Safety
Reports SeriesNo. 17LESSONS LEARNED FROMACCIDENTAL EXPOSURESIN RADIOTHERAPYThe following States are Members of the International Atomic Energy Agency.
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February 2000
STI/PUB/1084

SAFETY REPORTS SERIES No. 17

LESSONS LEARNED FROMACCIDENTAL EXPOSURESIN RADIOTHERAPYINTERNATIONAL ATOMIC ENERGY AGENCY
VIENNA, 2000

VIC Library Cataloguing in Publication Data
Lessons learned from accidental exposures in radiotherapy — Vienna :International Atomic Energy Agency, 2000.
p , 24 cm — (Safety reports series, ISSN 1020-6450; no 17)
STI/PUB/1084
ISBN 92-0-100200-9
Includes bibliographical references.
1. Radiotherapy—Exposure
2. Radiotherapy—Safety
I. International Atomic Energy Agency II Series

measures.

VICL

00-00236

FOREWORD
The medical use of radiation is unique in that patients are intentionally exposed
to radiation. The aim in radiation therapy is twofold: to deliver a dose and dose
distribution that is adequate for tumour control, but which also minimizes
complications in normal tissues. In therapeutic applications, the doses are high and a
deviation from the prescribed dose may have severe or even fatal consequences. Thereis therefore a great need to ensure adequate radiation protection and safety inradiotherapy by verifying that all personnel involved are appropriately trained fortheir duties, that the equipment used meets relevant international specifications forradiation safety and that safety culture is embedded in routine activities inradiotherapy departments.
Many individuals must interact and work together on highly technicalmeasurements and calculations, and therefore the potential for mistakes is great.
A review of the mistakes shows that most are due to human error. The International
Basic Safety Standards for Protection against Ionizing Radiation and the Safety ofRadiation Sources (IAEA Safety Series No. 115) require that a prompt investigation
be conducted whenever an accidental medical exposure of patients occurs. The reportof the investigation is to be disseminated to the appropriate parties so that lessons canbe learned to prevent similar accidents or mitigate their consequences in the future.
This Safety Report is a collection of a large number of events that may serve asa checklist against which to test the vulnerability of a facility to potential accidents,
and to provide a basis for improving safety in the use of radiation in medical
applications. A further purpose of this report is to encourage readers to develop aquestioning and learning attitude, adopt measures for the prevention of accidents, andprepare for mitigation of the consequences of accidents if they occur.
The IAEA officer responsible for preparation of this report was M. Oresegun of
the Division of Radiation and Waste Safety, Department of Nuclear Safety.

EDITORIAL NOTE
Although great care has been taken to maintain the accuracy of information containedin this publication, neither the IAEA nor its Member States assume any responsibility forconsequences which may arise from its use.
The use of particular designations of countries or territories does not imply any
judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of
their authorities and institutions or of the delimitation of their boundaries.
The mention of names of specific companies or products (whether or not indicated asregistered) does not imply any intention to infringe proprietary rights, nor should it beconstrued as an endorsement or recommendation on the part of the IAEA
Reference to standards of other organizations is not to be construed as an endorsementon the part of the IAEA

CONTENTS

1.

2.

3.

4.

INTRODUCTION

1

1.1.
1.2.
1.3.
1.4.

Background
Objective
Scope
Structure

12
2
2

REVIEW OF ACCIDENTS

3

2.1.
2.2.
2.3.
2.4.
2.5.

3
5
6
7

Definitions
Methodology for the review
Clinical considerations related to quality assurance
Radiation measurement systems
External beam therapy: Machine commissioning
and calibration
2.6. External beam therapy: Treatment planning,
patient setup and treatment
2.7. Decommissioning of teletherapy equipment
2.8. Mechanical and electrical malfunctions
2.9. Brachytherapy: Low dose rate sources and applicators
2.10. Brachytherapy (high dose rate)
2.11. Unsealed sources

22
36
39
40
59
61

CLASSIFICATION OF ACCIDENTS

66

3.1.
3.2.
3.3.
3.4.
3.5.

Classification of causes based on radiotherapy techniques
External beam therapy
Low dose rate brachytherapy
High dose rate brachytherapy
Use of unsealed sources

66
67
69
70
70

LESSONS LEARNED AND MEASURES FOR PREVENTION
OF ACCIDENTS

71

4.1. Resources: Personnel and equipment
4.2. Human factors
4.3. Training

71
71
73

10

4.4. Communication
4.5. Equipment
4.6. Human-machine problems
4.7. Improper decommissioning of equipment andunsafe storage of radioactive sources
4.8. Documentation
4.9. Integration of safety and quality assurance
4.10. Safety assessment
4.11. Regulatory control

74
75
78
79
80
81
82
86

APPENDIX: APPENDIX II OF THE BSS — MEDICAL EXPOSURE:
OPTIMIZATION OF PROTECTION
FOR MEDICAL EXPOSURES

87

REFERENCES

90

CONTRIBUTORS TO DRAFTING AND REVIEW91

RELATED IAEA PUBLICATIONS

93

1. INTRODUCTION

1.1. BACKGROUNDThis Safety Report provides a review of events that constitute incidents or
accidents related to the therapeutic use of ionizing radiation. The events concern
external beam therapy, brachytherapy and nuclear medicine. Most of the cases
involve patients, although some examples illustrate accidental exposure of medical
personnel or the general public. Most events involve radiation exposure; however, afew examples are included that illustrate injuries resulting from mechanical orelectrical failure of equipment.
The International Basic Safety Standards for Protection against Ionizing
Radiation and for the Safety of Radiation Sources (BSS) state that the regulatory
authority should require all parties to develop a safety culture that includes measures
to encourage a questioning and learning attitude and discourage complacency withrespect to safety [1]. Information on unusual events during operation of the facilities
which led or might have led to accidents provides background material that can beused to prevent accidents when such information is disseminated to regulators,
manufacturers, users and radiation safety specialists.
With regard to patients, the BSS require prompt investigation by licensees
in the event of an accidental medical exposure, defined by the BSS (p. 55) as
follows:"(a) any therapeutic treatment delivered to either the wrong patient or the wrong
tissue, or using the wrong radiopharmaceutical, or with a dose or dose
fractionation differing substantially from the values prescribed by the medical
practitioner or which may lead to undue secondary effects;(c) any equipment failure, accident, error, mishap or other unusual occurrence withthe potential for causing a patient exposure significantly different from the one
intended."
Following an investigation, the BSS requires the licensee to:"(a) calculate or estimate the doses received and their distribution within thepatient;(b) indicate the corrective measures required to prevent recurrence of such an
incident;

(c) implement all the corrective measures that are under [the local] responsibility;(d) submit to the Regulatory Authority, as soon as possible after the investigation
or as otherwise specified by the Regulatory Authority, a written report whichstates the cause of the incident and includes the information specified in (a)to (c), as relevant, and any other information required by the Regulatory
Authority; and(e) inform the patient and his or her doctor about the incident."

1.2.

OBJECTIVE

This Safety Report is a collection of a large number of events that may serve asa checklist to test the vulnerability of a facility to potential accidents and to provide
a basis for improving safety in the use of radiation. The plain presentation of manyfacts may permit a reader to ask whether events that took place at other institutions
might also occur at his/her own institution. A further purpose of this report is toencourage readers to develop a questioning and learning attitude, adopt measures forthe prevention of accidents and prepare for mitigation of the consequences ofaccidents if they occur.

1.3.

SCOPE

The events presented here have been reported to regulatory authorities andprofessional associations, published in scientific journals, or otherwise become
known through publication. No individuals, institutions or countries are identified inthis report. The rationale behind this anonymous method of presentation is toencourage individuals and institutes to report events of misadministration or accidents
without the fear of being professionally prosecuted. This report is not intended to be
an exhaustive analysis of individual accidents.

1.4.

STRUCTURE

Section 2 contains descriptions of the events. Each description consists of ashort account of what happened, followed by the initiating event and the relevant
contributing factors, as well as the remedial action taken, if known.
Section 3 discusses lessons learned from the events, with references to specific
events described in Section 2. Section 4 presents general principles for the preventionof rnisadministrations and accidents. Section 5 discusses recommendations for
implementing safety at a radiotherapy facility.

2. REVIEW OF ACCIDENTS
2.1.

DEFINITIONS

For the purposes of this report, the terms below are defined as follows, in linewith the BSS:
—Potential exposure is exposure that may result from an accident due to an eventor sequence of events of a probabilistic nature; the probability, while notnegligible, is significantly less than one.
— Normal exposure is exposure which is expected to be received under normal
operating conditions, including minor mishaps or errors whose probability ofoccurring is not significantly less than one.
The preceding two definitions encompass the full range of exposures from aradiation source and apply to occupational, public and medical exposure.
— Medical exposures are exposures incurred by individuals in the course ofdiagnostic examinations or treatment and exposures (other than occupational)
endured knowingly and willingly by individuals helping in the support andcomfort of patients undergoing diagnosis and treatment. Medical exposure also
includes exposures incurred by volunteers participating in programmes ofbiomedical research.
— Accident refers to any unintended event, including operating errors, equipment
failures or other mishaps, the consequences or potential consequences of which
are not negligible from the point of view of protection or safety.
Potential exposure is concerned with the potential for accidents in radiotherapy
or involving radiotherapy sources, the consequences of which are relevant to safety.
However, when an accident actually occurs it is no longer potential. It is real.
There are aspects of potential exposure that are unique to the medical use ofradiation sources since patients are intentionally exposed to direct radiation beams
and radiation sources are incorporated in their body as part of the diagnosis andtreatment. In therapeutic applications, the doses are extremely high, and a small
departure from the prescribed dose may have severe or even fatal consequences.
Potential exposure, in the context of the treatment of patients, concerns not only doses
significantly above the intended dose but also doses significantly below the intended
dose, since these too can have severe consequences.
The expression 'substantially differing from the prescription' deserves further
consideration to establish a common understanding in this review of accidental

medical exposures. The ultimate goal of radiotherapy is to deliver a specified
radiation dose to the prescribed target volume with the least dose to healthy tissues.
The demands for precision and accuracy are high, because any deviation in theradiation dose will increase the probability of a detrimental effect to the patient [2],as explained in the following paragraph.
The dose and dose distribution within the patient are aimed at maximizing the
local tumour control probability (TCP) while minimizing the normal tissuecomplications probability (NTCP). The margin for the overall error is very small(ideally below 5% overall deviation). A simplified example of this statement is
illustrated in Fig. 1, where the TCP and NTCP curves are presented in relation to thedose to the planning target volume. The probability of local tumour control withoutcomplications has a maximum value, which provides the basis for the justificationand optimization of the radiotherapy treatment (dose, dose fractionation and dose
distribution).
It is worth taking a more detailed look at Fig. 1: the slope of the sigmoid curves
is usually steep. In the simplified example, if the maximum is situated at a dose where
the slopes of the sigmoids are near 2, a deviation of 15% means a decrease in TCP oran increase in NTCP of 30%, which may jeopardize the justification of the decision
made. However, this is only one example and the slope of the curves varies depending
100 r

Q.

0

75

Tumour -o
OLONormal tissue —-f

50

25

Dose

FIG. 1. Tumour control probability (TCP) and the probability of normal tissue injury as afunction of radiation dose, in a hypothetical case. If normal tissue injury is to be avoided
altogether, the radiation dose cannot exceed A; the TCP is low. By accepting a relatively small
probability of normal tissue injury, the radiation dose can be increased to B, and the TCP issignificantly improved. A further increase in the radiation dose above C results in an increased
complication rate with very little improvement in TCP.

on the types of tumour and normal tissue as well as on the separation between the
TCP and NTCP. Therefore, there is no single and sharp limit for the deviation onwhich the decision that a deviation is 'substantial' enough to constitute an accidental
exposure from the point of view of the outcome can be based; nevertheless, forpractical reasons, the generic level of 20% has been chosen by some national
authorities to make an investigation mandatory (10% if the prescribed number offractions is equal to or less than three).
On the basis of these considerations, it is understandable that mistakes andequipment faults have an immediate influence on the outcome of the treatment, andthat most of the accidents in this report involve patients, in some cases with serious,
even fatal, consequences.
In addition to concern for patients undergoing radiotherapy treatment, potential
exposure from radiotherapy sources includes medical exposures to persons helping in
the comfort of patients, occupational exposures to staff and exposures to members of
the public.
Accidental exposures may occur to visitors helping in the support ofbrachytherapy patients if rules are misunderstood or violated or if a radiation source
becomes accidentally exposed (i.e. accidentally falls out of the patient).
Potential exposures to staff may result from equipment failure such as a stucksource in a teletherapy machine or a remote control after-loading device, or when abrachytherapy source is accidentally removed from a patient. Accidents with severe
consequences have also occurred during maintenance or source exchanges.
With regard to potential exposures to the public, the most important case of
exposure is due to loss of control or abandonment of sources. Very serious
consequences have also occurred with teletherapy units that are not well controlled or
are improperly decommissioned. A patient discharged from the hospital withbrachytherapy sources mistakenly left inside his or her body may not only risk death,but also expose members of the public. A particular problem is posed by old 226Ra
brachytherapy sources introduced in many countries several decades ago before any
regulatory control existed. The owners may have died, and the sources may remain
with members of the public who have no knowledge of the potential hazards theyentail.

2.2. METHODOLOGY FOR THE REVIEWThe description of an accidental exposure consists of a short account of theaccident itself, followed by the initiating events and the relevant contributing factors.
Consistent with its objectives, the emphasis in this Safety Report is on the collection
and collation of a large selection of events and contributing factors, rather than on an
exhaustive analysis of individual accidents.

Care has been taken to avoid making judgements. Rather, the plain presentationof a collection of facts was considered to be more effective towards increasingawareness, contributing to a questioning and learning attitude and adopting measures
for the prevention of accidents and mitigation of their consequences.
In some accident scenarios where the dosimetric units are relevant to theprimary cause of the accident, SI units have not been used. Instead the old units suchas millicurie (mCi) and milligram radium-equivalent (mg Ra-eq) were quoted as
used.
An initiating event is the action which triggers the chain of events (or event
sequence) which results in an accident. A contributing factor is any circumstance,
condition, action or omission which plays a part in the precipitation of an
accident.
Frequent contributors to accidents were: insufficient education in radiotherapy
physics; a lack of a set of procedures and protocols integrated into a comprehensive
quality assurance programme; and/or the lack of supervision over compliance withthe programme. Another finding was that training generally addresses only normal
situations and does not prepare radiotherapy staff for unusual situations, resulting ina lack of a 'safety culture'.
The review focuses on pure facts from which general lessons can be drawn andintegrated into the oncology service as part of a system for safety and quality.
Contributing factors have been indicated by a general statement (which is common toa number of accidents), followed by detailed information specific to that particular
event. As an example, for Event No. 26, the general statement: "lack of or ineffective
procedures, protocols and documentation" is followed by the more specificinformation: "incorrect tables were accepted for use".
However, overly general statements such as "lack of overall quality assurance"
have been avoided, since they would apply to most of the accidents and would convey
very little information. The reader will be able to conclude that quality assurance waslacking whenever procedures were not complied with (or did not exist at all), or
whenever safety provisions to prevent accidents (such as independent checks forsafety critical issues) were non-existent.

2.3. CLINICAL CONSIDERATIONS RELATED TO QUALITY ASSURANCEIn clinical institutions and hospitals where accuracy and precision are notmaintained within a narrow range, there can be a tendency to avoid normal tissuecomplications by lowering the prescribed dose. The consequence is a drastic decreasein the probability of tumour control. The only acceptable solution is improvement of
accuracy and precision so that the right doses can be prescribed. This practice ofprescribing lower doses than desirable cannot be considered as a potential exposure

or accident since the problem is associated with the prescription itself. This is an issueof ethics and is therefore not included in the methodology dealing with potential
exposures.

2.4. RADIATION MEASUREMENT SYSTEMSThe output of a therapy machine is measured by an ionization chamber andelectrometer with a calibration by, or traceable to, an authorized standard dosimetrylaboratory. Malfunction or improper use of this basic equipment can affect many
patients. Consistency among different sets of equipment requires frequent intercomparison. Constancy checks verify that equipment performs correctly over time.
Event No. 1: Dosimeter calibration report used incorrectly
The institution had its ionization chamber and electrometer calibrated for 60Co
at a standards dosimetry laboratory. The calibration certificate was in terms of doseto water, but was interpreted as specifying dose in air. The resulting error caused an
overdose to patients of approximately 11% for at least one year.
Initiating event— Calibration error: Incorrect use of a calibration certificate.
Contributing factors— Insufficient education, training or expertise: The physicist did not seem tounderstand the calibration certificate.
— Insufficient safety provisions (defence in depth): There was no independent
beam calibration by another physicist.
Event No. 2: Incorrect use of a plane parallel chamber
A new physicist at a hospital used a pancake chamber to calibrate several
electron beams. A label on the chamber, placed by the previous physicist, indicatedthe side on which the beam should be incident. Although the previous physicist had
used the chamber correctly, his labelling was incorrect and the new physicist used thechamber upside down in the beam, resulting in the following errors in dose delivery,
with the electron beams having the indicated energies:
6 MeV:
9 MeV:
12 MeV:

20% overdose,
10% overdose,
8% overdose,16 MeV:
20MeV:

Correct dose,
1% underdose.

Mailed thermoluminescence dosimeters (TLDs) from an independent
laboratory were used routinely by the institution for quality assurance of radiation
beam calibration. These dosimeters revealed the above discrepancies, which
prompted the physicist to investigate the cause. By the time the error was corrected,
the incorrect beam outputs had been used for part of the treatments of only a few
patients.
Initiating event— Incorrect calibration: Incorrect use of the plane parallel chamber to measure theoutput of the electron beams.
Contributing factors— Insufficient education, training or expertise: The physicist used a chamber
unfamiliar to him without verifying the proper technique.
— The previous physicist had labelled the chamber incorrectly.
— Deficient communication/transfer of essential information: Either there was
miscommunication, or a faulty method was transferred from one physicist toanother.
Remedial action— The calibration measurements were repeated, with the chamber properlyoriented in the beam. The label on the chamber was corrected.
Event No. 3: Error in correction for atmospheric pressure
An institution did not own a barometer and the physicist relied on the local
airport for measurement of atmospheric pressure. Pressure reported by the airport wascorrected to sea level, but the physicist interpreted the pressure report as appropriate
for the elevation of the site, about 1000 m. The use of the incorrect pressure resulted
in errors in the measurement of machine outputs, which caused a 13% overdose topatients. The error affected the calibration of all machines at this institution.
Initiating event— Calibration error: The use of incorrect pressure values to correct foratmospheric pressure resulted in an incorrect beam output.
Contributing factors— Shortage of instruments: The institution did not have its own barometer.

