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NBS TECHNICAL NOTE 978

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U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement: coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials heeded by industry, commerce, educational

institutions, and Government: provides advisory and research services to other Government

Agencies: develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices: and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities: provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

Headquarters and Laboratories at Gaithersburg. Maryland, unless otherwise noted: mailing address Washington, D.C. 20234.

: Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
Nationwide Survey of Cobalt-60 Teletherapy Dosimetry

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WATIONAX, BTTREAO

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TectKrviCal AJ<^£- SEP 14*1978

C.G. Soares

M. Ehrlich

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Center for Radiation Research U-&1S3

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

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Sponsored by

Radioactive Materials Branch

Bureau of Radiological Health

Rockville, Maryland 20852

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

U S NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued August 1978
National Bureau of Standards Technical Note 978

Nat. Bur. Stand. (U.S.). Tech. Note 978,40 pages (Aug. 1978) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No. OO3-0O3-O1968-2 Price $1.60

(Add 25 percent additional for other than U.S. mailing).
Contents

Page

1. Introduction 1

2. Administrative Procedure 1 2.1 Contacting the Prospective Participants 1 2.2 The Initial Survey 2 2.3 The Follow-up Survey 4

3. Description of Dosimetry System 4

3.1 Choice of Detector 4 3.2 Readout Technique 5

4. Dose Interpretation From TL Response 7 4.1 Method 7 4.2 Uncertainties in NBS Method

of Determining Absorbed Dose from Dosimeter Response 12

5. Discussion of Survey Results 12

5.1 Dose Interpretation from Response of Irradiated Dosimeter 12 5.2 Sources of Error in Delivered Dose 14

6. Comparison With Results of Others 18

7. Acknowledgement 19

8. References 20

Figures in text

Tables 21-36

Nationwide Survey of Cobalt-60 Teletherapy Dosimetry

C. 6. Soares and M. Ehrlich

Between September 1974 and December 1977 the National Bureau of

Standards, in cooperation with the Bureau of Radiological Health,

performed a study of the accuracy with which a prescribed absorbed

dose of cobalt-60 gamma radiation is delivered to a specified point

in a water phantom. Approximately two-thirds of the cobalt-60

teletherapy units in the U.S. were surveyed by mail, using a rugged

thermoluminescence dosimetry system. The dose given by participants

was evaluated from dosimeter response, and information supplied by

participants was used to check their computations of the dose delivered.

In this nationwide study, 83 percent of the units surveyed yielded

dose interpretations within 5 percent of the requested dose, 13 percent yielded differences between 5 and 10 percent, and 4 percent of

the dose interpretations differed by more than 10 percent from the

dose requested. Sources of discrepancies are discussed, and the

results of this survey are compared with those of other dosimetry

surveys.

Key words: Absorbed dose; cobalt-60 gamma radiation; computation

check; dose interpretation; mailings; survey; teletherapy; thermoluminescence dosimeters; water phantom.

1 . INTRODUCTION

Cobalt-60 gamma ray teletherapy sources are widely used for cancer therapy, both in

this country and around the world. The accuracy with which a prescribed absorbed dose is

delivered to a tumor has been shown to be crucial to treatment success. ' In order to assess the ability of cobalt-60 teletherapy users in the United States to deliver a prescribed dose of radiation to a water phantom, the National Bureau of Standards (NBS) has

carried out a measurement assurance program using mailed thermoluminescence dosimeters.

Other programs of this type have been successful in assessing the performance of cobalt-60

teletherapy users elsewhere (see sec. 6). The survey described in this paper includes

over two-thirds of the approximately 1000 teletherapy facilities currently in use in this

country.

