerns were addressed from the organizational perspective. In this 

way, authors acquired a systemic view of aviation safety, including the interactions not only 

among people, but also between people and technology, and between people and the 

organizations to which they belong. Most importantly, and for the first time the role was 

scrutinized, and the relationship between, management and safety.”

In response to this realization, the industry has put in place programs to reduce these events, 

including the PEAT process. For instance, the ICAO presently provides guidance in human 

factors for aviation professionals in the following 12 areas:

1. fundamental human factors concepts

2. flight crew training: Cockpit resource management and line-oriented flight training

3. training of operational personnel in human factors

4. operational implications of automation in advanced technology flight decks

5. investigation of human factors in accidents and incidents

6. ergonomics

7. human factors in air traffic control

8. human factors, management and organization

9. human factors in CNS/ATM7 systems

10. human factors in aircraft maintenance and inspection

11. human factors in cabin safety

12. cross cultural factors in aviation safety

 7 Communications, Navigation, Surveillance / Air Traffic Management.

G-1

APPENDIX G

REFERENCES FOR APPENDICES

[95] U.S.Department of Energy (DOE), “Root Cause Analysis Guidance Document,” 

DOE-NE-STD-1004-92, 1992.

[96] 10 CFR Part 50 Appendix B, Criterion XVI, “Corrective Action”.

[97] U.S.Department of Energy (DOE), “Accident Investigation and Prevention,” 

DOE-HDBK-1208-2012, Volume I, “Accident Analysis Techniques,” July 2012.

[98] The Joint Commission. (2011, January 4). Sentinel Event Policy and Procedures. 

Retrieved from Joint Commission: 

http://www.jointcommission.org/Sentinel_Event_Policy_and_Procedures/

[99] The Joint Commission. (2012). Sentinel Event Data: Root causes by event type 

2004-2012. Retrieved September 11, 2013, from Jointcommission.org: 

http://www.jointcommission.org/assets/1/18/Root_Causes_Event_Type_04_4Q201

2.pdf

[100] Cook, R.I., D.D.Woods, and C.Miller, “A Tale of Two Stories: Contrasting Views on 

Patient Safety,” Boston: National Patient Safety Foundation, April 1998.

[101] U.S. Nuclear Regulatory Commission, “Report to Congress on Abnormal 

Occurrences: Fiscal Year 2002,” NUREG-0090, Vol.25, April 2003.

[102] U.S. Nuclear Regulatory Commission, “Report to Congress on Abnormal 

Occurrences: Fiscal Year 2004,” NUREG-0090, Vol.27, April 2005.

[103] U.S. Nuclear Regulatory Commission, “Report to Congress on Abnormal 

Occurrences: Fiscal Year 2007,” NUREG-0090, Vol.30, April 2008.

[104] U.S. Nuclear Regulatory Commission, “Report to Congress on Abnormal 

Occurrences: Fiscal Year 2008,” NUREG-0090, Vol.31, May 2009.

[105] Kotaite, A. (16 February, 2001). Address by the President of the ICAO Council. 

Retrieved from: http://www.icao.int/Pages/biography-president-kotaite-addresslinacre.aspx



NUREG-2170

June 2017

Technical

A Risk-Informed Approach to Understanding Human Error

in Radiation Therapy

J. Wreathall, W.S. Brown, L. Militello, S.E. Cooper, C. Lopez, C. Franklin

1. The WreathWood Group, 4157 MacDuff Way, Dublin, OH 43016

2. Brookhaven National Laboratory, Upton, NY 11973

3. Applied Decision Science, LLC, 1776 Mentor Ave., Suite 424, Cincinnati, OH 45212

4. U.S. NRC, Office of Nuclear Regulatory Research, Division of Risk Analysis, Washington, DC 20555

5. Same as #3; 6. Same as #4

Division of Risk Analysis

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555

A critical part of the U.S. Nuclear Regulatory Commission (NRC) mission is licensing and regulating the use of byproduct materials, 

including in radiation therapy. More than half of the reported misadministration events in the medical use of byproduct materials in 

radiation therapy are attributed to human error. 

In support of the NRC recommendation to continue to apply and extend a risk-management framework (as discussed in NUREG 

2150 [1], A Proposed Risk Management Regulatory Framework), this report discusses an approach to improve patient safety that is 

based on human reliability analysis (HRA), traditionally part of probabilistic risk assessments (PRAs). Using HRA and related 

disciplines, four key elements for improving patient safety are highlighted: 1) improved understanding of human error in radiation 

therapy; 2) improved ability to anticipate errors; 3) effective strategies for supporting humans in managing complexity; and 4) 

methods to evaluate and monitor the effectiveness of corrective actions. This report also includes reviews of studies conducted by 

the NRC using human reliability analysis methods and recommends the continued application of these methods. Successful safety 

improvements from commercial aviation and nuclear power industries are examined and offered as examples of the feasibility of this 

approach. It is important to emphasize that target audiences for this document include, but are not limited to, decision makers, 

regulators, designers, and external vendors engaging with the risk-informed framework.

Radiation Therapy

Medical

Risk

Human Reliability Analysis



NUREG-2170 A Risk-Informed Approach to Understanding Human Error in Radiation Therapy June 2017