— Insufficient awareness:• Physicists relied on pressure data from an airport without knowing that thedata referred to sea level (requesting both station pressure and pressure
corrected to sea level should eliminate ambiguity).
• Physicists did not consider altitude as having a bearing on atmosphericpressure.
Remedial action— The institution purchased its own barometer which measured local pressure.
Event No. 4: Error in correction for atmospheric pressure
Atmospheric pressure was determined using data from nearby weather stations.
The physicists concerned did not realize that these data were actually corrected to sea
level. This happened at four institutions, and six physicists were involved.
In at least two of the treatment centres the same incorrect pressure wasobtained redundantly. At three of the centres the overdose was 13-14% and at one
centre the overdose was 21%. Many patients were treated at each institute. The
21% overdose by a 60Co unit continued for about ten months until a different
physicist calibrated the machine and informed the radiation oncologist of themiscalibration. The radiation oncologist chose to continue with the older andincorrect calibration.
Initiating event— Incorrect calibration: The use of incorrect pressure values to correct foratmospheric pressure resulted in an incorrect beam output.
Contributing factors— Shortage of instruments: The institutions did not have their own barometers.
—Insufficient awareness:• Physicists relied on pressure data from an airport without knowing that thedata referred to sea level (requesting both station pressure and pressure
corrected to sea level should eliminate ambiguity).
• Physicists did not consider altitude as having a bearing on atmosphericpressure.
Event No. 5: Lack of consistency between dosimetry at affiliated institutions
Three radiation oncologists and two physicists worked at three affiliatedhospitals. Each person worked in at least two, sometimes three, of the hospitals. There
were two sets of calibration equipment (ionization chambers and electrometers),

usually assigned to a particular hospital. The two sets of equipment were neverintercompared although they were used by both physicists.
A review by an outside physicist showed a discrepancy of up to 15% between
measurements made with the two sets of equipment. After discussion with thephysicists and radiation oncologists, one physician stated that he had noticed adifference in clinical results at two hospitals and had allowed for the difference byprescribing 70 Gy to the prostate in one hospital and 60 Gy in the other hospital foran equivalent treatment.
Initiating event— Incorrect calibration: An invalid calibration factor was used.
Contributing factors— Lack of or ineffective procedures: Different sets of equipment, which differedin calibration, were used, without intercomparison, at two hospitals served by
the same radiation oncologists and physicists.
— Ineffective communication or transfer of essential information: Physicians
failed to inform physicists that apparently there were differences in clinicalresults at two hospitals presumed to be using the same dosimetry techniques.
Rather than discuss the problem with the physicists, at least one clinician
dealt with the discrepancy by using a 'clinical prescription correction
factor'.

2.5. EXTERNAL BEAM THERAPY: MACHINE COMMISSIONING
AND CALIBRATION
Event No. 6: Incorrect calibration procedures
A new physicist adopted the calibration procedures used by the previous
physicist. The previous physicist had calibrated all beams for a linear accelerator
using 200 monitor units, with the exception of one beam, which he calibrated at
300 monitor units. For the latter beam, he used a 'conversion factor' to convert from
300 to 200 monitor units. The new physicist calibrated all beams at 200 monitor units;
however, he applied the previous physicist's conversion factor to one beam although
it was now unnecessary. The miscalibration resulted in an overdose of 50% forpatients treated with that beam.
Initiating event—Incorrect calibration: Use of an inappropriate correction factor.

10

Contributing factors— Lack of or ineffective procedures and protocols: A physicist used data andprocedures established by a previous physicist without understanding them.
— Ineffective communication or transfer of essential information: Poor handing
over of procedures from one physicist to another.
— Lack of safety provisions (defence in depth): There was no independent beam
calibration by another physicist.
Event No. 7: Incorrect calibration of machine output
An institution had a new 60Co teletherapy source installed. The installer left acertificate specifying the exposure in rontgen per minute (R/min) at 1 m for the largest
field available. The institution used the installer's output, with corrections for inverse
square and decay, as the output in centigray per minute for all field sizes. When fieldsize dependence, backscatter, rontgen to centigray conversion, timer, etc., wereconsidered, the correct calibration revealed that patients received doses that wereapproximately 15% too low; the magnitude of the error depended on the field size.
Initiating event— Incorrect calibration of the beam: The machine 'calibration' was based on the
manufacturer's certificate of source strength for conditions inappropriate fortreatment.
Contributing factors—Lack of education in radiotherapy physics: There was poor understanding of thefactors needed to convert from the installer's statement of source strength to thedata used clinically.
— Lack of procedures and protocols: The physicist did not measure the machine
output.
— Lack of safety provisions (defence in depth): There was no independent beam
calibration by another physicist.
Event No. 8: Calibration error after a source change in a 60Co unit
After a source change in a 60Co unit, the physicist incorrectly calibrated theoutput. An incorrect time of 30 s instead of 18 s (0.3 min) was used to calculate theabsorbed dose. This resulted in a 166% overestimation of the exposure time. Over a
period of about one month, 115 patients were treated and received overdosage,
resulting in severe health effects including fatalities. Inconsistencies in the dosimetry
were detected by the IAEA/WHO postal dose quality audit and by an external expert,
but this did not trigger any significant improvements in the procedures.

11

Initiating event— Incorrect calibration of the beam: The dose rate used clinically was incorrect
because of a mistake in the exposure time during calibration.
Contributing factors— Insufficient education on radiotherapy physics.
— Lack of procedures, protocols and documentation: The absence of documented
procedures allowed the mistake to go unnoticed.
— Lack of safety provisions (defence in depth):• There was no independent calibration by another physicist of a new beam
(after a source change).
• There was no comparison of the calibration for compatibility with the source
manufacturer's certificate of source strength or output.
• The treatment times were not reviewed for consistency with previous
treatments.
— Lack of awareness of physicians and of the person in charge of the dosimetry:
Inconsistencies previously detected and notified were not investigated.
Event No. 9: Incorrect calibration of machine output
Electron beams of 7 and 11 MeV were calibrated incorrectly, resulting inunderdosage of 17-18%. On the same machine, a photon beam was calibrated
incorrectly, resulting in overdosage of 5%. In addition, there was a drift in the beam
output over time, up to 7%. Mailed TLDs indicated a potential problem, and a review
by an independent, outside physicist revealed that the calibrations were incorrect.
Apparently, the incorrect calibrations had been used clinically for at least 11 months.
During that time, there was no record of quality control performed by the institution's
physicist. The cause of the incorrect calibrations was unknown.
Initiating event— Incorrect calibration of the beam: The cause was unknown.
Contributing factors— Insufficient safety provisions (defence in depth): There was no independent
beam calibration by another physicist.
— Lack of or insufficient quality control: Over an 11 month period no checks werecarried out on the beams.
Remedial action— Calibrations were corrected as a result of an external review by an independent
physicist. At the time of this report, it is not known whether the deviation inbeam output has been corrected.

12Event No. 10: Calibration error after changing a ^Co teletherapy source
Physics staff calibrated the output of a new source installed in a 60Co unit.
About three and a half months later, nursing staff began to notice that the skinreactions of some of the patients treated on the machine were not healing as rapidly
as would be expected. These concerns were communicated to the physics staff, whoreviewed the output tables in clinical use as well as the original calibration; theyreported that the data were correct. There was no explanation for the slow healing of
the patients, which continued to be observed.
An intercomparison exercise, organized by a national association of medical
physics, led to the discovery of a miscalculation in the original calibration. Over thefive month period, the 207 patients treated on the 60Co machine had received a dosethat was 25% higher than prescribed.
Initiating event— Incorrect calibration of a beam (the specific reason for the miscalibration was
not reported).
Contributing factors— Lack of safety provisions (defence in depth):• There was no independent beam calibration by another physicist.
• There was no comparison of the calibration for compatibility with the source
manufacturer's certificate of source strength or output.
— Inefficient quality control:• A verification of the calculations did not reveal the error. It is possiblethat the time for a calibration measurement was recorded incorrectly.
• Subsequent reviews of the calculations by the local staff were ineffective,
despite indications of clinical problems.
— Insufficient investigation of unusual patient response: There was no persistentsearch for the causes of the slowly healing skin effects.
Event No. 11: Incorrect calibration of a machine with asymmetric jaws
A linear accelerator with asymmetric jaws was calibrated with the detector
positioned in the penumbra region and did not represent the dose in the centre of the
field. The result was that patients received an overdose of 27%.
Initiating event— Incorrect calibration of the beam: The calibration was made in the
penumbra.

13

Contributing factors— Insufficient education and training in radiotherapy physics: There seemed to bea lack of understanding of the features of an asymmetric beam.
— Insufficient defence in depth: There was no independent beam calibration byanother physicist.
— Lack of or inefficient procedures and protocols: Procedures for thecommissioning of a new machine were not appropriate.
— Insufficient awareness and alertness.
Remedial action—The machine was calibrated correctly and new data placed in clinical use.
Event No. 12: Incorrect decay of a 60Co source and fabrication of records
A 60Co unit was calibrated correctly initially, but subsequently corrected for
decay in the wrong way and no re-measurement of the output was made. The overdose
to patients increased progressively over a period of 22 months, reaching 10% in
5.5 months and as much as 50% in the subsequent 16.5 months. During the latter
period, 426 patients were treated. Of the 183 patients who survived beyond one year,
34% had severe complications in various sites, including the brain, spinal cord, skin,oropharyngeal mucosa, colon and rectum, which led to the death of some patients.
Initially the incorrect machine output was attributed to a faulty measuring system.
The physicist at the institution produced ten calibration documents, which turned out
to be fabncated, that showed that the machine output used clinically was correct.
When some patients showed symptoms of overexposure, the hospital asked
consultant physicists to investigate the dosimetry. Their findings were that:(a)(b)

Both the 60Co unit and the measuring system apparently had been functioning
correctly during the 22 month period.
The hospital physicist had fabricated all but one of the reports of measurementof machine output. The output used clinically had been calculated, without acheck by measurement, and the calculation was incorrect.
Initiating event— Incorrect calculation of output: The output was based on an incorrect decay ofsource output.
Contributing factors— Apparently, insufficient staff resources were devoted to dosimetry and quality
control: The beam output was not checked over a 22 month period, since thephysicist was assigned to a new accelerator.

14

— Insufficient defence in depth: There was no independent check of decay curves.
—The physicist falsified records in an attempt to justify his calculations.
Remark— The number of staff and training needs should be reviewed whenever theworkload increases, a new machine is purchased or a new technology ortreatment technique is introduced.
Event No. 13: Use of incorrect tables for decay of a 60Co source
A physicist generated tables of decay factors to correct the output of a 60Co
irradiator. The tables were generated by a computer, with the year incorrectly entered
as one year earlier. Consequently, for a three month period, patients received a dose
approximately 12% less than that prescribed. The error was detected during a review
of dosimetry by an outside physicist.
Initiating event— Incorrect calculation of output (it was based on incorrect decay of source output).
Contributing factors—Insufficient safety provisions (defence in depth):• There was no independent check of decay curves.
• The treatment times were not reviewed for consistency with previous
treatments.
Event No. 14: Lack of communication regarding units of output
of a treatment machine
A physicist calibrated a 60Co unit in terms of rontgen/mmute at a 5 cm depth ina Masonite phantom. The radiation oncologist, who performed all treatment planningand dosimetry calculations, assumed that the output was in rad/minute to aminiphantom (muscle) at the depth of maximum dose (0.5 cm). The magnitude of theerror, depending on the field size, can reach an overdose of 20-25%.
Initiating event— Incorrect clinical use of a beam calibration.
Contributing factors—Lack of or ambiguous assignment of responsibilities: The entire dosimetry
system, from calibration of machine output to calculation of treatment time,
was not the responsibility of one person (preferably the physicist).

15

— Ineffective communication/transfer of essential information: There was a lackof communication between the physicist who calibrated the machine and thephysician who performed treatment planning.
—Lack of or inefficient procedures, protocols and documentation: The
technique used by the physicist for calibrating the machine output, in terms ofrontgen in a Masonite phantom, was unusual and does not follow accepted
protocols.
Remedial action— A review of dosimetry by an outside physicist revealed the misunderstanding
concerning the machine outputs and corrections were made.
Event No. 15: Radiation leakage from the collimator of a linear accelerator
A patient who had been treated for prostatic cancer was seen a few days after
he had finished his treatment course. The patient had developed a severe skin reaction
on the ventral part of the right arm, the right part of the thorax and the right part of
the head. In addition, he had lost some hair from his head. The skin reaction wasinvestigated further, leading to the diagnosis of radiation dermatitis caused by an
estimated dose of 10-20 Gy. The treatment had consisted of a course of 25 x 2 Gy
with 25 MV photons, using a three beam technique (anterior plus lateral-oblique
beams), produced by a 14 year old accelerator.
During the radiotherapy treatment period of this patient, the accelerator showeda slow start at 'radiation on' and radiation was sometimes delivered at a dose rate thatwas less stable than normal, especially at some gantry angles; sometimes the starting
procedure failed completely. Qualified hospital and manufacturer's technicians madea thorough investigation of the origin of these problems, resulting in some additional
maintenance and an increase in the frequency of quality control checks. The
functioning of the accelerator was monitored almost daily. No deviations from thenormal beam properties was observed. The interlock system had never been set off
during patient treatment.
Detailed observations of the skin reaction of the patient revealed that a skinreaction was not seen behind the right ear and that the oral mucosa did not show any
reaction. It was therefore concluded that the skin reaction could have been caused
only by low energy radiation.
The most probable position and direction of the apparent source of radiation
leakage was reconstructed from the region of the skin reactions in relation to thegantry and collimator position of the accelerator under the treatment setup. From
this reconstruction it was concluded that the source of radiation leakage was locatedat the upper end of the beam guidance tube close to the last part of the bending
system.

16

It was thereby demonstrated that, despite the relatively high degree of control
and the presence of safety systems, this old accelerator could be prone to producing
a large amount of radiation leakage which was harmful to patients.
Initiating event— Unknown, but thought to be a misadjustment of an accelerator parameter which
made the accelerator operate in a very unusual mode.
Contributing factor— Possibly, advanced age of the installation. The only unusual symptom wasbeam instability. Operating procedures called for this to be investigated but did
not recognize it as a reason to stop operation of the machine when the beam
properties appeared normal.
Event No. 16: Treatment with an accelerator operated in the PHYSICAL
mode
During clinical use of a linear accelerator, a problem arose with the selection ofelectron and X ray energies. A technician from the manufacturer reported that the
problem was associated with a microswitch required to select the scattering foil forelectron beams or the beam flattening cone for photon beams. About one month later,
on two consecutive days, the same problem reappeared briefly. The machine was not
repaired at this time. Four days later the problem returned permanently and themachine could be operated only at two X ray energies and one electron energy(10 MeV). Two weeks later, it was impossible to start the accelerator in the
MEDICAL mode. The electronics engineer informed the chief radiation oncologist
about the problem. The radiation oncologist asked the electronics engineer to alter themachine so that patients could be treated in the PHYSICAL mode.
The electronics engineer switched the linear accelerator to the PHYSICAL
mode and made the following provisions for treatment:
(1)
(2)
(3)

(4)

He covered the push buttons that are operative in the MEDICAL mode withpaper;
In co-operation with the technician, he measured the absorbed dose understandard conditions for all beams;
He prepared a written report describing procedures for the treatment ofpatients under the PHYSICAL mode and fixed it to the console of the linear
accelerator;
He instructed two radiographers regarding operation of the linear accelerator
for treatment in the PHYSICAL mode and observed the treatment of the first
four patients.

17

Thirteen patients were treated with apparently no problems, the last patient
being treated with a 10 MeV electron beam. Data for the next patient were set on the
console for treatment with 20 MV X rays, dose rate 300 MU/min, without any
problem. The treatment started but was terminated by a timer after 21 s; the timer was
the third independent safety system designed to switch off the beam if two lonizationchamber monitors failed. (According to the radiographer, the audible dose rate
indicator beeped with a very low frequency.) When the treatment was terminated,
only a few monitor units were displayed on both monitors; the radiographer therefore
immediately called the electronics engineer to check the machine. When the patient
was removed from the treatment room, the radiographer observed a skin reaction on
the patient, which indicated a high degree of overexposure. Treatment was
immediately suspended.
The electronics engineer used an ionization chamber to measure the accelerator
output under the same operating conditions used for treatment. He found that there
was an extremely high dose in the centre of the field, caused by a failure to deploy
the X ray target, monitor chamber and beam flattening cone, leaving the 10 MeV
electron scattering foil and (disconnected) electron monitor chamber in position. Thismalfunction was due to the failure of a power supply, which supplied the interlock
systems, that should have been active in the non-clinical mode. Hence, no indicationof the wrong positioning of the essential elements in the beam line appeared on thecontrol panel. Operation in the PHYSICAL mode had disabled interlocks that could
have detected this dangerous condition.
Initiating event— Treatment in non-clinical, PHYSICAL mode (decision by the radiation
oncologist after several failures with interruption of treatments).
Contributing factors— Equipment failure: The accelerator failed to operate in the clinical (MEDICAL)
mode.
— Maintenance problems: Intermittent, unresolved equipment failures, the causeof which was not identified.

Event No. 17: Mishandling of equipment failure
Staff operating a linear accelerator noted that the machine would not produce
an electron beam. They reported the fault to a maintenance technician from the
manufacturer, who was working on the 60Co unit located in an adjacent room. The
technician performed corrective maintenance but, after the repair, the analog displayof electron energy permanently indicated 36 MeV regardless of the energy selected.

18

The energy selection keys correctly indicated the energy selected. The analog
indication of the 36 MeV was interpreted as a faulty meter displaying only 36 MeV.
Patients were treated under the above conditions for ten days before theproblem with the energy display was reported to the Department of Physics andRadiation Protection. Treatments were then terminated. Physicians began to correlate
the poor tolerance and severe reactions observed in some patients with themalfunction of the machine.
A dosimetric check performed the next day showed that the energy of theelectrons was 36 MeV, regardless of the energy (7, 10 or 13 MeV) selected on
the console. The manufacturer was informed and sent technicians to repair themachine.
The failure to select the electron beam energy was due to a short circuit of thesystem that selects the trajectory of the electron beam. The path curvature is afunction of both the electron energy and the intensity of the magnetic field generated
by the coils. Instead of correcting the basic cause (the wrong coil current resulting
from the short circuit), the energy of the electrons was manually adjusted to match thewrong current. The correct trajectory was obtained for the electron beam, but at thewrong energy (permanently 36 MeV). The analog indicator of the energy (whichindicates the current of the bending coil and depends on the selected energy) was
permanently indicating 36 MeV, even between treatments, owing to the short circuitin the control of this current.
When the maintenance technician had selected the manual mode of energy
adjustment, he effectively put the machine into a non-clinical mode in which it would
ignore the energy selected on the console. Therefore, the energy of the electrons wasalways 36 MeV, with the beam concentrated in the centre of the field. The result was
treatment with electrons that delivered a dose several times higher and irradiated a
volume deeper than prescribed. The malfunction affected 27 patients.
Initiating event— Incorrect maintenance: Misadjustment of the beam energy.
Contributing factors— Ineffective communication/transfer of essential information and noncompliance with procedures: Physicists were not notified immediately about
the malfunction of the machine (treatments were resumed without a check of
the beam output). Thus procedures for transferring the machine to and from themaintenance technician were not followed.
— Incorrect interpretation of conflicting signals: The reading on the analog meter
indicating the actual energy was ignored.
•—It was possible to operate the accelerator with the energy selection disabled(equivalent to 'non-clinical mode').

19

— Shortage of instruments for quality control: Instruments for quick (daily)
constancy checks of beam output and energy, compatible with patient
workload, were not available.
Event No. 18: Dose on central axis incorrect because of a loose
wedge mounting mechanism
Wedge factors were measured on a 60Co unit with the beam in the vertical
position. When the machine gantry was rotated 90° for treatments with a horizontal
beam, a loose wedge mounting mechanism allowed wedge filters to shift so that thedose distribution and the central axis wedge factor were incorrect. The error was aslarge as 8%, with the dose to the patient too high across the beam for one horizontal
machine position and too low for the other horizontal beam position.
Initiating event— Deficiency in the design of accessories of the machine: Mechanical deficiency
related to the wedge holder.
Contributing factor— Lack of or ineffective procedures for commissioning: Wedge factors were notmeasured with a horizontal beam at the time of commissioning of the machine(the use of accepted protocols with documentation of the data would avoid
omitting measurements).
Event No. 19: Holder for port film left in beam during treatment
A lucite plate 1.25 cm (0.5 in) thick used to hold port films was left in the beamduring treatment with a linear accelerator. There was no interlock system to detect thepresence of the plate. An independent check of beam output with TLDs indicated thefollowing per cent relationship between the measured and expected output:

20_ , . ,
Radiation beams

Institution s expected dose/,., —,*,„*
dose measured by TLD (%)

Photons: 10 MV
Electrons: 6 MeV
9 MeV
12 MeV
16 MeV
20 MeV

90
10
26
30
46
57

Further investigation by the institution's physicist revealed that the output
measured by the TLDs was correct when the plate was in the beam.
Initiating event— Mistake in the treatment setup: A holder for portal films was left in the beamduring treatment.
Contributing factor— Equipment design deficiency: There was no interlock to detect the presence ofthe portal film holder.
Remedial action—The linear accelerator was retrofitted with an interlock to detect the presence ofthe portal film holder.
Event No. 20: Design error in accelerator control software
A linear accelerator equipped with computer controlled electron and photon
beams was in clinical operation. Several serious incidents led to the recognition of aproblem whenever an initial instruction for 25 MV X rays was changed quickly to anelectron beam instruction. The computer process for setting and verifying themachine parameters required about 20 s to complete after the X ray mode wasselected. If an electron mode was selected before the computer process for X rays wascompleted, the monitor displayed electron mode but operated from hybrid
instructions. The resulting beam consisted of 25 MeV electrons with high beam
current appropriate for X rays but without beam scanning, with no X ray target andno X ray beam flattener in the beam. The resulting electron dose at the position of
maximum dose was 160-180 Gy before the beam cut off automatically, in one secondor less.
When the above sequence of events first occurred it was not recognized as afault by the manufacturer of the accelerator. One month later the same sequenceoccurred at another hospital and was attributed to microswitch failure. Six months
later a third hospital reported a similar experience with yet another patient. The
manufacturer made some modifications in response to the assumed microswitch
failure.
Three months later, following a fourth occurrence at another hospital, thepossibility of a malfunction was rejected by the manufacturer. One month later afurther accident in the same department led to an investigation by the hospitalphysicist. The fault was demonstrated and software errors were suggested as thecause. The physicist verified that other machines of the same type functioned in thesame way. One additional accident occurred before all accelerators of that type were21

withdrawn from clinical use. Of six patients treated during the machine malfunction,
two died, apparently of overexposure.
Initiating event— Equipment malfunction due to operation in extreme conditions (it operated on
hybrid instructions after the operator changed too quickly from the X ray mode
to electron mode).
Contributing factors— Insufficient factory testing: Computer controlled radiation equipment was notfactory tested for extreme or unusual operating conditions that could occur inpractice.
— Staff operating the linear accelerator requested the electron mode before
the computer had completed processing of the previously requested X raymode.
— Insufficient follow-up by the manufacturer: The reported incidents were
misinterpreted or dismissed by the manufacturer until, after similar incidents insix different hospitals, a hospital physicist identified and demonstrated thenature of the fault.

2.6. EXTERNAL BEAM THERAPY: TREATMENT PLANNING,
PATIENT SETUP AND TREATMENT

Event No. 21: Inconsistent sets of basic machine data
For one machine, an institution had two sets of data for output factors, depth
dose and other basic parameters. The two sets of data differed by 10%, with one set
being correct. The data were used interchangeably for a period of time, with the resultthat some patients were underdosed by 10%.
Initiating event— Incorrect data for patient dose calculations
Contributing factors— Lack of or ineffective procedures, protocols and documentation: The institution
had two sets of data that were inconsistent.
— Insufficient safety provisions (defence in depth): An incorrect table of values
was used without verification.

22

Event No. 22: Incorrect data for tissue maximum ratios
Tables of tissue maximum ratios were generated for a 10 MV photon beam,
based on the institution's measurements. Inspection of the tables during a review of
dosimetry by an outside physicist after several months of use revealed aninconsistency in the trend of data with increasing depth and field size. Checks against
the original measurements showed that at least 15 entries in the table were incorrect,
some by as much as 13%.
Initiating event— Incorrect data for patient dose calculations.
Contributing factors— Insufficient safety provisions (defence in depth): Tables of measured tissue
maximum ratios were not verified against published values before clinicaluse.
Remedial action— The tables were corrected. Charts of patients affected by the incorrect data werereviewed and the calculations corrected.

Event No. 23: Insufficient understanding of the treatment planning system(TPS) algorithm
A treatment planning computer was used to calculate 60Co treatment plans
involving wedges. The technologist and dosimetrist were not sure whether thecomputer calculation included the wedge factor. They asked the consultant
physicist and were told that the wedge factor was not included in the computer
calculation and should be accounted for manually. Three patients were treatedin this way. While preparing to treat an additional patient following thesame treatment protocol, a hand calculation of the treatment time indicated alarge discrepancy with the computer generated value. The computer alreadyincluded the wedge factor, so that this factor had been applied twice. A review ofcases revealed misadministration for three other patients, who received
overexposures of close to 11, 12 and 14% for prescribed doses of 62, 65 and 50 Gy,
respectively.
Initiating event— Incorrect data for patient dose calculations (the wedge factor was applied
twice, once by hand and once by the treatment planning computer).

23

Contributing factors— Lack of or ineffective procedures, protocols and documentation:
Commissioning with incomplete validation of the computer TPS.
— Insufficient safety provisions (defence in depth): There was no regular manual
checking of computer calculations.
Event No. 24: Incorrect basic data in a TPS
Basic data used in a TPS differed from measured data for a particular linear
accelerator; the inconsistency was not detected during commissioning of the planningsystem. The result was that patients received a 15% overdosage. The data had beenentered into the computer by the hospital's previous physicist and the reason for theerrors was unknown.
Initiating event— Incorrect data for patient dose calculations (incorrect basic data used in a TPS).
Contributing factors— Lack of or ineffective procedures, protocols and documentation:• Inadequate commissioning (TPS).
• Inadequate transfer of responsibilities and/or taking over by newly appointed
staff, in this case the physicist.
— Insufficient safety provisions (defence in depth): No manual checks of thetreatment plans were made.
Remedial action— The data in the TPS were corrected.
Event No. 25: Incorrect depth dose data
During installation of a linear accelerator, an institution contracted the servicesof the manufacturer to measure depth dose data. The institution's physicist laterchecked the data and found an 8% discrepancy between his measurements and those of
the manufacturer for some field sizes and depths. He concluded that the manufacturer's
data were correct and used them clinically. A review by an outside consultant
physicist revealed that the physicist's measurements were correct. During a period ofseveral months, some patients received doses that were 8% lower than prescribed.
Initiating event— Incorrect data for patient dose calculations: The manufacturer provided basic
data that were incorrect for some field sizes and depths.

24

Contributing factors—Lack of or ineffective procedures, protocols and documentation: Incorrectcommissioning (data generated by the manufacturer were accepted fortreatments, although the institution's physicist found an 8% discrepancy when
he compared these data with his measurements).
— Insufficient education, training or expertise: A known discrepancy was not
resolved.
— Lack of or ineffective assignment of responsibilities: The physicist did not takeresponsibility for all aspects of dosimetry.
Event No. 26: Incorrect calculation of treatment times
Tables of treatment times used clinically were incorrect; the reason for theerrors was unknown. Thirteen patients who were prescribed 60Co teletherapy
treatments received doses that were from 15 to 40% lower than the doses intended.
Initiating event— Incorrect data for patient dose calculations: Basic data (tables of treatment
times) were incorrect.
Contributing factors—Lack of or ineffective procedures, protocols and documentation: Incorrecttables were accepted for use.
—Insufficient safety provisions (defence in depth):• There was no independent verification of basic data (time charts used to
calculate treatment times).
• There was no independent check of treatment times calculated for individual
patients.
Remedial action— A new departmental policy required a check by an independent person of daily
treatment times for each patient. The check was performed on a weekly basis
or whenever a treatment time was changed.
Event No. 27: Misapplication of distance correction
An institution treated most patients with a constant source-skin distance (SSD)
technique, although some patients were treated with a constant source-axis distance(SAD) or isocentric technique. For an isocentric treatment plan, the dosimetrist
assumed that it was necessary to use a correction factor to convert from SSD to SAD.
The physicist checked the method. However, the computer program for isocentric25techniques had already allowed for this factor. This practice of manually correcting for
isocentric treatments continued for nine years, until the systematic error was discovered.
Over the nine year period, all patients treated isocentrically received incorrect
doses due to the double application of the inverse square correction. Although thesepatients were a small percentage of all patients, at least 1000 patients received doses
less than prescnbed. The magnitude of the error depended upon the depth of theisocentre, but for some patients the error may have been 30%.
Initiating event— Incorrect patient dose calculations: The SSD was entered manually into a
computer treatment planning program for isocentnc techniques, although theprogram allowed for that already.
Contributing factors— Insufficient education, training or expertise: The training should include theparticular piece of equipment TPS.
— Lack of or ineffective procedures, protocols and documentation: Incorrectcommissioning of the treatment planning computer, with poor understanding of
the computer algorithm.
— Insufficient safety provisions (defence in depth): Lack of independent checking
and verification procedures.
Event No. 28: Incorrect calculation using the inverse square law
A patient was prescribed a 60Co teletherapy treatment of 20 fractions of 2 Gyeach. The prescribed SSD was 70 cm; the SSD used for most patients was 80 cm. The
physicist who calculated the dose used an incorrect inverse square correction factor
that led to a given dose which was 70% too high. Consequently, the patient received
3.4 Gy per fraction instead of the intended 2 Gy for the first eight fractions. The
calculations were not checked until after the eighth fraction, when a junior physicist
discovered the mistake. The dose for the remaining fractions was reduced tocompensate for the excess dose delivered during the earlier treatments.
Initiating event— Incorrect data for patient dose calculations (an incorrect inverse squareconversion factor was used).
Contributing factors— Insufficient awareness: The treatment was unusual (the treating distance wasunusual, 70 cm rather than 80 cm SSD), without there being sufficientawareness of the difference.

26

— Insufficient safety provisions (defence in depth): Lack of an independent checkof the treatment plan.
— Lack of or ineffective procedures, protocols and documentation: Treatment
times were not checked until the patient received eight fractions of histreatment.
Remedial action—A procedure was introduced which required that calculations be checked before
the third fraction of treatments consisting of more than three fractions. For
treatments requiring three or fewer fractions, calculations were to be checkedby the first fraction.
Event No. 29: Incorrect calculation of treatment times for open
and wedged beams
One field was treated with a combination of open and wedged beams on a
Co unit. The treatment times were reversed when calculating the dose distribution for the total treatment, with the result that the patient received an overdoseof 10%.
60

Initiating event— Incorrect patient dose calculations (treatment times were reversed for open andwedged beams).
Contributing factor— Insufficient safety provisions (defence in depth): Lack of an independent checkof the treatment plan.
Event No. 30: Error in the wedge factor
During a review of past treatment data, 12 wrong treatments were discovered.
Some patients were found to have received doses 10-18% higher and some 10-27%
lower than the prescribed doses. The overdoses resulted from erroneous informationin the treatment planning computer reference data. All these treatments, with one
exception, involved the use of wedge filters.
The treatment discrepancies were first discovered when a therapy technologist
examined the files of previously treated patients in order to practice hand calculated
dosimetry for an upcoming board certification test. The radiation oncology staff thenbegan recalculation of doses to all patients who had received wedge related
treatments since the introduction of that type of treatment two years earlier. The staff
also re-ran the original computer calculations, which showed that administered doses

27had deviated significantly from prescnbed doses. In 11 of 12 wrong treatments theplanning staff failed to use a computer program 'wedge normalization factor' inmaking initial dose calculations. The use of this wedge normalization factor isdescribed in the manufacturer's computer instruction manual. Instead of using thisgenerated factor, the planning staff used different measured wedge factors.
Initiating event— An incorrect procedure was used for computerized treatment planning
involving wedges.
Contributing factors— Lack of or ineffective procedures, accepted protocols and documentation.
— Insufficient safety provisions (defence in depth):• Lack of an independent check (of dosimetry involving the use ofwedges).
• Computer calculations were also not manually checked.

Event No. 31: Wedge factors used twice in the calculation of treatment times
A physicist began employment at a new institution, which had the same TPS he
had used in his previous place of employment. At the new institution, the wedge
factors were included in the computer calculations; at the previous institution, thewedge factors were applied manually for each patient. The physicist used, in error, the
TPS in the same way as he had used it in his earlier position. In other words, heapplied the wedge factor at the new institution manually for each patient, whichmeant that this factor was used twice for the calculation of treatment times. For one
patient the result was a 53% overdose for a boost (wedged) field, resulting in a total
dose that was 17% higher than prescribed.
Initiating event— Incorrect patient dose calculations (a wedge factor was applied twice in thecalculation of the treatment time).
Contributing events— Insufficient education, training or expertise: Lack of understanding of the TPS
operation.
— Insufficient safety provisions (defence in depth):• Application without verification of a calculation adapted from anotherhospital.
• The computer calculations were not manually checked.

28

Event No. 32: Failure to include intended wedges in the treatment setup
A patient was prescribed a 60Co teletherapy treatment of 16.6 Gy. The treatment required two fields using 30° wedges. The physicist failed to record the wedges
in the setup portion of the patient's chart and the wedges were not used during
treatment. When the error was detected, the patient had received a dose of 23.0 Gy
to the reference point on the central axes instead of the prescribed 16.6 Gy, an
overdose of almost 40%. In addition to the overdose at the reference point, the dose
distribution was non-uniform, with the hottest region receiving a much larger
overdose.
Initiating event— Incorrect patient dose calculations: The physicist failed to specify wedges in the
setup instructions.
Contributing factors— Insufficient safety provisions (defence in depth):• There was no independent verification of the treatment parameters.
• There was no inspection of the resulting isodose distribution.

Event No. 33: Misunderstanding of a complex treatment plan given verbally
A patient was prescribed a 60Co teletherapy treatment to two different treatmentsites. The two sites were to be treated daily, with the first site receiving 2.4 Gy each
day for 20 days and the second site receiving 2.5 Gy each day for 10 days. The twotechnologists involved in this procedure misunderstood the physician's verbal
instruction concerning the treatment plan and failed to recognize the different numberof treatment fractions for the two sites. Therefore, the second site received an
additional four days of treatment before the error was detected. The patient received
10 Gy more than was prescribed, an overdose of 40%.
Initiating event— Incorrect treatment setup.
Contributing factors— Lack of or ineffective procedures, protocols and documentation:• There were no written procedures for treatment prescription.
• Unclear verbal prescription.
— Unusually complex treatment involving two sites that were to receive different
doses in different fractions.

29Remedial action— A new protocol required that a 'stop' mark be placed in each patient's
daily treatment log, indicating the projected final day of treatment for eachsite.
Event No. 34: Incorrect identification of patient
The radiation technologist called the patient's name but did not confirm his
identity by looking at his photograph in the record. The radiation technologist had notseen the patient and, therefore, did not realize that another patient had responded.
When he was being positioned for therapy, freckles on his back were mistaken fortreatment positioning tattoos and were used for the setup. The radiation technologist
did not recognize the absence of treatment tattoos in the patient's lumbar/sacral spine
area despite a photograph. The patient indicated that his setup was not correct and theradiation technologist called the oncology physician to venfy that the treatment wascorrect. The physician verified that the treatment was correct on the chart but did not
speak to or examine the patient. The patient was then administered 2.5 Gy from ateletherapy unit to the lumbar/sacral spine, i.e. the wrong site.
Initiating event— Incorrect treatment setup: A patient responded when another patient's namewas called.
Contributing factors— Non-compliance with procedures:• For patient identification: There was no check of the patient's photograph or
identity bracelet, if there was one.
• For site identification: There was no confirmation of anatomical marks forbeam location.
— Lack of awareness: No one heeded the patient's warning and objection to being
treated at the wrong site.
Event No. 35: Treatment setup based on another patient's chart
A patient was prescnbed a 60Co teletherapy dose to the brain but received anunintended dose of 0.45 Gy to the lung. For instructions regarding setup, thetechnologist picked up the chart of another patient, who was prescnbed a lungtreatment. The technologist positioned the patient without confirming his identity
against the chart and proceeded with the lung treatment. During the treatment, asecond technologist noted the discrepancy between the patient's name and the chartand terminated the treatment.

30

Initiating event— Incorrect treatment setup: The technologist used another patient's chart to set
up the treatment.
Contributing factors— Inattention or lack of awareness.
— Non-compliance with procedures for patient identification.
Event No. 36: Calculation error after a change of treatment regimen
A 60Co teletherapy treatment was prescribed to the larynx, a total of 76.8 Gy
in 64 fractions of 1.2 Gy each, two fractions per day. After 18 treatments, theoncologist revised the treatment plan. He reduced the field size and moved theradiation field off the spinal cord. The technologist made a mistake in thecalculation of the new treatment time and the patient received twice the prescribed
dose for the next 22 fractions. Instead of the intended 26.4 Gy for the second partof the treatment, the patient received a dose of 52.8 Gy. The physicist reviewed
the treatment plan and patient chart on a weekly basis, but failed to notice themistake until the third review. The treatment was immediately suspended. By then,
40 fractions had been administered. It was estimated that the patient received a total
dose of about 75 Gy in 40 fractions rather than the prescribed dose of 76.8 Gy in
64 fractions.
Initiating event— Incorrect patient dose calculation: The technologist miscalculated the treatment
time for the revised treatment plan.
Contributing factors— Insufficient safety provisions (defence in depth): Lack of an independent checkof the treatment plan (treatment times).
— Procedures followed mechanically without sufficient awareness: Ineffective
weekly checking of the patient's chart.
Event No. 37: Confusion of fractional dose and total dose
A child was prescribed 60Co teletherapy treatment to the brain. The total
prescribed dose was 3 Gy, to be delivered in two fractions of 1.5 Gy each. The
dosimetrist calculated the dose for 3 Gy per treatment. Three other individuals
reviewed the calculations before the treatment started and failed to notice the error.
As a result, the patient received a total dose of 6 Gy. The error was discovered by astudent therapy technologist who reviewed the treatment plan.

31

Initiating event— Incorrect patient dose calculation: The dosimetrist misunderstood the total dose
for the fractional dose in a two fraction treatment.
Contributing factors— Ineffective communication/transfer of essential information.
— Procedures followed mechanically without sufficient awareness: Three persons
failed to detect the error.
— The nature of the treatment combined with insufficient awareness favoured themistake.
• The treatment was unusual (fractionation).
• The treatment was very short (two sessions), which increased the risk of notdetecting the error before treatment was completed.
Event No. 38: Incorrect positioning of treatment beams
A patient was prescribed 60Co teletherapy to both the right hip and thethoracolumbar spine, to be completed on different days. After the patient completed
the thoracolumbar spine treatment, the technologist mistakenly continued treatment
to the same region for a total of 12 Gy. The staff technologist involved in thistreatment failed to notice the statement in the patient's chart that the treatment to thespinal area had been completed. Six more treatments were given. The mistake was notdetected during the routine chart review by a physicist.
Initiating event— Incorrect treatment setup: The technologist did not check the patient's chart
carefully before setting up a field and administering radiation.
Contributing factor— Inattention or insufficient awareness.
— Insufficient safety provisions (defence in depth): The patient setup was notchecked by another person (the technologist worked alone).
Event No. 39: Wrong tattoo used in treatment setup
A patient was prescribed a 60Co teletherapy treatment to the lumbar spine but
received an unintended dose of 2.5 Gy to the thoracic spine. The patient had
previously received teletherapy treatments over the thoracic spine, and the tattoo(anatomical marker) was still visible. The technologist setting up the treatment failed
to review the patient's chart, which indicated the proper treatment, and used the tattoo
from the previous treatment.

32

Initiating event— Incorrect treatment setup: The technologist used a tattoo from a previous
treatment to set up a new series of treatments.
Contributing factors— Ineffective procedures: The patient had two tattoos, one of which was from aprevious treatment that was near the site of the tattoo for the current treatment— Inattention or insufficient awareness: The technologist did not carefully review
the patient's chart for the location of the treatment site.
Remedial action— After this incident, technologists were instructed to have a physician ordosimetrist present during patient setup.
Event No. 40: Treatment of the wrong patient and lack of communication
because of language problems
A patient was scheduled to receive treatment to the head and neck area using alinear accelerator. The patient spoke little English and the oncologist did not speak thepatient's language. The oncologist asked the patient which area of the body was being
treated and the patient pointed to his head. Without reviewing the patient's chart, thephysician assumed that he was scheduled for treatment with a 90Sr plaque andadministered a dose of 10 Gy to the surface of the right eye.
Initiating event— Incorrect treatment setup: Incorrect patient identification.
Contributing factors— Non-compliance with procedures for patient identification: The oncologist did
not check the patient's chart.
— Inattention or insufficient awareness: The radiation oncologist relied on thepatient to locate the treatment site.
— Ineffective communication/transfer of essential information: The patient andthe oncologist did not speak the same language.
Event No. 41: Incorrect labelling of a simulator film
A patient was to receive a 60Co treatment of 2.16 Gy to the left posterior neck.
However, the treatment was delivered to the right posterior neck. Because thetreatment simulation was performed in the prone position rather than in the routine
supine position, the right side of the simulator film was marked as the left and the33patient was set up incorrectly. In reviewing the simulator film, the oncologist failed
to notice that the treatment simulation was to the wrong side of the patient's neck.
Initiating event— Incorrect simulation/localization: Incorrect labelling of the localization film.
Contributing factors— The treatment was simulated in an unusual position.
— Insufficient safety provisions (defence in depth): There was no check of theorientation of the anatomical site relative to the film.
Event No. 42: Field improperly located on patient
A patient was prescribed a treatment of 3 Gy to the cervical spine. The
technologist looked at the patient's chart but still mistakenly set up the treatment fieldon the thoracic spine. The patient received 2.87 Gy to the thoracic spine instead of theprescribed 3 Gy to the cervical spine The patient had previously received treatment
to the thoracic spine, leading the technologist to assume that the thoracic treatment
was continuing.
Initiating event— Incorrect treatment setup: Failure to adequately examine the patient's chart
before administering treatment.
Contributing factor— Insufficient safety provisions (defence in depth): There was no independent
check of the chart and the setup.
Event No. 43: Field improperly located on patient
A patient was prescribed 60Co teletherapy of 2 Gy per fraction to themediastinum. The technologist marked the beam setup incorrectly as a brain treatment
and the patient received ten fractions to the brain before the error was detected,
even though the technologist was to have been supervised by qualified staff members.
Initiating event— A technologist set up a new patient and marked the wrong beam position.
Contributing factors— Ineffective communication/transfer of essential information: The prescriptionof the oncologist was misread, by the technologist, ten different times.

34

— Inattention or insufficient awareness: Checks by three staff members failed todetect the setup mistake.
Event No. 44: Incorrect positioning of patient
A patient was to receive 60Co treatments of 10 fractions, 3 Gy each, to the rightscapula. After the second fraction, the oncologist reviewed the port film and noticed
that 80% of the intended target area had been missed. A chart review indicated thatduring simulation of the treatment, the oncologist had placed a mark on the patient's
chest as indicated by the ceiling laser. However, for treatment the back pointer on theteletherapy unit was used to position the beam. Since the back pointer and ceilinglaser locate the radiation beam at different angles, the tissue volume treated was
medial to the intended site.
Initiating event— Incorrect treatment setup.
Contributing factor— Ineffective communication/transfer of essential information:• Poor communication between the radiation oncologist and radiation
technologist during simulation.
• Procedures were misunderstood: Procedures for simulation were
unsuccessful in ensuring that the technologist understood the treatment planof the oncologist.
Remedial action— The oncologist changed the original prescription to include two additional
treatment fractions in order to bring the total treatment dose to 30 Gy.
Event No. 45: Improper positioning of the patient for treatment
A patient with metastatic lung disease received a bone scan in the prone position(face down). A metastatic lesion was found in the left hip, for which the patient was toreceive 27 Gy. For radiation therapy, the patient was positioned in the supine position(face up). The orientation of the bone scan was misinterpreted and the patient was
treated on the right hip rather than the left hip. The treatment continued for two weeks,
until a resident oncologist discovered the error while reviewing the patient's chart.
Initiating event— A nuclear medicine bone scan was misinterpreted with regard to laterality of a
hip lesion.

35

Contributing factors— Ineffective procedures, protocols and documentation:• Different imaging conventions (for X ray and nuclear medicine images).
• The radiotherapy staff was not familiar with imaging conventions in the
nuclear medicine department and did not review the report of the bone scan.
• The prescnption for radiotherapy did not indicate clearly which hip was tobe treated.
Event No. 46: Machine incorrectly set for rotation rather thanfor a stationary fieldA patient was prescribed a stationary field treatment of 40 Gy to the pelvis, in
20 fractions of 2 Gy each For the 14th fraction, the technologist, who was working
alone, performed the setup but did not notice that the machine was still set for therotational technique used for the previous patient. She also failed to visually check theoperation of the machine after the beam-on switch was activated. Therefore, thepatient received 0 8 to 1.0 Gy and 0.6 to 0.7 Gy to the left and right sides of the pelvis,
respectively, both of which were outside the region of the intended treatment.
Initiating event— Incorrect treatment setup (machine settings).
Contributing factors— Design deficiency. The machine design allowed the use of parameters from theprevious treatment.
— Insufficient safety provisions (defence in depth): There was no independent
check of the machine settings.
— Ineffective procedures or non-compliance with procedures: The technologist
did not observe the patient during irradiation.

2.7. DECOMMISSIONING OF TELETHERAPY EQUIPMENT
Event No. 47: Abandonment of a 137Cs machine
A private radiotherapy institute moved to new premises, leaving behind a 137Cs
teletherapy unit without notifying the licensing authority. Because of partial
demolition of the building, the teletherapy unit became totally unsecured. Two people
entered the building and removed the source assembly from the radiation head. They
tried to dismantle the source assembly at home and in the attempt the source capsule
was ruptured. Because the radioactive source was in the form of caesium chloride,36

which is highly soluble and easily dispensed, there was considerable contaminationof the environment, resulting in external irradiation and internal contamination ofseveral persons.
After the source capsule was ruptured, remnants of the source assembly weresold for scrap to a junkyard owner. He noticed that the source material glowed bluein the dark. Several persons were fascinated by this and over a period of days
friends and relatives came to witness the phenomenon. Fragments of the source thesize of rice grains were distributed to several families. Five days later, a number of
persons showed gastrointestinal symptoms that were not recognized initially as
being due to exposure to radiation. However, one of the persons irradiatedconnected the illnesses with the source capsule and took the remnants to the publichealth department. This action started a chain of events that led to investigation of
the accident.
Many individuals received external and internal radiation exposure exceeding
acceptable limits. In total, some 112 000 persons were monitored; 249 persons were
contaminated either internally or externally. Some suffered very high internal andexternal contamination owing to the way they had handled the caesium chloride
powder, such as daubing their skin, eating with contaminated hands, and handlingvarious objects. Four persons died within four weeks of admission to hospital, having
received total body doses of at least 5 Gy.
Initial radiation surveys were conducted on foot over the contaminated areas.
Seven main sites of contamination were identified, including the junkyards
concerned, some of them with dose rates of up to 2 Sv-h~I(~200 R/h) at 1 m. Aerial
surveys were conducted over 67 km2. Of 159 houses monitored, 42 required
decontamination. The final volume of waste stored was 3500 m3 or 275 lorryloads. The radioactivity accounted for in the decontamination was estimated at
44.4 TBq, compared with the known activity of the source before the accident of
50.9 TBq.
Initiating event— The abandoned machine was stolen and dismantled; the source capsule wasbroken.
Contributing factors— Lack of compliance with regulations:• There was no safe, formal and complete decommissioning of the facility.
• The 137Cs teletherapy unit, no longer in use, was abandoned andunsecured.
— The source was not recognized by the general public as being dangerous.
— The chemical form of the source facilitated the spread of radioactive
contamination.

37

Event No. 48: Illegal import, storage and disposal of a teletherapy unit
A teletherapy unit with a 37 TBq 60Co source was purchased and imported
without complying with all the existing import requirements. It was stored in awarehouse for six years, when its scrap value attracted the attention of a maintenancetechnician, who dismantled the head of the unit and removed the cylinder containing
the 60Co.
The technician removed the cylinder and other metal parts from the unit, loaded
them into a pickup truck, drove to a junkyard and sold the parts as scrap. Before
arriving at the junkyard, he deliberately ruptured the cylinder containing the source.
The pickup truck thus contained a large quantity of radioactive material from thesource.
The source consisted of about 6000 tiny pellets (1 mm x 1 mm in diameter) of
60
Co, a metal with magnetic properties and low mechanical resistance. When thecylinder ruptured, several pellets were dispersed and remained in the truck when theheavy parts were unloaded at the junkyard.
When the ruptured cylinder was moved by cranes, together with the other metal
pieces, the 60Co pellets were spread over the junkyard, attracted by the crane's
magnetic field, and mixed with the other metal materials. Consequently, pellets andpellet fragments were also transferred to the vehicles used for transporting scrap tovarious foundries. The main purchaser of the scrap was a firm that manufactured
construction reinforcing rods and connecting rods for motor vehicles. Owing tomechanical defects, the truck that was contaminated with 60Co pellets remainedparked on the street for 40 days. The truck was then moved to another street where it
stood for a further ten days.
It was later found that scrap contaminated with 60Co had been used by steel
production plants to manufacture reinforcing rods and metal table bases. A lorrytransporting contaminated rods passed near a nuclear laboratory where radiation
detectors were used to monitor removal of radioactive material. The detectors not
only indicated the presence of radioactivity, but also activated a camera thatphotographed the contaminated vehicle. Two days later the authorities ascertained theorigin of the contaminated rods.
An extensive investigation showed that 30 000 table bases and 6600 t ofreinforcing rods had been made from the contaminated material. Aerial surveys of anarea of 470 km2 were conducted, resulting in the recovery of 27 cobalt pellets. Visitswere made to 17 636 buildings to determine whether contaminated material was usedin their construction Acceptable radiation levels had been exceeded in 814 buildings,
that were then partly or completely demolished.
The accident exposed approximately 4000 people to radiation, about 80 ofwhom received doses greater than 250 mSv. Apparently, five people received doses of
3-7 Sv over a two month period.

38

Initiating event— A person dismantled an insecurely stored head of a 60Co teletherapy unit andbroke the source capsule.
Contributing factors— Non-compliance with regulations:• A radiotherapy unit was illegally imported and transported.
• The unit remained in storage for six years under unsafe and unsecured
conditions.
— A technician did not recognize the potentially dangerous situation.
— Radioactive parts of the unit were sold as scrap for industrial processing.

2.8. MECHANICAL AND ELECTRICAL MALFUNCTIONSEvent No. 49: Mechanical failure of a 60Co unit
A spider gear in the drive unit of a vertically mounted 60Co unit was not
fully engaged. As a result, the head of the unit fell on the patient, causing a fatal
injury.
Initiating event— Failure of equipment (fault in spider gear in the drive unit).

Event No. 50: Accidental fall of lead blocks
During treatment with a linear accelerator, a lead block weighing 1.4 kg fell
from a height of 0.5 m, striking the patient, after the therapist accidentally knocked it
off the accessory mount.
Initiating event— Inadequate setup of accessories (lead block accidentally knocked off themount).
Contributing factors— Inadequate securing of block.
—Positioning of lead block, which had the potential of falling and hitting patients
if it fell.

39

Event No. 51: Electrical failure of a simulator
Failure of a diode on the limit switch allowed the image intensifier on asimulator to over-travel. The power cables to the intensifier were subsequently
pinched by the unit covers, causing an electrical shock to the patient.
Initiating event— Equipment failure.
Contributing factor— Radiotherapy machines involve other risks (electrical, mechanical, chemical) to patients and staff in addition to those arising from radiation.
These should always be recognized and appropriate inspection should beensured.
Event No. 52: Mechanical failure of cassette holder assembly
When the gantry on a simulator was being rotated, the cassette holder assembly,
weighing approximately 9 kg, detached and fell to the floor, glancing off the head of
the patient. The accident was caused by two sheared cassette holder screws whichwere to have been replaced by the manufacturer.
Initiating event:— Inadequate setup of accessories.
Contributing factor— Delayed maintenance of the sheared cassette holder screws.

2.9. BRACHYTHERAPY: LOW DOSE RATE SOURCES
AND APPLICATORS
Event No. 53: Incorrect dose calculations due to the use of incorrectsource strength units
To calculate brachytherapy dose distributions, a physicist used a treatment
planning computer that required input of the source strengths in terms of exposure
rate at a distance in special units of R-cm"2^"1 He specified the source strength in SI
units, reference air kerma rate in (j.Gy-m~2-h~'. Therefore, five patients received doses
14% higher than prescribed.

40

Initiating event— Incorrect patient dose calculations: The physicist specified the source activities
in units which were inconsistent with the manufacturer's calibration.
Contributing factors— Lack of harmonization of source strength units: Different units were used by
the manufacturer of the TPS and by the physicist.
— Insufficient training/understanding of TPS software.
— Insufficient safety provisions (defence in depth): There was no independent
check of the computer calculations either by another person or by the use of amanual method.
Event No. 54: Inconsistent units of source activity
A patient received a brachytherapy implant of the prostate gland using I92Ir
seeds encased in nylon ribbon. The prescribed dose to the prostate was 32.6 Gy. Twomonths later, while reviewing the shipping documents associated with the implant,
the dosimetrist noted a discrepancy in the units of activity between what she hadordered and what she had received. The hospital had ordered ribbons containing
0.79 mCi per seed. This is equivalent to 29 MBq. However, the vendor had delivered
brachytherapy ribbons containing 0.79 milligram radium-equivalent (mg Ra-eq)1 per
ribbon. This is equivalent to 60 MBq. When the shipment was received, thedosimetrist had checked the prescription order against the activity received and notedthat the numbers (0.79) matched; she failed to notice that the units differed.
Therefore, the dose to the prostate was 56.7 Gy rather than the prescribed 32.6 Gy, an
overdose of 74%.
Initiating event—Incorrect data provided by the vendor for dose calculation.
Contributing factors— Lack of harmonization of source strength units: Different units were used by
the manufacturer of the sources and by the physicist.
— Insufficient safety provisions (defence in depth):• The dosimetrist did not verify consistency between the activity of thesources ordered and those received.
• The strength of the source was not measured at the hospital.

1 mg Ra-eq = 2.06 mCi.

41

Event No. 55: Sources of incorrect activity
A patient was prescribed a brachytherapy dose of 28.8 Gy to the prostate, using
a transpenneal interstitial implant containing 70 192Ir seeds encased in ten nylon
ribbons. The hospital ordered 192Ir brachytherapy ribbons containing seven seeds per
ribbon with an activity of 0.79 mCi (29 MBq) per seed. However, the vendor
delivered ribbons containing seven seeds each but with an activity of 0.79 mg Ra-eq(50 MBq) per seed. Ten catheters containing the sources were implanted in thedesired treatment site.
During the course of the treatment two catheters became displaced and were
removed by the physician. As the treatment progressed the patient became confused
and removed four additional catheters, which were recovered by an attending nurse.
The treatment was discontinued prior to the planned time of removal. The error in thestrength of the sources was discovered during a recalculation procedure after removalof the catheters. Because of the higher strength of the sources and the early removalof the catheters, the administered dose was estimated to be 35.2 Gy rather than
28 8 Gy, an overdose of 22%. The overdose was enhanced by the fact that theradiation was delivered in a time shorter than that prescribed. In addition, removal ofsources at different times altered the dose distribution.
Initiating event— Incorrect data provided by the vendor for dose calculation.
Contributing factors— Lack of harmonization of source strength units: Different units were used by
the manufacturer of the sources and by the hospital.
— Insufficient safety provisions (defence in depth):• The dosimetrist did not venfy the consistency between the activity of thesources ordered and those received (the number 0.79 was checked but not
the unit of activity).
• The strength of the source was not measured at the hospital.

Event No. 56: Implant of the wrong 137Cs source
A patient was prescribed brachytherapy of the cervix, using two 137Cs sources.
The prescnption required source strengths of 20 and 25 mg Ra-eq for 26 h. After thetreatment was completed and the sources had been returned to the storage facility,
it was discovered that a 5 mg Ra-eq source had been used instead of the prescribed
25 mg Ra-eq source. Therefore, the patient received about 56% of the intended
dose.

42

It was reported that the 137Cs sources were appropriately colour coded and thatthe storage drawers in the safe were clearly labelled with the strength as well as thecolour code of the sources. Nevertheless, one drawer contained sources of two
different strengths. It was concluded that the mix-up was due to human error, leading
to the selection of a source of the wrong strength.
Initiating event— Use of the wrong source.
Contributing factors— Lack of or ineffective procedures, protocols and documentation:• The labelling of the sources was ineffective.
• A storage drawer contained sources of different activities.
• There was no verification of source activity prior to treatment.
Event No. 57: Use of the wrong 137Cs sources
A patient was prescribed a gynaecological brachytherapy treatment of 27 Gy
using five 137Cs sources with a total activity of 4322 MBq. The individual loading
the applicators selected two sources with incorrect activity owing to faded colour
coding on the sources. The total activity used was 5937 MBq. Consequently, thepatient received an overdose. The error in the selection of sources was not detectedby the individual responsible for performing an independent verification of source
activity.
Initiating event— Use of the wrong source.
Contributing factors—Lack of or ineffective procedures, protocols and documentation: The colour
coding for source identification was faded.
— Insufficient awareness: The sources were verified prior to the implant, but theerror was not detected.
Event No. 58: Treatment time based on the wrong isotope
A patient was prescribed a brachytherapy treatment of 32.5 Gy using a 137Cs
low dose rate afterloading unit. The physicist calculated the treatment time using thecharacteristics of 192Ir rather than data for 137Cs. The patient received a dose of 22 Gy,an underdose of about 30%. The error resulted because the console button which
activated calculation of the treatment plan was selected by mistake.

43

Initiating event— Incorrect patient dose calculation (calculation of the treatment time was basedon 192Ir rather than 137Cs).
Contributing factor— Insufficient safety provisions (defence in depth): There were no independent
checks of the treatment plan and calculation of treatment time.
Remedial action— The hospital modified the isotope selection system to require more than one
step when selecting an isotope and instituted independent verification of alltreatment plans and calculations.
Event No. 59: Treatment planning based on the wrong isotope
A patient was treated with combined interstitial brachytherapy andhyperthermia. Both 125I and 192Ir seeds were discussed as potential brachytherapy
sources with respect to cost and availability. The decision was taken to use 125I.
However, for the final dose calculations, the dosimetrist used 192Ir instead of 125I.
Therefore, the patient received 5 Gy instead of the prescribed 25 Gy, or 20% of theintended dose. The error was discovered during a review of the patient's record at aroutine brachytherapy conference.
Initiating event— Incorrect patient dose calculation: The dosimetrist based the treatment plan on
the wrong radionuclide.
Contributing factors— Ineffective communication/transfer of essential information: There was amisunderstanding between the radiation oncologist and the dosimetrist.
— Insufficient safety provisions (defence in depth): There was no effective
independent check of the treatment plan.
Event No. 60: Error in the calculation of dose from an eye plaqueA patient was prescribed brachytherapy for ocular melanoma by means of acustom made eye plaque containing 125I seeds. The prescription was for 300 Gy to beadministered to the base and 143 Gy to the apex of the tumour.
The physicist designed the eye plaque and calculated the dose to the prescribed
points. However, before implantation he modified the plaque to have a different
radius of curvature. He changed the co-ordinates for placement of the 125I seeds in the44

plaque but failed to change the associated points for calculation of the dose to thepoints of interest.
Four days after application of the eye plaque, while he was planning the treatmentof another patient, the physicist suspected that he had made a mistake in the earlier case.
He retrieved the relevant data from the computer, reviewed the calculations, andconfirmed that an error had been made. The patient's eye plaque was then removed.
As a result, the patient received a dose of about 590 Gy (an overdose of 97%) to
the base of the tumour and 195 Gy (an overdose of 36%) to the apex of the tumour.
Initiating event— Incorrect patient dose calculation: The physicist altered the eye plaque withoutchanging the dose calculations.
Contributing factors— Insufficient safety provisions (defence in depth): The dose calculation was notverified by a second person.
— Lack of or ineffective procedures, protocols and documentation: The procedure
for the calculation was not well defined.
Event No. 61: Incorrect source strength
Intracavitary radium tubes that were 35 years old, were stored in the nuclear
medicine hot laboratory. No certificates of activity were available. The sources were
put back into clinical use, despite the fact that a note attached to the door of the safe
stated that they were not to be used for therapy. The sources were identified
incorrectly, with activities interchanged, so that the 10 mg source was assumed to be
15 mg and the 15 mg source was used as 10 mg. In addition, the filter was assumed
to be 0.5 mm Pt when it was actually 1 mm Pt. The interchange of sources could
result in either over- or under-dosage for an individual patient, as well as in anincorrect dose distribution. The error in regard to filter thickness resulted in a dose to
the patient that was approximately 7% lower than prescribed.
Initiating event— Incorrect source identification: The sources had been labelled with incorrect
activities.
Contributing factors— Non-compliance with procedures:• Sources removed from use were used clinically, contrary to instructions.
• Sources were used for which there were no manufacturer's certificates ofactivity.

45

— Insufficient safety provisions (defence in depth): Source activities were notverified by a measurement.
Event No. 62: Error in calculating the time of removal
A patient was prescribed a brachytherapy dose of 40 Gy, using a 137Cs implant
for a period of 50 h. However, an error was made in calculating the time of removal
and the sources were left for an additional 22 h. The resulting overdose wasapproximately 44%.
Initiating event— Incorrect patient dose calculation. There was a miscalculation of the time forthe removal of the implant.
Contributing factor— Insufficient safety provisions (defence in depth): There was no independent
check of the time of removal.
Event No. 63: Wrong treatment distance in dose calculation
A patient was prescribed endobronchial brachy therapy treatment using 192Ir
seeds to a dose of 30 Gy to the treatment area. The physicist performed calculations
for a treating distance of 1 cm instead of the prescribed distance of 1.5 cm.
The brachytherapy calculation was based on an old calculation form with analgorithm that was incorrect for this procedure. There was only a manual
calculation of dose to one point, without a full isodose distribution. The patientthus received a dose of 20.5 Gy at 1.5 cm, approximately 32% less than theprescribed dose.
Initiating event— Incorrect patient dose calculation: The physicist used a form and a calculation
method that were obsolete.
Contributing factors— Lack of or ineffective procedures, protocols and documentation:• Copies of the obsolete form had not been withdrawn and were still available
for clinical use• The documentation of the dosimetry calculations did not include the treating
distance.
— Insufficient safety provisions (defence in depth): There was no independent
check of the calculations.

46

Event No. 64: Treatment based on an obsolete treatment plan
The radiation oncologist prescribed a dose of 30 Gy from 137Cs sources bymeans of a brachytherapy afterloading device. Before initiating treatment, thedosimetrist revised the original treatment plan and the oncologist approved therevision. The physicist who loaded the 137Cs sources into the applicator used theoriginal instead of the revised treatment plan. As a result, the patient received 14.6 Gyrather than 30 Gy, or 49% of the prescribed dose.
Initiating event— Incorrect patient dose calculation: The patient's treatment was based on a planthat was later revised.
Contributing factor—Insufficient safety provisions (defence in depth): There was poor
communication between the oncologist, dosimetrist and physicist.

Event No. 65: Failure to implant all sources as planned
A patient was prescribed a brachytherapy treatment of 24 Gy using an implantthat combined 192Ir and 137Cs sources. The licensed radiation oncologist was notpresent during insertion of the sources. The oncology resident who performed theimplant inserted only the 192Ir sources and left the 137Cs sources in the transport
container. Therefore, the patient received a dose to the treated area of 10 Gy insteadof 24 Gy, or 42% of the prescribed dose.
Initiating event— Incomplete source setup: The oncology resident failed to carry out thetreatment as prescribed.
Contributing factors— Insufficient education, training or expertise: The oncology resident performing
the procedure was inadequately trained and was not given proper supervision.
— Lack of or ineffective procedures, protocols and documentation: There were noprocedures to account for the total activity in the implant or the activity
remaining in the carrier.
— Ineffective communication/transfer of essential information: The oncologist did
not ensure that the prescription was clearly understood.
— The implant was unusual in that it combined two different isotopes.

47

Event No. 66: Sources with incorrect activity
A review of source activity by an independent consulting physicist showed thatsome 137Cs sources which had been in clinical use for several years had a lower
measured strength than the stated strength used to calculate treatment times. The
nominal, stated and measured source strengths were as follows:

Source strength (mg Ra-eq)a
Nominal activity(mg Ra-eq)

Stated activity(mg Ra-eq)

Measured activity(mg Ra-eq)

Measured/stated(mg Ra-eq) (%)

5
10
10
10
20
25

4.8
9.1
9.1
9.5
176

3.4
8.4
8.6
9.4
164

22.7

21.1

71
92
95
99
93
93

a

1 mg Ra-eq = 76 MBq

Upon investigation, the hospital's physicist found that when the institution
switched from radium to 137Cs, a conversion factor was derived for use with a source
calibrator that was based on an incorrect assumption regarding the filtration of theradium sources. As a result of this and possibly other errors, sources were used that
had a measured activity lower than the stated activity, many by more than 5% andsome by as much as 29%. Furthermore, the activity of the nominal 10 mg sources
differed among themselves to such an extent that they should not have been usedinterchangeably.
Initiating event— Incorrect supplier data for patient dose calculations.
Contributing factors— Incorrect calibration: An incorrect factor was used in checking the source
strength in a calibrator.
— Insufficient safety provisions (defence in depth):• Sources grouped in a batch of the same nominal strength (10 mg) were notchecked for consistency.
• Sources were used clinically without correct assessment of theirstrengths.

48Remedial action— The stated strengths of sources used clinically were corrected. Sources of thesame nominal strength were batched and those that deviated more than 2%from the average were withdrawn from clinical use.
Event No. 67: Treatment based on incorrect source activity
Three new 137Cs sources were ordered, to contain a nominal strength of 20 mg
Ra-eq each. When they were received, the manufacturer's certificates stated that thestrength of each was 17.5 mg Ra-eq. After a period of clinical use, measurement of
the source activities showed that two sources contained 15.7 and 20.7 mg Ra-eq,
respectively. The sources had been used interchangeably, with treatment times basedon a nominal activity of 20 mg Ra-eq each. Therefore, one source resulted in an
overdose of 4% and the other caused an underdose of 22%.
Initiating event—Incorrect supplier data for patient dose calculations: Sources used clinically had large and variable differences between nominal and measured
activities.
Contributing factors— Lack of or ineffective procedures, protocols and documentation:• Sources were accepted from the manufacturer although their stated strengths
were 12.5% lower than the strength ordered.
• Sources were accepted from the manufacturer although their activities did
not meet the criterion for interchangeable use.
• Sources with a wide range of activities were not individually identified andlabelled.
• Source strengths used clinically were as ordered and not as stated on the
manufacturer's certificates.
Event No. 68: Clinical use of a source with essentially no activity
An external review of a large inventory of 137Cs sources showed agreement
within 1% between the measured and stated strengths, with the exception of one
source. A source with a stated strength of 20.8 nig Ra-eq was found to have a strengthof only 0.20 mg Ra-eq. The source strengths had not been checked upon receipt,
although the sources had been used clinically for three months. It was not possible todetermine for which patients the low strength source had been used. However,depending upon the total activity and geometry of an implant, this source would cause
a lower dose than prescribed and a distorted dose distribution.

49

Initiating event— Incorrect supplier data for patient dose calculations: One 137Cs source ofessentially no activity was used clinically.
Contributing factor— Lack of or ineffective procedures, protocols and documentation: There was noverification of source activity prior to accepting the source from the
manufacturer.
Remedial action— The defective source was returned to the manufacturer.
Event No. 69: Loss of a 137Cs brachytherapy source
When preparing for a brachytherapy procedure, the medical physicist noted that
there were only two 137Cs sources in the source storage drawer instead of the
expected three. The activity of the missing source was 1960 MBq. The radiation
safety officer and the physicist made a physical search and radiation survey with aGeiger-Miiller counter and a scintillation detector but did not locate the source. A
review of the brachytherapy source inventory records revealed that the source had notbeen returned to the storage safe after completion of a brachytherapy procedure twomonths earlier.
At this facility, all sealed sources are colour coded according to their nominal
activity; the 20 mg Ra-eq sources are colour coded m white. The licensee speculated
that the 20 mg source may have been mistaken for a cut piece of white nylon spacer
and, therefore, inadvertently placed in the trash can. Personnel monitonng devices
had not indicated unusual levels of radiation exposure, although one technologist who
had been involved in the earlier procedure did not wear a ring badge. It was notpossible to determine whether there was unusually high exposure to personnel. It was
concluded that the source probably had been disposed of in a local landfill and wasburied at a depth of approximately 8-13 m below the surface, and that no radiation
survey of the patient had been conducted following removal of the sources; thepossibility that a lost source remained in the patient could not be eliminated.
Initiating event— The staff did not return all brachytherapy sources to the source storage area
promptly after removal from the patient.
Contributing factors— Lack of or ineffective procedures, protocols and documentation: Theprocedures to account for the return of sources to the safe were inadequate.

50

— Insufficient safety provisions (defence in depth):• Monitoring of the patient and the treatment area after removal of theapplicator was not performed or was ineffective.
• No radiation survey of disposable waste material was performed aftersources were removed from a patient.
Event No. 70: Inaccurate implantation and loss of sources
A patient received a permanent implant of 125I seeds to the prostate. The
physician localized the sources by means of a transrectal ultrasound guided probe.
The prescribed dose was 120 Gy from 58 seeds, each with a nominal activity of
UMBq.
Shortly after the implant, radiographs and computerized axial tomographyscans revealed that two of the seeds had been excreted in the urine. In addition, 21 of
the 56 remaining seeds were located in tissue surrounding the prostate rather than theprostate gland. Calculations showed that the tumour received a dose of 50 Gy insteadof 120 Gy, 42% of the intended dose.
Initiating event— Incorrectly performed medical implant.
Contributing factors— Either the equipment used to implant the seeds did not work properly or theoperator was not sufficiently trained in its use.
— Insufficient safety provisions (defence in depth): Precautions for the recovery
of lost sources were insufficient.
Event No. 71: Loss of 192Ir sources
Seven 192Ir seeds, each of a strength of 266 MBq, were obtained for thetreatment of a patient with lung cancer. It was decided that only five of the seven
seeds would be needed to deliver the prescribed dose. Therefore, the nylon ribbon
containing the seven seeds was cut into two pieces. The two ribbons, one containing
two seeds and the other containing five seeds, were placed in a transport container andtaken to the patient's room. The five seed ribbon was implanted into the patient andremoved 10 h later. The two seed ribbon was left in the storage container in thepatient's room during the 10 h treatment period. At the time of source removal, theradiation oncologist counted the seeds removed from the patient and verified that they
matched the number of seeds implanted. After removal, a radiation survey of thepatient and the patient's room was conducted and showed no detectable radiation
above background.

51

Three weeks later, an inventory of seeds, carried out in preparation for theirreturn to the supplier, revealed that the ribbon containing the two seeds was missing.
A search revealed two seeds in a crack between the carpet and the wall in the patient's
room where the brachytherapy procedure had taken place. It is assumed that the seeds
were pushed into the crack when the room was vacuumed after the patient was
released. If so, the seeds remained in the room for 22 d before being recovered. The
room had remained empty for three weeks, after which time another patient wasadmitted to the room; this patient and his wife stayed in the room for the next 15 h,
at which time the sources were found and removed
Initiating event— Source not returned to the safe: two 192Ir seeds were left unsecured and were
subsequently lost.
Contributing factors— More sources were sent to the patient's room than were required for the
implant.
— An implant was performed in a patient's room rather than in an operating suiteor treatment room.
— Insufficient safety provisions (defence in depth):• Sources removed from the patient were accounted for, but not the total
number of sources sent to the room.
• There was no monitoring of the room after the patient completed
treatment and left the area.
Event No. 72: Use of a leaking 125I source
A 333 MBq source of 125I was used in an implant to the brain. After the
implant, it was discovered that the source was leaking and a review of records
indicated that it was one of 36 seeds that had been used in previous treatments. It waspossible that the source had been ruptured during removal from an earlier implant,
although the manufacturer had specified that the seeds could be reused after
temporary implants. The accident was discovered because personnel had becomeaware of the potential for such an event through a notice describing a similar event inanother hospital.
Initiating event— A leaking 125I source was re-used for therapy.
Contributing factors— Insufficient safety provisions (defence in depth):52• The leakage of the source was not detected before implantation.
• Available information on a similar incident that had occurred elsewhere had
not triggered a checking procedure, although personnel were alert to thepossibility after the implant.
Event No. 73: Physical sizes of sources and applicators incompatible
A patient was prescribed a treatment with 137Cs tubes. The technologist
mistakenly used sources that were physically too small to fit the applicator. In fact,
because of their size, the sources had been withdrawn from clinical use. As a result,the sources slipped out of the applicator and irradiated normal tissue. The error was
discovered at the midpoint of the treatment, at which time the incorrect sources were
replaced by proper sources and the treatment continued. The dose to normal tissue
was estimated at 4-5 Gy.
Investigation showed that the technologist who loaded the applicator had neverperformed the procedure before and the supervising technologist had not loaded an
applicator for eight years.
Initiating event— Incorrect treatment setup: Sources were used that were not suitable for theapplicator and, therefore, could not be properly secured.
Contributing factors— Insufficient education, training or expertise: The personnel handling sources
and applicators lacked proper training.
— Lack of or ineffective procedures, protocols and documentation: Sources
withdrawn from clinical use were not sent elsewhere for disposal nor properly
labelled and stored.
Remedial action—The hospital arranged for safe, remote storage of sources no longer in use,posted a map of the source storage vault indicating the type of source at eachlocation and enhanced source accountability practices.
Event No. 74: Sources became dislodged before the calculated
time of removal
A patient received an endobronchial implant with 192Ir seeds. During the night,
a ribbon containing 25 seeds became dislodged, although the endotracheal catheter
remained in place. At about midnight, a nurse noticed the dislodged ribbon but took
no action. At about 02:00 on the same night, the same nurse returned and taped the53

ribbon to the side of the patient's face. At approximately 04:15, another nurse called
the radiation safety officer, who removed the ribbon using a remote handling tool.
Dose estimates showed that the side of the patient's face received
approximately 10 Gy, with 2.8 Gy to the eyes. The scalp received 3.5 Gy, since thepatient at one point folded the ribbon into her hair The nurse who handled the ribbon
received an estimated 0.2 Gy to her fingers. The tumour received a much lower dosethan prescnbed, although no quantitative estimate was possible.
A few days later the patient was readmitted to the hospital complaining ofburning eyes and visual sensitivity to light. An ophthalmologist diagnosed her
condition as keratoconjunctivitis, probably radiation induced.
Initiating event— Sources dislodged from the catheter in the patient.
Contributing factors— Insufficient education, training or expertise: Owing to the lack of training fornurses caring for brachytherapy patients, a nurse taped the sources to thepatient's face.
— Ineffective communication/transfer of essential information: Neither the patient
nor the nurse understood the nature of the treatment; they did not recognize thatthe seeds were radioactive.
— The event occurred at night, when there was less bedside nursing care; theribbon taped to the face might otherwise have been noticed earlier.
Event No. 75: Improper handling of dislodged sources
A patient was implanted with a total of 1776 MBq of 192Ir in two nylon ribbons.
The ribbons were inserted into two catheters that extended from the patient's
abdomen into the common bile duct. The procedure was scheduled to last 20-23 hand deliver 15-20 Gy to a colon tumour obstructing the common bile duct.
After the 192Ir ribbons were inserted into the catheters, the implant site wasdressed and the nursing personnel were instructed not to change the dressing. Nursing
staff on the next shift were not aware of these instructions since they were not written
on the patient's chart. Therefore, owing to excessive drainage of bile at the implantsite, the dressing was changed and reinforced with additional absorbent several times
during the evening and early the next morning.
At about 04:00, the nurse on duty noted that the dressing was completely
displaced and replaced the dressing. The nurse noticed the two nbbons of iridium
seeds but, not knowing what they were, coiled the ribbons in her hand and taped themto the patient's abdomen. A routine radiograph of the patient late next morning
showed that the seeds were no longer in the catheters. The ribbons were removed54

from the surface of the patient's abdomen by a physician at approximately noon on
the same day.
It was estimated that the patient received approximately 11.5 Gy to the target
tumour site, between 1.7 and 10 Gy to the skin of the abdomen, 20 Gy to the liver andsmall bowel, 0.1 Gy to the kidney, 0.5 Gy to the colon and 0.03 Gy to the testes. The
nurse who coiled the ribbons and taped them to the patient's abdomen received
approximately 0.08 Gy to her hands.
Initiating event— Inadequate instructions to brachytherapy nurses: Sources were displaced by thenurses from their implanted position.
Contributing factors— Lack of formal training in radiation safety for workers in the nursing unit.
— Ineffective communication/transfer of essential information:• Instructions to nursing staff were not in writing.
• Oral instructions to nurses on duty were not transmitted to the next shift.
• A suitable nurses' manual for implants was lacking.
• Neither the patient nor the nurse understood the nature of the treatment; they
did not recognize that the seeds were radioactive.
Event No. 76: Source dislodged before implant was terminated
For treatment of the uterine cervix, a patient was to receive a 42 h insertion of
applicators loaded with 137Cs sources. The tandem applicator contained three sources(15, 10 and 10 mg Ra-eq) and the two ovoids each contained a 15 mg Ra-eq source.
After the sources were inserted, the attending nurse discovered a 15 mg Ra-eq sourcein the patient's bed. The patient later developed an ulceration beneath her nght thigh.
Investigation showed that the source had apparently fallen out of the applicator as it
was inserted; the reason was not known. The patient became uncooperative andrefused further treatment.
Initiating event— Inadequate source application: A 137Cs source fell out of an applicator during
treatment.
Contributing factors— The sources were not well packed and secured in the patient during insertion of
the applicators.
— Training of nurses in charge of brachytherapy patients was deficient.
— Ineffective procedures.

55

— Routine nursing care of the patient failed to detect a loose source in her bed.
Remedial action— Members of the nursing staff were given refresher training in radiation safety.
Procedures were changed to require the presence of two individuals during
the insertion of brachytherapy sources. In addition, all tandem and ovoid
applicators were to be taped securely with tamper resistant tape.

Event No. 77: Source removed prematurely by patient
For cancer of the utenne cervix, a patient was treated with applicators
containing radium sources, which were to remain in place for a total of three days.
The radiation oncologist and physicist came to the ward to remove the applicators andfound them hanging on an intravenous stand next to the patient's bed. The patientreported that she had removed them sometime during the previous day because they
were uncomfortable. The nursing staff had not noticed that the applicators had been
removed. It was not possible to accurately estimate the underdosage, but the patient
probably received a dose close to 30% lower than prescribed.
Initiating event— The patient removed the applicators containing the radioactive sources.
Contributing factors— Inadequate training of nursing staff.
— Failure to communicate the nature of the treatment to the patient.
— Failure of nursing staff to notice that the applicators had been removed by thepatient.
Remedial action— Nursing staff received written instructions concerning their responsibility tocheck that applicators are in place in a patient.

Event No. 78: Misadministration due to patient interference
A patient with cancer of the uterine cervix was treated with an applicator
containing 137Cs sources. During a routine check, the nursing staff found that thepatient had removed the applicator, causing unintended exposure of her legs. The
licensee estimated that the treatment area received 17% less than the prescribed dose
and the patient's legs received some 6 Gy.

56

Initiating event— The patient removed the applicators containing the radioactive sources.
Contributing factors—The patient's removal of the applicators was not anticipated by the nursing staff.
— Ineffective communication: Failure to communicate the nature of the treatment
to the patient.

Event No. 79: Brachytherapy equipment failure
A patient was prescribed 137Cs brachytherapy treatment for uterine cancer, anda remote afterloading device was used for this purpose. During the procedure, amalfunction of the device disconnected the tube used to transfer the source from theshielded storage unit to the patient. The disconnection resulted in a 137Cs source being
located near the upper part of the patient's leg, rather than properly inserted into thepatient. Estimates of the dose to the leg were highly uncertain, ranging from 0.23 to
237 Gy, as the length of time of the exposure was unknown. Later examinations of
the patient revealed no physical signs of radiation damage, indicating that the
exposure was at the lower end of the range.
Initiating event— Equipment failure (disconnecting the source from the drive mechanism).
Contributing factors—Possibly poor design and construction.
— Inefficient procedures: Staff failed to check key parts, such as the tube
connector, prior to treatment.

Event No. 80: Mishandling of source ribbons
A patient was scheduled to have an endobronchial implant of two ribbons
containing 35 192Ir seeds with a total of 2516 MBq. One ribbon contained 20 seeds
and the other 15. During the insertion the physicist mistakenly gave the attending
physician the inactive end of the 15 seed ribbon (the portion that did not include
radioactive sources) and the physician inserted the inactive end into the patient. The
other ribbon, containing 20 seeds, was inserted correctly. The physician cut off theremaining lengths of the ribbons and gave them to the physicist. The physicist,
assuming that these pieces of ribbon contained no radioactive material, coiled themand held them in her hands. One of these pieces contained 15 seeds. After completion57

of the procedure, the physicist discarded the unused pieces of nbbon in a wastebasket
located in a waiting room across from the patient's room The dose rate in the waiting
room was approximately 0.63 mSv per hour, a radiation level well above theregulatory limit of 0.02 mSv in any one hour in unrestncted areas.
The implant was performed at 14:30 and the patient was scheduled to have atumour dose of 15 Gy. The physician decided to remove the ribbons from the patientearlier than planned because the dose rate was higher than he normally administered.
The ribbons were removed at 20.30. Neither the physicist nor the hospital's radiation
protection officer was present during the removal procedure.
The following morning at approximately 08:30, the medical physicist
inventoried the sources removed from the patient and found that one of the ribbons
contained no seeds. She immediately informed the radiation protection officer, who
conducted a search for the missing radioactive material At approximately 11 '00 in
the morning two pieces of nbbon were found in the wastebasket. It was determinedthat the dose to the medical physicist's hands was approximately 2.7 Gy, assumingthat she held the ribbon for about 5 mm. The physician stated that the patient received
approximately half the prescnbed dose
Initiating event— Incorrect treatment setup (the physicist failed to correctly identify the activeend of one ribbon).
Contributing factors— Lack of or ineffective procedures, protocols and documentation.
• There was no survey of all radioactive sources before implanting them into
the patient.
• There was no radiograph of the implant after insertion of the ribbons.
— Insufficient safety provisions (defence in depth):• Unused parts of nbbons or other material potentially containing radioactive sources were not checked for radioactivity before being discarded aswaste.
• Neither a radiation protection officer nor a physicist was present during theremoval of the sources from the patient.
• Accounting for radioactive materials was delayed until it was of little
value.
Event No. 81: Mechanical failure of brachytherapy equipment
Upon removal of an intracavitary applicator from a patient, it was noted that theweld of a pivot point had broken. This left a sharp, semicircular piece of stainless steel
in the patient, which had to be removed surgically58

Initiating event— Equipment failure: An applicator broke owing to mechanical failure of a pivot
point.
Contributing factor— Lack of or ineffective procedures, protocols and documentation: The applicator
was not properly inspected during cleaning and sterilization.

2.10. BRACHYTHERAPY (HIGH DOSE RATE)
Event No. 82: Delivery of tumour dose to the wrong site becauseof a defective catheter
A patient was prescribed a brachytherapy dose of 35 Gy to one lung using 192Ir
seeds. A kink in the catheter used to insert the seeds caused the seeds to be positioned
26 cm from the target site. The error was not discovered until the end of the treatment
time. Because of the error, the hypopharynx received 35 Gy and the prescribed target
site received only 0.1 Gy.
Initiating event— Inadequate treatment setup: A kink in the catheter prevented sources frommoving to the prescribed treatment site.
Contributing factor—Lack of or ineffective procedures, protocols and documentation:• There was no quality control of the catheter, such as physical inspection,
before its use.
• There was no radiographic verification of the location of the source, which
could have detected the error at the beginning of treatment.
Remedial action— For future implants, dummy seeds were used to test the proper movement ofsources through the catheter and radiographs were taken to ensure the correct
location of the active sources after insertion.
Event No. 83: Using a high dose rate prescription for the wrong patient
A patient was undergoing a series of five treatments for cancer of the nasal
septum, using a high dose rate 192Ir afterloading unit. The first four treatments werecompleted without incident. However, prior to the fifth treatment the operating

59

physicist picked up a patient's chart located next to the control panel without realizingthat it was the chart for another patient. As a result, he entered programme
information for the wrong patient. While the treatment was under way, a student
technologist inquired about the length of time to complete the treatment. The
prescribing physician and the operating physicist responded with different times. The
physician, realizing there was an error, terminated the treatment immediately. The
erroneous information had led to the 192Ir source being positioned so that the patient's
lips were exposed for about 1 min, for a dose of approximately 0.75 Gy. The planned
treatment would have delivered 0.25 Gy to the lips. (In this case, the overexposure of
normal tissue was not serious, however, this type of error has the potential for harmto the patient.)
Initiating event— Deficient patient identification: The physicist entered treatment information for
the wrong patient.
Contributing factors— Poor housekeeping' The treatment chart for the wrong patient was placed next
to the machine console.
— Insufficient safety provisions (defence in depth): There was no independent
check of the information used for the treatment.
Event No. 84: Malfunction of high dose rate equipment
A patient was to be treated with a high dose rate unit containing 159 GBq of
I92jr jne prescribed dose was 18 Gy in three treatments. Five catheters were placedin the tumour and the source was stepped through the preprogrammed positions ineach catheter. During the first treatment, the physician experienced difficulty placing
the source wire into the fifth catheter. Personnel disregarded the alarm from an area
radiation monitor because the unit console indicated 'safe'. The staff assumed that thearea radiation monitor was malfunctioning. In fact, the source wire had been broken
and the source had remained in the patient.
The patient, with the source still in the catheter, was transported back to thenursing home. The source remained in the patient for almost four days, at which time
the catheter containing the source fell out. Because of this misadministration, thepatient received 16 000 Gy at 1 cm from the source instead of the prescribed 18 Gy.
The nursing home staff placed the catheter in an area used to store non-radioactive
medical waste. The waste was removed later by an incinerator company. The source
was discovered when it tripped a radiation monitor located at the incinerator.
The patient died shortly after removal of the source; it was clear that radiation
exposure had been a major contributing cause of death. The loss of the source resulted

60

in radiation exposure to 94 individuals, including persons at the cancer clinic and thenursing home, ambulance staff, and workers at the waste disposal company.
Initiating event— Equipment failure: While it was inside a catheter in the patient, the source broke
free from the source wire.
Contributing factors— Staff ignored a monitor alarm and assumed that a 'safe' indication on the
console was correct (previous false alarms of the external monitor contributed
to this assumption).
— Insufficient safety provisions (defence in depth):• There was no spare or backup portable monitoring equipment to verify thealarm.
• The patient and work area were not monitored before and after treatment.

2.11. UNSEALED SOURCES
Event No. 85: Treatment of the wrong patient because two patients
had the same name
A therapeutic dose of 370 MBq of 131I was prescribed to a patient for treatmentof hyperthyroidism. The physician who was familiar with the patient was notavailable and asked another physician to administer the isotope. In arranging fortransportation, a porter noted that the patient was listed as being assigned to a bed that
she believed was occupied by another patient. The porter asked the nuclear medicine
secretary to check the discrepancy. The secretary referred to a list for the patient's
name, obtained the bed assignment area from a computer file and changed the request
form. The secretary did not know that there were two patients in the hospital with thesame first and last names, one for iodine administration and the other for treatment ofa lung disease. In addition, the secretary did not know that the computer program thatgenerated the patient list did not print (and in fact deleted) duplicate entries: the nameof the patient who was to undergo treatment for hyperthyroidism was thus not printed
on the list. The physician who administered the dose picked up the request form andthe 131I dosage and went to the nursing station on the floor of the patient with the lungproblem. The physician did not inform the nursing staff that he was about toadminister a therapeutic dose to one of their patients and went to the patient's room.
There, he asked the patient's name and verified the name on the wristband, but did
not cross-check the patient's identification number on the wristband with the number61

on the request form. The physician completed the request form and returned thepatient's folder to the nurse's station.
Within five minutes of the administration of 131I, the nurses discovered the error
and informed the physician and the radiation protection officer. As a remedy, a doseof 1000 mg of potassium iodide was administered immediately, followed by threesubsequent doses of 1000 mg each at four hour intervals. The estimated radiation doseto the patient's thyroid was between 1 2 and 1 4 Gy.
Initiating events— Incorrect patient identification.
Contributing factors— Two patients in the hospital had the same first and last names.
— Computer software, designed to avoid duplicate entries, deleted the name of
one patient— Administration of 131I treatments was not restricted to designated rooms orwards where the nursing staff had specialized training.
— Deficient procedures or non-compliance with procedures:• The physician did not communicate with the nursing staff at the ward, but
went directly to the patient's room to administer the iodine.
• Hospital protocol for the identification of patients was not followed withregard to verifying the patient's hospital number.

Event No. 86: 131I treatment of the wrong patient
A therapy dose of 333 MBq of 131I was given inadvertently to the wrong
patient (patient A instead of patient B). Patient A was to receive 740 MBq of"Tcm for a diagnostic bone scan; the 99Tcm was administered and the patient
was seated in the waiting room Patient B, who was scheduled to receive a 131I
hyperthyroidism treatment, arrived, completed an interview, signed a consent
form and sat in the waiting room pending the iodine treatment. The technologist prepared a dose of 333 MBq of 131I and called patient B; however,patient A responded. The technologist explained the 131I treatment, scheduled
a follow-up appointment, and administered the dose to patient A. The patientthen questioned the technologist, and it became evident that the wrong patient
had been treated. Patient A was immediately informed of the error and his
stomach was pumped, retrieving 118 MBq of the material. The patient wasgiven potassium perchlorate and Lugol's solution to release 131I trapped in the
thyroid and to block further uptake. Patient A's dose to the thyroid was estimatedat 8.2 Gy.

62

Initiating event— Incorrect patient identification.
Contributing factors— Ineffective communication: One patient responded to another patient'name.
—Non-compliance with procedures: Patient identification procedures were notfollowed.
Event No. 87: Wrong dose of 131I
A 60 year old woman was referred to the nuclear medicine department forthyroid ablation following a thyroidectomy for cancer. The physician prescribed6475 MBq of 131I to be administered orally.
The hospital received from the distributor the patient's prescribed amount of
131
I in one vial, together with a second vial containing 5180 MBq of 131I. A
technologist assayed both vials and placed them together in a fume hood located in
the nuclear pharmacy. Both vials were in their original lead shields with the contents
correctly labelled.
When the physician was ready to administer the 131I, the technologist who had
assayed the vials was not available and another technologist went to the pharmacy toobtain the radiopharmaceutical. The administering technologist picked up both vials
and, without reviewing the labels, assumed that both vials were required for theproper dose. The technologist did not consider the use of two vials for one
administration to be unusual since this was a common occurrence at this facility. After
reviewing the dosage record, the physician instructed the technologist to administerthe 131I. Without reviewing the labels on the containers, the physician assumed thatthe use of two vials was correct.
The mistake was discovered the next day when the nuclear pharmacist received
a request for 925 MBq of 131I and could not find the second vial. The resulting
investigation determined that the vial had been used the previous day.
Initiating event— Two unsealed sources were administered in place of one.
Contributing factors— Ineffective communication: Improper identification of radionuclides, startingwith receipt of the material, through the preparation of doses, to administration.
— Insufficient safety provisions (defence in depth): There was no independent
check of the material to be administered.

63

Event No. 88: Radiopharmacy error
A patient was referred for treatment of Graves' disease with 555 MBq of 13II.
The radiopharmacist assumed that the dosage to be delivered was 1073 MBq rather
than 555 MBq, since a 1073 MBq dose was routinely used for Graves' disease inthat hospital. Therefore, he requested a 1073 MBq dose from a commercial
radiopharmacy. The dose received was 1058 MBq, labelled as such. When theradiopharmacist logged the dosage into the computer after it had been measured by adose calibrator, he failed to take note of the dose of 550 MBq in the referring
physician's prescription. In addition, the physician who administered the isotope did
not check the prescription. As a result, the patient's thyroid received about 319 Gy
instead of the intended 167 Gy, an overdose of 91%.
Initiating event— The wrong source was administered: Incorrect activity of 131I.
Contributing factors— Insufficient awareness/alertness: The radiopharmacist assumed that theprescribed dose was the usual dose, without verification.
— Lack of or ineffective procedures, protocols and documentation:• The radiopharmacist failed to verify the dose received against thatprescribed.
• The nuclear medicine physician failed to verify the activity before headministered the isotope.

Event No. 89: Confusion regarding the activity of 131I
A patient was prescribed 370 MBq of 131I for a thyroid treatment. A capsulecontaining 370 MBq was ordered, but the distributor shipped a capsule containing
444 MBq. Personnel receiving the capsule did not note the discrepancy. Prior toadministration, the capsule was assayed in a dose calibrator. However, because thetechnician was expecting a reading of 370 MBq, he misread 444 MBq as 370 MBq.
The administration of 444 MBq resulted in an overdosage of 20%.
Initiating event— Incorrect source for patient treatment: The supplier shipped a capsule with anactivity higher than had been ordered.
Contributing factors— Lack of or ineffective procedures, protocols and documentation:64• Hospital staff did not check the activity stated by the supplier against theactivity ordered.
• The calibrator scale was misread.
— Insufficient safety provisions (defence in depth): Lack of independent
verification of the 131I dose against the prescription.

Event No. 90: Dose to an ineligible patient
A patient was administered 180 MBq of 131I for a whole body scan. The scan
indicated an unusually high breast uptake of 13!I and it was discovered that the patient
was a nursing mother. Before administering the dose, both the physician and nuclear
medicine technologist failed to confirm that the patient was not breastfeeding.
Consequently, the infant received an estimated 300 Gy to the thyroid and 0.17 Gy to
the whole body and will require thyroid hormone medication for life to ensure normal
growth and development.
Initiating event— A therapy dose of 131I was given to a nursing mother.
Contributing factors— Insufficient awareness: The technologist was distracted and forgot to ask thepatient a standard list of questions.
— There was a staff shortage on the day of administration.

Event No. 91: Radioactive spills from a patient during resuscitation efforts
A therapy dose of 7400 MBq of 131I was administered to an 87 year old patientin an effort to relieve oesophageal compression caused by metastatic thyroid
carcinoma. The patient had a gastrostomy tube and a Foley catheter in place.
Approximately 34 h after receiving the dose, the patient had a cardiopulmonaryarrest. Sixteen staff members attempted to resuscitate the patient; their efforts
included insertion of a pacemaker. Blood and urine contaminated with radioactivity
were spilled but the clothing of those present was not checked for activity. Althoughcontamination was extensive, subsequent thyroid bioassays showed no uptakes by the
involved staff. Monitoring of personnel showed that the highest reading was 0.3 mGyfor one of the nurses.
Initiating event— The patient experienced cardiac failure shortly after thyroid ablation therapy.

65

Contributing factors— There were no contingency procedures in place for emergency situations
involving patients with radioactivity.
— There had been no training exercises simulating emergencies.
— Shortage of instruments: Radiation monitoring instruments and decontamination utensils were not available.
Event No. 92: Administration of a wrong dose of 131I
A patient was to be administered 259 MBq of 131I The isotope was in the formof two capsules, 130 MBq each, which were labelled correctly. Previous doses of thislevel had been administered in the form of one capsule. When the vial was inverted
by the technologist, only one of the two capsules fell out and she assumed that thiswas the entire dose. Later, when disposing of the vial shield, the technologist
discovered the other capsule. As a result, the patient received only 50% of theprescribed dose.
Initiating event— One of two 131I capsules remained stuck in the vial.
Contributing factors— Deficiencies in procedures, or procedures were not followed• Staff failed to check the vial label to ascertain the number of vials and theprescribed dose.
• There was no measurement of the vials before and after administering thedose.

3. CLASSIFICATION OF ACCIDENTS

3.1

CLASSIFICATION OF CAUSES BASED ON
RADIOTHERAPY TECHNIQUESIn order to gain an overview of the initiating events and contributing factors of
the cases reviewed in this Safety Report, and to facilitate the practical application of
the lessons to be learned, it is illustrative to identify the accidents following thechronological order of the treatment process. The following subsections attempt to do
this. The numbers given in parentheses after some of the steps refer to the events
described in Section 2.

66

3.2. EXTERNAL BEAM THERAPY
3.2.1. Events related to equipment, thus affecting many patients
Accidents related to problems with equipment, such as calibration of the beam
output, involve all patients treated with the beam until the problem is discovered. The
most important events were those resulting in an error in the determination of doserate, and therefore wrong irradiation times for patients treated under these conditions.
In the three worst cases, 115, 207 and 426 patients were involved, with dosedeviations of up to 60% and many deaths. In addition, there were two major accidents
related to maintenance of accelerators, one of them involving 27 patients (several ofwhom died as a direct result of radiation exposure).

Radiation measurement system: These include lonization chambers andelectrometers.
Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Calibration of reference system
Intercomparison with secondary system
Routine use

1
5
2,3,4

Treatment machine
Step at which the event occurred (chronological order)

Event Nos (see Sec. 2)

Commissioning (acceptance)
Calibration (annual)
Constancy check (daily, weekly)
Malfunction of machine
Incorrect use

7,8,10,11,14,25
6, 9, 12
13
15, 16, 17, 18,20,49,51
19,20,23,24,48,50

Simulator
Step at which the event occurred (chronological order)

Event Nos (see Sec. 2)

Malfunction

52, 53, 54 (malfunction)

Treatment planning system
Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Commissioning and input of basic data
Routine use

21, 22, 24, 25, 30
23,26,27,28,29,31

67

3.2.2. Events involving individual patients
Some of the accidents reviewed occurred due to different conventions for theincorporation of different diagnostic imaging modalities that are used in treatment planning, including those from simulators, computer tomography scanners,
nuclear medicine, ultrasound, etc. Also, accidents were caused by incomplete or
inaccurate documentation of patient charts for all aspects of treatment planning,
including isodose curves, use of wedges, special blocking and placement andorientation of beams, since these factors are used to calculate beam-on times for all
fields.
There were cases of treatments to the wrong person or the wrong anatomicalsite, and of incorrect tumour dose and overdose to normal tissues because of
ineffective institutional protocol for patient identification, such as a photograph of thepatient, unique patient hospital number, ID bracelet and verbal confirmation ofpatient identification.

Prescription
Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Miscommumcation of prescnption
Error in use of images

2, 3, 4, 33, 37, 45
41, 44, 45

Treatment planning
Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Documentation
Calculation of treatment time or monitor units
Incorrect use of treatment planning system

32
28, 29, 36
27, 31, 36, 37

Execution of treatment
Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Patient identification
Documentation of patient setup

34,35
32, 33, 38, 39, 40, 42, 43,
45,47
19,46
47

Incorrect operation of treatment machine
Final review at completion of treatment

68

3.3. LOW DOSE RATE BRACHYTHERAPY
3.3.1. Events related to equipment, thus affecting many patients
Cited in this review are accidents which occurred when source characteristics,
such as source strength, isotope and filter thickness, are not verified by measurement
upon receipt from a supplier and each time they are used, by identification of source
marking. Mistakes arose when strengths of sources were quoted in two nonequivalent units, e.g. mg Ra-eq or mCi, and these were used erroneously orinterchangeably, when users did not properly distinguish between nominal andmeasured source strength, and when suppliers provided source activities different
from those ordered. Other causes of accidental exposures were when sources
withdrawn from clinical use were not clearly labelled as such and taken out ofcirculation, as well as the improper use of leaky sources.

Step at which the event occurred (chronological order)

Event Nos (see Sec. 2)

Verification of activity
Verification of batch uniformity
Verification of identification markers
Storage and inventory
Performance of leakage test
Applicators: commissioning and maintenance

55, 56, 57, 58, 69, 70
68, 69, 70
58, 5958,71,73
74
75, 833.3.2. Events involving individual patients
Brachytherapy requires an individual treatment plan based on the geometry of
the target site and prescribed dose with the treatment time based on the geometry of
the implanted or inserted sources. Serious misadministrations occurred when thesource geometry or activities were not correct or the correct placement of sources was
not maintained throughout the treatment time. Severe under-irradiation of the tumourand over-irradiation of normal tissues were also reported when sources weredislodged during treatment. In a few reported cases of accidents, nursing staff lacked
the required special training in radiation safety needed to perform nursing care withminimal exposure to staff and visitors and to deal with emergencies. Patients were notinformed that sources were radioactive and should not be removed or tampered withduring treatment.

69

Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Prescription documentation and communication
Planning of application including activity, geometry,anatomical site, estimation of dose rate and treatment time
Identification of patient
Informing the patient
Documentation
Radiographic localization of sources
Sources correctly in place for duration

67
55, 56, 60, 61, 62, 65, 66

Source identification
Final calculation of dose and treatment time
Correct delivery of prescription
Correct treatment time
Monitoring of patient after removal of sources

3.4.

85
74, 79, 80
62, 65, 67
72, 8257, 72, 76, 77, 79, 80, 81,
83
58, 59, 63, 67
64
78, 81, 82
82
82

HIGH DOSE RATE BRACHYTHERAPY

3.4.1. Events related to equipment, thereby affecting many patients
High dose rate units, because they involve very high activity sources, have apotential for damage to patients as a result of misadmimstration Cited in this report
are cases of misadmimstration owing to malfunctioning equipment, such as cathetersthat do not allow full movement of sources Also cited are cases of inadequately
trained personnel operating equipment, resulting in misadministration of treatment
Because high dose rate brachytherapy is relatively new compared with low dose ratebrachytherapy, the total number of high dose rate events is lower.

Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Training of personnel
Malfunction of equipment

77, 7881, 84, 86

3.5. USE OF UNSEALED SOURCES
Step at which the event occurred (chronological order)

Event Nos (see Sec 2)

Planning and dispensing
Administration
Safety and contamination

89, 90, 91
87, 88, 89, 92
9270

4. LESSONS LEARNED AND MEASURESFOR THE PREVENTION OF ACCIDENTS
The purpose of radiotherapy is to achieve maximum control of the tumour withminimum complications to normal tissue. This objective sets very high requirementson systematic accuracy and precision in relation to the total dose, dose distribution
and dose fractionation. In order to satisfy these requirements, both systematic andrandom errors must be kept within a few percentage points during the entire process,from the time of prescnption to the completion of the radiation treatment, a chain of
events that involves a number of persons.
A review of the initiating events and contributing factors described in Section 2
confirms that most of the accidents could have been prevented by consistent
application of the requirements cited in Appendix II (medical exposures) andAppendix IV (potential exposures) of the BSS. The lessons learned from theaccidents are discussed below.

4.1

RESOURCES: PERSONNEL AND EQUIPMENT

A radiotherapy department needs to have sufficient staff in relation to thenumber of patients and types of treatment modalities; guidelines have been
recommended by national and international organizations. A shortage of staff usuallyleads to an increase in errors. In addition, a reassessment of staff and training has tobe undertaken when the workload increases (more patients) or new equipment ispurchased, or when a new technology or treatment modality is introduced. The failure
to reassess staff workload led to a large scale accident in which a 60Co beam was used
for a period of 22 months without quality control checks because the only medical
physicist available was assigned to a new accelerator.

4.2. HUMAN FACTORSRadiotherapy is very much dependent on human performance. A large numberof steps, tasks and subtasks have to be performed many times every day and differ
only slightly from one patient to the next. Rarely are all steps in radiotherapy
performed by a single individual; many people work together, each contributing asmall part of the entire process. Many workers in several disciplines must interact inhighly technical measurements and calculations, and the potential for mistakes islarge if care is not taken. Even well trained and well qualified personnel make errors,
owing to inattention to details, lack of alertness, or lack of awareness, especially if71they have to work in less than ideal conditions. In some of the accidents, even thereview of charts and calculations failed to detect a mistake.
4.2.1. Identification of safety critical steps and activities
A list of safety cntical activities and steps in the radiotherapy process is given
below. The list is not intended to be exhaustive and is derived from the examples
discussed in this report:(a)Commissioning of radiotherapy machines and validation of treatment planningsystems,(b) Periodic calibration and constancy checks of beams and sources,(c) Decommissioning of radiation sources and devices containing sources,(d) Identification of the patient,(e) Identification of the treatment target site ('localization'),(f) Preparation of the treatment plan,(g) Formal prescription of the treatment plan,(h) Simulation of the treatment and placement of body marks (tattoos),(i) First treatment session,(j)
Daily patient positioning and selection of parameters and treatment accessories,(k) Accumulation of doses in the patient's chart,(1) Modifications during the course of treatment,(m) Radiopharmaceutical administration,(n) Maintenance of radiotherapy equipment and devices,(o) Removing sources from the patient after a brachytherapy treatment andreturning them to storage,(p) Identification of long lived brachytherapy sources kept in storage,(q) Measurement of activity (dose rate) of short lived brachytherapy sources
obtained for single treatments.
The assignment of functions and lines of authority and responsibility should
ensure that there are no gaps or ambiguities which leave safety critical tasks insufficiently covered. At this stage, the necessary degree of human redundancy is established.
4.2.1 1. Measures for the prevention and mitigation of consequences
Measures in this group are:(a)72

Clear and concise written instructions to prepare staff for unusual situations, ingeneral as well as on a case by case basis. Clear job descriptions, assignment ofresponsibilities and lines of authority.

(b)Anticipation of unusual, unplanned situations following a review of allprocedures. A drill or rehearsal of measures to be taken may be appropriate forsome types of accident. A review of all procedures to identify those which
could lead to an accident in an unusual or unplanned situation.

Special care needs to be taken when there are complex or unusual treatment
plans or when there is a change in procedures, such as:
(1)
(2)
(3)

A change of supplier of radioactive material;
A non-typical dose, an unusual target area or a treatment with the patient in an
unusual position;
A complex treatment, for example, treatment of more than one anatomical siteon the same day.

4.3. TRAINING
Oncologists, physicists and other paramedical staff necessarily require formal
training to effectively carry out their duties. In addition, opportunity for continuing
their education by attending professional meetings and refresher courses is necessary
in a field as dynamic as radiotherapy. Acquisition of new equipment often requires
attendance at training sessions provided by manufacturers. Membership in
professional organizations provides a valuable mechanism for communication withcolleagues through meetings, journals and other publications. These contacts promotefurther informal interaction, which is particularly important for a physicist working
alone.
Nurses and ancillary personnel who interact with patients have to be trained inradiation safety. They need to be cognizant of techniques for reducing exposure tothemselves, other workers and visitors. They have to be able to recognize
emergencies, such as unintended removal of radioactive sources, and know which
procedures are appropriate in such an event.
Training has to cover not only normal situations, but also the identification ofabnormal situations (case histories) and unusual events, as well as measures for
accident prevention.
4.3.1. Training for special or unusual situations
Experience has shown that in an unusual situation, for example after an
equipment failure or when conflicting signals appear, the behaviour of staff is often
unsafe. Wrong assumptions are made, leading to accidental exposures. This may be
because the staff member makes the interpretation that matches his or her experience,73

expectation or convenience. Expectations are usually based on expenence gained
dunng normal operation of equipment or routine procedures. Additional specifictraining is needed to cover special or unusual situations, for example how to deal withconflicting signals on a console, a change of supplier, or complex types of treatment.
The inclusion of unusual situations, the discussion of case histones and contingency
plans, and exercises in training coupled with penodic refresher training will help staff
recognize potential problems.

4.4

COMMUNICATION

Communication between staff members is essential for all aspects of treatment,
since many persons with various responsibilities must interact. Mistakes may bemade owing to lack of adequate communication, incorrect information, or poor
understanding of correct information.
The review of events reveals the following sources of error:
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)

(i)

Failure to transmit information.
Transmitting incorrect communication.
Communication to the wrong person.
Correction of a problem by an unqualified person without help or review.
Oral communication, either in person or by telephone, without written confirmation, resulting in misunderstanding.
Mistakes in reading or transferring information.
Unreadable or confusing handwritten communication, informal expressions or
use of jargon that is not understood by everyone in the same way.
Misunderstanding of communication in a foreign language: This may include
manufacturers' instructions for the use of equipment, as well as communication
between staff and between staff and patients
Incomplete or poorly written instruction manuals for complex equipment such as treatment machines and treatment planning computers Of
particular concern are instructions that do not cover unusual or special
applications.

Not explicitly cited in this review of accidents, but often playing a role, are thefollowing circumstances:
(1)
(2)
(3)

74

Interpersonal problems among staff;
Noisy working conditions, leading to distraction and loss of concentration;
Insufficient access to, or non-availability of, personnel in essential positions at
cntical times

4.4.1. Measures for the prevention and mitigation of consequences
In any institution, one person, usually a medical physicist, should have overall
responsibility for all aspects of the dosimetry system. It is his or her responsibility to
see that the lines of communication concerning dosimetry of patients are working
properly. Other measures are:
(a)
(b)
(c)
(d)
(e)
(f)

Identification and posting of all rules for communication critical to safety.
Clear assignment of responsibilities of the staff, including written job descriptions
An organizational structure that clearly identifies the 'chain of command', i.e.,each worker's supervisor and the persons he/she supervises
Checklists that describe all steps of routine procedures as well as actions to betaken when accidents occur.
Clear and concise written procedures for safety critical communications.
Signatures on documents critical to safety. For example, prescnptions are to be
written and signed by the physician. Basic data, treatment plans, daily logs ofpatient therapy, etc., are to be signed or initialled by the person performing orchecking that step in the process.

Some examples of critical communications relevant to safety are given in Table I.
Once the safety critical communications have been identified, responsibilities
for these communications are assigned. Formal procedures are established for safety
critical communications, design of forms and checklists. Rules for clear, concise andcomplete communication and for developing protocols, drawings, charts, language
and clearances with signatures are drawn up.

4.5.

EQUIPMENT

Equipment failure accounts for only a moderate percentage of all accidents
cited in Section 2. However, the consequences can be severe and many patients may
be affected when equipment does malfunction. Contributing factors are:
(a)
(b)
(c)

(d)

Insufficient redundancy in the design of equipment (single fault criterion,interlock failure);
Software problems;
Hardware incompatibilities in equipment and accessories (wedge or shielding
block incompatible with coding system, or ionization chamber that does not fit
an electrometer);Possibility of operating the equipment in a 'non-clinical mode' with the key inthe usual 'beam-on' position.

75

TABLE I. EXAMPLES OF COMMUNICATIONS CRITICAL TO SAFETY3Content of communique or
memorandum

From

To

Prescription for treatment

Radiation oncologist,
nuclear medicine physician

All staff involved mplanning or delivering
treatment

Input data for preparation oftreatment plan

Simulation radiographer

Planning radiographer,dosimetrist

Proposed treatment plan andcalculation, prescribed doseRadiotherapy or nuclear
medicine technologist

Medical physicist

Proposed treatment plan andconfirmed calculation

Medical physicist

Radiation oncologist

Patient immobilization orshielding aids

Radiation oncologist

Mould room technician

Confirmed plan

Radiation oncologist

Medical physicist orradiation technologist

Equipment fault report

Radiotheraphy technologist

Medical physicist

Request for maintenance

Medical physicist

Maintenance organization

Instruction to discontinue
treatment

Medical physicist

Radiation oncologist,
radiotherapy technologist

Maintenance complete

Maintenance organization

Medical physicist

Check equipment and venfy
that it is ready for use

Medical physicist

Technologist' In many organizations, a person may perform more than one function

4.5.1. Measures for the prevention and mitigation of consequences
In Appendix II of the BSS, which deals with medical exposures, therequirements in paragraph II. 11 for the design of equipment used in medical exposure
are in pertinent part as follows:
"(a)

76

failure of a single component of the system [must] be promptly detectable sothat any unintended medical exposure of patients is minimized; and(b) the incidence of human error in the delivery of unintended medical exposure be
minimized."

Methods for good engineering are well developed. One basic element is the
single fault criterion, which leads to redundancy. In other words, failure of one
component should not cause an accident; an accident should be possible only if
two or more systems or components fail simultaneously. A faulty component
should be repaired immediately in order to maintain redundancy and prevent
accidents [3, 4]:
(1)

(2)

(3)

(4)

(5)

(6)

(7)

Analysis of common failures is used to ensure that one common failure would
not nullify the redundancy. For this purpose, redundancy is complemented byindependence.
Components and systems have to be designed to be fail-safe, so that a failure
does not lead to an unsafe situation. For example, the source driver of a gamma
teletherapy unit is designed so that the radioactive source returns to the shielded
position if there is a cut-off of electrical power (see the BSS, Appendix II,
paragraphs 11.11-13 and 15).
Prototype tests need to include special or extreme conditions of equipment
operation. Tests need to involve clinical personnel and others who use
equipment routinely and not be limited to engineers and personnel who design
and install equipment.
Equipment is to be designed so that when it is in 'non-clinical mode' (reductionof interlocks), it is impossible to enter a 'beam-on' command from the regular
key in the control console.
Validation of the equipment and its operation, under clinical conditions, has tobe made before initiating treatment of patients. Detailed documentation
describes the tests performed and the personnel responsible. Commissioning ofequipment for clinical use is the responsibility of the institution's medical
physicist.
There is a need for manufacturers of radiation therapy equipment to promptly
and thoroughly investigate any reported incident or unusual event and notify
authorities and other users of their findings and appropriate preventive
measures. A formal system of reporting and disseminating information frommanufacturers to users is essential.
Machine malfunctions are not to be corrected until the full dosimetnc
consequences to patients who were treated during the period of the malfunction are known. Measurements while the machine is malfunctioning
may be necessary in order to estimate an incorrect dose delivered to apatient.

77

4.6

HUMAN-MACHINE PROBLEMS

4.6.1. Problems of human-machine interface
Personnel need to understand the functioning of equipment, including thesignificance of alarms, signals, and displays that indicate a malfunction.
4 6.1 1. Measures for the prevention and mitigation of consequences
(a)

(b)
(c)

Inconsistent signals are investigated and the one indicating the more serious
consequence is assumed to be correct until proven otherwise. For example, if aradiation monitor gives off an alarm indicating that a beam is on, but theconsole display shows that the beam is off, it is more prudent to assume that thebeam is on, not that the alarm is malfunctioning.
The human-machine interface in the clinical environment is considered in thedesign and testing of equipment.
Specific training to recognize and respond to abnormal signals and display
interlocks, as well as contradictory signals.

4.6.2. Bypassing of interlocks and operation in a 'non-clinical mode'
Operation in a 'non-clinical mode' sometimes is the response to frequent or
unresolved maintenance problems. However, circumventing interlocks and othersafety devices in order to continue with patient treatments increases the possibility ofaccidents, which can lead to serious or even fatal consequences.
4 6.2 1. Measures for the prevention and mitigation of consequences
(a)
(b)
(c)
(d)

(e)

78

Plan and implement an efficient maintenance programme and avoid
improvisation as far as practicable.
Never treat patients with equipment operating in the 'non-clinical mode'.
Never try for a 'quick-fix' or 'patch-job' in order to continue treatment ofpatients.
Withdraw from clinical use equipment that is malfunctioning until formal
repair is complete and its operation is checked in a thorough and deliberate
way.
Design equipment so that it is possible to turn 'beam on' in the 'non-clinicalmode' only by using special tools or computer codes. It is only possible tooperate the machine in the non-clinical mode for maintenance or other nonchnical work (BSS, Appendix II, paragraphs 11.15 (c) and (d)).

4.6.3. Maintenance problems
Mistakes made in maintenance can and have affected many patients and have
led to severe, even fatal, consequences.
4.6.3.1. Measures for the prevention and mitigation of consequences
(a)

(b)

(c)

Maintenance personnel have to be cognizant of the potential consequences ofany manipulation, alteration or adjustment of components of a machine for thesafety of patients and personnel who use the machine clinically.
Before a machine is released for clinical use following maintenance or repair, acheck of the output, as well as any other operating parameters that could havebeen affected, has to be made. Formal procedures for handing over the machine
after such maintenance or repair to the physicist must be in place for necessary
checks before clinical use.
Instructions for the use of a machine that has been altered must be clearly andunambiguously documented in writing. Machine functions are only to bealtered by or with the authorization of the manufacturer. Unauthorized
modifications may compromise safety and may also nullify a manufacturer's
warranty obligations and product liability.

4.7. IMPROPER DECOMMISSIONING OF EQUIPMENT
AND UNSAFE STORAGE OF RADIOACTIVE SOURCES
Unauthorized handling, dismantling or abandonment of teletherapy isotope
units has led to large scale serious consequences, including fatalities and widespreadenvironmental contamination [5].
4.7.1. Measures for the prevention and mitigation of consequences
(a)

(b)

(c)
(d)

Governmental licensing needs to regulate the importation, storage,commissioning, operation, temporary suspension, decommissioning anddisposal of radiation sources.
There have to be clear, unambiguous and authorized procedures for the
temporary or indefinite secure storage of radioactive sources until safe disposal.
Formal control has to be in place during this period of time.
Equipment no longer in use has to be disabled in a way that prevents connection
to an electrical power supply.
Sealed sources found to be damaged or leaking are to be stored in a way thatprevents their clinical use and contamination of the environment.

79

(e)
(f)
(g)

Obsolete brachytherapy sources have to be disposed of safely, in accordancewith governmental regulations.
Radioactive sources used in nuclear medicine are to be disposed of safely, in
accordance with governmental regulations.
Empty source containers have to be clearly labelled and stored in a location
separate from containers with sources.

48. DOCUMENTATION
Documentation is essential for high quality and safe radiotherapy and to foster
communication among staff and continuity of practice over time [6, 7].
Some of the accidents reviewed have a strong component of misinterpretationof instructions, displays and interlocks as well as signals relating to unusual events.
Many maintenance mistakes are related to a lack of comprehensive training andcontinuing education, especially in dealing with unusual situations.
4.8.1. Measures for the prevention and mitigation of consequences
A log book is needed for each major item of equipment, such as 60Co units,linear accelerators, TPSs, iomzation chambers and electrometers, in which
information concerning calibration, repairs, maintenance and checks of performanceare routinely recorded Each entry has to be dated and signed by the responsible
person.
For individual patients, all aspects of treatment have to be documented as apermanent part of each patient's chart and retained for at least as long as specified by
local laws and regulations. The prescription has to be written by a physician.
Treatment plans, generated by manual or computer methods, need to include isodose
distributions, with placement of beams, any wedge filters or beam blocks. The basic
data and TPS used to generate the plan, including data such as depth dose, tissue-air
ratios, wedge factors, etc., have to be clearly written down. All details required for the
setup of the patient and machine time or monitor units for each field are to be clearlyshown. A diagram of the patient with location of the beams is essential, and aphotograph of the patient in the treatment position is useful for more complex
treatments, particularly those involving blocked or irregular fields. Each patient's
treatment is to be detailed in a daily log, with beam times or monitor units listed.
Original documentation concerning equipment or a patient's treatment isnever to be altered If errors are detected that require correction later, comments
may be added to the record but the onginal documentation is not to be changed.
Addenda to documentation must include the signature or initials of the person
responsible.

80

Comprehensive training is complemented by familiarization with the
manufacturer's documents. In this respect, as indicated, the BSS recommends that
performance specifications and operating and maintenance instructions be provided
in a major world language understandable to the users, in compliance with therelevant International Electrotechnical Commission (IEC) and International
Organization for Standardization (ISO) standards, and that this information betranslated into local languages whenever appropriate. In addition, the operating
terminology (or its abbreviations) and operating values must be displayed on
operating consoles in a major world language acceptable to the user. The
requirements imply that instructions be understandable, without ambiguity, to users
of all categories, such as radiation oncologists, technologists, medical physicists andmaintenance engineers.
It is emphasized that accompanying documents, according to the IEC
definition, should be a part of the equipment (see IEC-601-1, subclause 6.8 [3]) andthat equipment is incomplete if the accompanying documents are not delivered.
According to IEC-601-1, the accompanying documents include at least:(a)(b)

Instructions for use, including accessories, interlocks and displays, as well as
any maintenance to be performed by the user;
Technical descriptions, including, among other information, a statement thatthe supplier will make available on request circuit diagrams, component partlists, calibration instructions or other information that will assist appropriately
qualified user personnel to repair those parts of equipment designated by the
manufacturer as reparable;
In addition, it is strongly recommended that the user maintain:

(1)
(2)
(3)

Records of repairs, revisions of the equipment and faults and lessons learned;
Manufacturer's newsletters;
Manufacturer's notes on investigations of malfunctions of the equipment,
accumulated experience, preventive recommendations and, most importantly,
warning notes on incidents with similar machines in other facilities.

4.9. INTEGRATION OF SAFETY AND QUALITY ASSURANCETechnical and managerial principles need to be applied to prevent unintentional
exposures and to mitigate the consequences, should they occur. These principles startwith a systematic assessment of both equipment and treatment procedures, followedby the application of 'defence in depth' measures and the integration of prescription
and delivery of treatment in an overall approach. This approach involves the attitudes

81

of all the persons involved in radiation therapy — physicians, physicists,
technologists, nurses, equipment maintenance engineers and clerical staff. All need todemonstrate a 'culture of safety'.
Independent checks, internal and external, are an important means of avoiding
errors or detecting them before they cause injury to patients.
External independent checks may be formal, such as those performed by anoutside physicist from an authorized reviewing organization, as required by clinical
trials. External reviews also may be informal, where colleagues intercompare
lonization chambers and other measuring systems. In addition, mailed dosimetersfrom reviewing organizations provide an inexpensive means of checking machine
output and other basic machine parameters.
Within an institution, having a second person check data and repeat calculations
or measurements is an important way of finding mistakes before they result inincorrect doses to patients. Internal independent checks need to be part of the routine
flow of work m a dosimetry system. Independent verification procedures can beeffective only if physicists and other staff work together as colleagues, in a spint of
goodwill and co-operation. Staff at all levels need to feel free to challenge the workof any other person if they feel that an error has been made that would cause detriment
to a patient.
Independent checks are successful only if each person involved is free to point out
errors and discuss problems as well as accept correction of his or her own errors. It is
crucial that interaction be conducted in a professional manner, bearing in mind thebest interests of the patient. A professional attitude has to be maintained by all categories of radiotherapy staff, independent of their vantage point in the review process

4.10. SAFETY ASSESSMENT
The mam requirements for a safety assessment — out in the BSS in
Appendix IV, 'Potential Exposure: Safety of Sources' [1] — are also applicable toradiotherapy sources and installations.
A safety assessment requires a systematic approach to identify events that canlead to potential exposures and to estimate the probability and magnitude of such
exposures There are two approaches to the safety assessment: deterministic andprobabilistic methods.
The deterministic method is often used first to provide safety, with appropriate
safety margins, in systems where accident scenarios can be identified. Appropriate
measures may include changes in equipment design or operating procedures, with
relevant training of personnel and formulation of contingency plans.
The probabilistic method is used to identify weaknesses in safety measures that
may be overlooked by the deterministic approach, because they may not yet have82

resulted in an actual accident. The method identifies and calculates the expected
frequency of initiating events that may lead to potential exposure and calculates the
expected detriment due to initiating events for comparison with acceptance criteria.
The method also establishes the relative importance of individual events andquantifies the probability of their leading to the detrimental effect.
Probabilistic and deterministic methods should not be viewed as being mutually
exclusive, but rather as complementary techniques.
4.10.1. System for quality management
Integrated therapy systems involve both quality and safety. In fact, most of themeasures to monitor quality serve equally to detect any deviation concerning safety
since the parameters to be controlled are often the same. The quality control programme can be designed to combine both quality and safety. For example, test toolsthat perform constancy checks serve to monitor quality and safety at the same time.
To meet the requirements of radiotherapy, many centres are implementing
techniques used in other disciplines where the prevention of errors is an everyday
issue. While the differences between industry and medicine should be allowed for,appropriate elements of these techniques known as the 'Quality Management System'and defined by ISO Standard 9000 can be incorporated into the planning andmanagement of radiotherapy services. The system includes the calibration of sources,
clinical dosimetry, quality assurance of medical exposures and records, as well as
regular and independent quality audits, as listed in Appendix II of the BSS. A detailed
description of quality assurance programmes and quality control tests has been set out
in many recent publications [8-13].
4.10.2. Monitoring effectivenessThe vulnerability of a facility to accidental or erroneous exposures needs tobe assessed in order to ensure that the probability of their occurrence remains
low and that remedial action would be effective. Some methods for assessment are:
(a)

(b)
(c)

Reviewing records of normal conditions and minor mishaps to detect any
abnormal conditions or occurrences that may increase the probability ofaccidents;
Checking the layers of the defence in depth system to detect if any are notfunctional, thereby making the system vulnerable;
Developing a system of investigation and analysis of accidents and unusual
events, including initiating events and contributing factors, and disseminating
information on lessons learned;

83

(d)
(e)
(f)
(g)

Testing the facility for vulnerability against initiating events and contributing
factors emerging from item (c);Internal and external audits and mtercomparisons to detect weaknesses andindicate corrective measures;
Evaluation of mistakes made, with a review of steps for mitigation of theireffects and future avoidance;
Evaluation of protocols for patient follow-up after therapy to detect abnormal
radiation reactions that may suggest errors in exposure.

4.10.3. Defence in depth
The term defence in depth is defined in the BSS as "the application of morethan one single protective measure for a given safety objective such mat the objectiveis achieved even if one of the protective measures fails". There are layers ofoverlapping safety provisions, such as physical components, procedures andcombinations of both. A low overall probability of failure can be more readily
achieved by a combination of independent protective layers such that the probabilities
are multiplicative.
Examples of defence in depth are:(1)
(2)

A treatment plan that is produced by a technologist, checked by the medical
physicist and rechecked by in vivo dosimetry, andThe output of a linear accelerator that is controlled by an lonization chamber,checked by a second ionization chamber preset to 7-10% higher dose, and a
backup timer set to the prescribed time +15%.

4.10.4. Safety culture
An essential managerial principle for all individuals and organizations is toestablish a 'safety culture' encompassing both personal attitudes and habits of thought
as well as organizational policies and priorities.
Good practices are essential but not sufficient. There is a need to go beyond theimplementation of a good practice, so that both personal attitudes and organizational
policies are onented towards the same goal: that all duties be carried out correctly,with alertness, full knowledge, sound judgment and a proper sense of accountability.
Even some of the provisions for defence in depth, such as redundant reviews andchecks, may fail to detect a problem. The basic principles of safety culture are that:"all duties important to safety should be carried out correctly, with due thought andfull knowledge, sound judgment and a proper sense of accountability" [14].
Management should provide the environment, training and provisions to maintain andexercise alertness and awareness of the staff.

84

Each person responsible for any aspect of patient treatment needs to have a
critical view of his or her own and other people's work. Freedom to challenge thework of others in a spirit of goodwill points up potential accidents before they
happen. It also discourages complacency amongst staff.
Furthermore, it implies a learning attitude by all concerned, whereby both
operating experience and the results of new research are used to reassess and improvesafety.
4.10.5. Emergency preparedness plans
Despite preventive measures, accidents or unusual events may occur. The safetyassessment will identify possible accident scenarios and situations, andcountermeasures for mitigation are designed. Some of the actions in the emergency
response plan need to be taken immediately, without hesitation or mistakes. To
achieve this, personnel need to be trained and simulation drills carried out. A clear
and concise list of actions and responsible officers is posted in relevant places. Thereis written documentation of rules for action and training, including simulating
exercises, for the emergency response plan. The development and periodic review ofthe plan is part of the system of protection for radiotherapy.
4.10.6. Investigation of accidental medical exposuresand dissemination of information on accidents and unusual events
In a facility where an accident occurs, corrective measures can be applied only
if a prompt investigation is made. With regard to accidental medical exposure, the
BSS promulgates specific investigative action, calculation of dose and corrective
measures, as well as the requirement to report the incident to the Regulatory
Authority and to inform the patient (BSS, Appendix II, paragraphs 29, 30).
In addition to the investigation of accidents, which leads to corrective andpreventive measures, the dissemination of operational experience can avoid accidents
elsewhere. The number of reported events in a single country is usually insufficient
to provide a significant number of lessons in a reasonable time. However, acompilation of accidents at the international level would allow all countries to benefit
from the lessons from each of them. Moreover, the experience gained from unusual
events that did not culminate in accidents can contribute to the knowledge needed to
avoid accidents. The dissemination of this information can be used to test thevulnerability of many facilities worldwide and improve preventive measures without
waiting until an accident occurs.
Therefore, a reporting system which allows for anonymous reports of cases thatare otherwise not published is necessary for such reports to reach a wider community.
An international reporting system of unusual events and operational experience is in85

place in other fields, such as nuclear power, and a similar approach can equally serve
to prevent accidents in radiotherapy.

4.11. REGULATORY CONTROLThe assessment of safety, together with measures to reduce the likelihood of
unwarranted or unjustified radiation exposure, requires the incorporation of safety
into the management system and the development of a safety culture as defined in
Section 4.10.4. Furthermore, these measures need to be ensured by means ofappropriate regulatory control. This implies a regulatory framework involving
authorization, inspection and enforcement. Radiotherapy is a practice that is subject
to licensing. Licensing requirements include mandatory safety assessment of all
aspects of the acquisition, use and disposal of radiation sources, including
construction, importation, storage, commissioning and decommissioning, whether
temporary or permanent. In addition, plans for the prevention of mistakes and for
emergency preparedness are to be assessed before licences are issued. In this regard,an IAEA technical document [15] gives detailed guidance on safety assessment
requirements for the authorization and inspection of radiotherapy facilities.

86Appendix
APPENDIX II OF THE BSS —MEDICAL EXPOSURE: OPTIMIZATION OF PROTECTION
FOR MEDICAL EXPOSURESDESIGN CONSIDERATIONSGeneral"11.11. The requirements for the safety of sources specified in other partsof the Standards shall also apply to sources used in medical exposure, where
relevant, and, in particular, equipment used in medical exposure shall be so designed
that:
fa)
(b)

failure of a single component of the system be promptly detectable so that any
unplanned medical exposure of patients is minimized; andthe incidence of human error in the delivery of unplanned medical exposure be
minimized.
11.12. Registrants and licensees shall:

(a)

(b)

(c)
(d)

taking into account information provided by suppliers, identify possible
equipment failures and human errors that could result in unplanned medical
exposures;take all reasonable measures to prevent failures and errors, including theselection of suitably qualified personnel, the establishment of adequate
procedures for the calibration, quality assurance and operation of diagnostic
and therapeutic equipment, and the provision to personnel of appropriate
training and periodic retraining in the procedures, including protection andsafety aspects;
take all reasonable measures to minimize the consequences of failures anderrors that may occur; anddevelop appropriate contingency plans for responding to events that may occur,
display plans prominently, and periodically conduct practice drills.

11.13. Registrants and licensees, in specific co-operation with suppliers, shallensure that, with regard to equipment consisting of radiation generators and that
containing sealed sources used for medical exposures:

87

(a)

(b)

(c)

(d)
(e)
(f)

(g)

whether imported into or manufactured in the country where it is used, theequipment conform to applicable standards of the International
Electrotechmcal Commission (IEC) and the ISO or to equivalent national
standards;
performance specifications and operating and maintenance instructions,
including protection and safety instructions, be provided in a major world
language understandable to the users and in compliance with the relevant EEC
or ISO standards with regard to 'accompanying documents', and that thisinformation be translated into local languages when appropriate;
where practicable, the operating terminology (or its abbreviations) andoperating values be displayed on operating consoles in a major world language
acceptable to the user;
radiation beam control mechanisms be provided, including devices that indicate
clearly and in a fail-safe manner whether the beam is 'on' or 'off';
as nearly as practicable, the exposure be limited to the area being examined ortreated by using collimating devices aligned with the radiation beam;
the radiation field within the examination or treatment area without any
radiation beam modifiers (such as wedges) be as uniform as practicable and thenon-uniformity be stated by the supplier; andexposure rates outside the examination or treatment area due to radiation
leakage or scattering be kept as low as reasonably achievable.""Requirements for radiation generators and irradiation installations
radiotherapy

for11.15. Registrants and licensees, in specific co-operation with suppliers, shallensure that:
(a)

(b)

(c)

radiation generators and irradiation installations include provisions forselection, reliable indication and confirmation (when appropriate and to theextent feasible) of operational parameters such as type of radiation, indicationof energy, beam modifiers (such as filters), treatment distance, field size, beamorientation and either treatment time or present dose;
irradiation installations using radioactive sources be fail-safe in the sense thatthe source will be automatically "shielded in the event of an interruption of
power and will remain shielded until the beam control mechanism is reactivatedfrom the control panel;high energy radiotherapy equipment:(i) have at least two independent 'fail to safety' systems for terminating theirradiation; and

(d)

(e)
(f)

(ii) be provided with safety interlocks or other means designed to prevent theclinical use of the machine in conditions other than those selected at thecontrol panel;the design of safety interlocks be such that operation of the installation during
maintenance procedures, if interlocks are bypassed, could be performed only
under direct control of the maintenance personnel using appropriate devices,
codes or keys;
radioactive sources for either teletherapy or brachytherapy be so constructedthat they conform to the definition of a sealed source; andwhen appropriate, monitoring equipment be installed or be available to givewarning of an unusual situation in the use of radiation generators andradionuclide therapy equipment."

89

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[2]
[3]

[4]

[5]
[6]

[7]

[8]

[9]
[10]

[11]

[12]
[13]

[ 14]

90

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS,INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOURORGANISATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICANHEALTH ORGANIZATION, WORLD HEALTH ORGANIZATION, International
Basic Safety Standards for Protection against Ionizing Radiation and for the Safety ofRadiation Sources, Safety Series No. 15, IAEA, Vienna (1996)
PEREZ, C A , BRADY, L W, Principles and Practice of Radiation Oncology, 2nd edn,
J B Lippmcott Company, Philadelphia and New York (1992)
INTERNATIONAL ELECTROTECHNICAL COMMISSION, Safety of Medical
Electrical Equipment, IEC, Part 1, General Requirements, Publication 601-1, IEC,
Geneva (1981)
INTERNATIONAL ELECTROTECHNICAL COMMISSION, Medical Electrical
Equipment, Medical Electron Accelerators — Functional Performance Characteristics,
Publication IEC-976, IEC, Geneva (1989).
INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in
Goiama, IAEA, Vienna (1988)
INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Dose and Volume Specification for Reporting Intracavitary Therapy in
Gynaecology, Report 38, ICRU, Bethesda, MD (1985)
INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Prescribing, Recording, and Reporting Photon Beam Therapy, Report 50,
ICRU, Bethesda, MD (1993)
BELLETI, S , et al, Quality assurance in radiotherapy The importance of medical
physics staffing levels Recommendations from an ESTRO/EFOMP joint task group,Radiother Oncol 41 1 (1996) 89-94
WORLD HEALTH ORGANIZATION, Quality Assurance in Radiotherapy, WHO,
Geneva (1988).
AMERICAN ASSOCIATION OF PHYSICISTS IN MEDICINE, Comprehensive QA
for radiation oncology Report of Radiation Therapy Committee Task Group 40, Med.
Phys 204(1994)581-618
FRENCH SOCIETY OF RADIATION ONCOLOGY AND FRENCH SOCIETY OF
HOSPITAL PHYSICISTS, Recommendations for a Quality Assurance Programme inExternal Radiotherapy, ESTRO Physics for Clinical Radiotherapy Booklet No 2,
Garaant (1995)
DEPARTMENT OF HEALTH, A Quality Management System for Departments ofRadiotherapy, Publication PL/CMO 10, London (1991)
AMERICAN ASSOCIATION OF PHYSICISTS IN MEDICINE, Medical Accelerator
Safety Considerations Report of the AAPM Radiation Therapy Committee, Task GroupNo 35, Med Phys 204(1994) 1261-1275
INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, Safety Culture, Safety
Series No 75-INSAG-4, IAEA, Vienna (1991)

[15]INTERNATIONAL ATOMIC ENERGY AGENCY, Safety Assessment Plans for
Authorization and Inspection of Radiation Sources, IAEA-TECDOC-1113, IAEA,
Vienna (1999).

91

CONTRIBUTORS TO DRAFTING AND REVIEW
Andreo, P

International Atomic Energy Agency

Cohen, M

McGill University, Canada

Gultresa, J

Centre de Dosimetrfa, Spam

Haywood, J.

South Cleveland Hospital, United Kingdom

Novotny, J

Hospital Na Homolce, Czech RepublicOresegun, MInternational Atomic Energy Agency

Ortiz-L6pez, P.

International Atomic Energy Agency

Seng, D

Nuclear Regulatory Commission, United States of America

Stovall, M

University of Texas, United States of America

van de Geijn, J

Consultant, United States of America

Vivanco, J.

Hospital General de Astunas, Spain

Zufiiga-Bello, P

Consejo Nacional de Ciencia y Tecnologia, Mexico

Technical Committee Meeting
Vienna, Austria- 17-21 March 1992

Consultants Meetings
Vienna, Austria 16-20 October 1993, 26 November-7 December 1995,
18-29 November 1996

92

RELATED IAEA PUBLICATIONS
Accidental Overexposure of Radiotherapy Patients in San Jose", Costa Rica (1998).
Lessons Learned from Accidents in Industrial Irradiation Facilities (1996).
Lessons Learned from Accidents in Industrial Radiography, Safety Reports Series
No. 7 (1998).

93No. 9, June 1999

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