2. ADMINISTRATIVE PROCEDURE

2.1 Contacting the Prospective Participants

Lists of teletherapy licensees were supplied to NBS by the Bureau of Radiological

Health (BRH) at a rate of one list of about 50 licensees every six weeks. Each licensee

was sent a form letter (table 1 ) which explained the purpose of the survey and asked for

voluntary participation. Table 2 is a copy of the questionnaire which was included for the

recipient to indicate whether or not he was willing to participate. This form was to be

returned to NBS in a stamped self-addressed envelope which was enclosed. If there was no response from the licensee after four weeks, a follow-up form letter was sent (table 3),

including the same questionnaire and return envelope. This approach was quite useful in

obtaining additional participants. Almost half of the licensees who were sent follow-up

letters agreed to participate. _,_
Over the three years of the survey, there were 1020 licensees' names and addresses

supplied by BRH. All but 144 licensees returned the questionnaire. Of those returning the

questionnaire, 656 indicated willingness to participate*, while 74 refused participation.

The remaining 146 indicated that they did not have a cobalt-60 teletherapy unit at the time

of inquiry.

2.2. The Initial Survey

The initial survey comprised 24 mailing cycles, each approximately six weeks long and

consisting of the following phases:

a. Mailing Phase

Each mailing involved between 35 and 50 sets of six dosimeters. Approximately ten

days before the dosimeters were to be mailed, licensees who had expressed willingness to

participate were sent an announcement (table 4) that the dosimeters were to be mailed soon. Then, at the beginning of the six-week mailing cycle, one set of dosimeters was mailed to the participants for each cobalt-60 teletherapy beam to be surveyed. Included with each set

was a copy of the announcement sent the previous week, instructions for irradiation of the

dosimeters, and an information form, for the participant to complete.

The instruction form is shown in table 5. The participants were asked to compute the

time necessary to deliver 3.0 grays (300 rad) at a depth of 1 centimeter in a large water

phantom for a 10 cm x 10 cm field size and their normal treatment distance. They were then

to irradiate 5 of the 6 dosimeters in air, on top of the cardboard shipping carton, for this

time at this distance. The sixth dosimeter was to be left blank as a control. The essentials of the prescribed irradiation geometry are shown in figure 1. Participants were

asked that the irradiations be performed under conditions duplicating as closely as possible the routine patient irradiations, a nd that the individuals carrying out the irradiations be those routinely involved in ;nt set-up and treatment.

The three-page information form ( i 6) was in two parts. In the first part (page 1)

the participant was asked to supply time dnd date of irradiation, distance, field size,

irradiation time, and identification of the person doing the exposures and the person

supplying the information. This page could be readily completed by a technician and was

to be returned with the dosimeters. The second part (pages 2 and 3) was to be filled out

by the physicist in charge. It required information about beam calibration and parameters

used by the participant to determine the absorbed-dose rate used for the dosimeter irradiations. This latter information was used by NBS to check the participant's absorbed-dose

computation

.

b. Evaluation and Reporting Phase

After the dosimeters had been returned to NBS by the participants, dose interpretations

from dosimeter responses were made, according to the procedure described in section 4.1.

*Some of these licensees had more than one cobalt-60 teletherapy unit to be surveyed.

-2-
PRESCRIBED IRRADIATION GEOMETRY

DOSIMETER

BLOCK—>*

300 rods 12F*

SSD

TECHNIQUE

SSD

SAD

TECHNIQUE

SAD

±

MAILING CARTON

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Figure 1. Prescribed Irradiation Geometry. The participant was to

employ his usual distance and technique (either source-to- surface,

SSD, or source-to-axis, SAD).

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This generally occurred during the fifth and the sixth week of the mailing cycle. Upon

completion of this evaluation, each participant was sent the absorbed-dose interpretation

obtained from the response of each dosimeter involved in the particular mailing, with his own dosimeters identified. Also included were: the value of absorbed dose as computed by

NBS from the calibration information (if supplied by the participant), tables of factors

and methods used in this computation, and some general information about the survey. The

text of the report is given in table 7. Table 8 shows the various methods used to compute

absorbed-dose rate at a 1-centimeter depth in a water phantom. The factors employed in the

computation are listed in table 9. Copies of the reports were given to the Bureau of

Radiological Health but without participant identification. (Performance of all participants is known only to NBS and is treated as confidential information.)

2.3 The Follow-up Survey

As a part of the last mailing cycles of the initial survey and in one additional

mailing cycle, follow-up studies were performed. They were:

a. Statistical Verification of Survey Results

A sub-sample of the original participants was selected for further work. The chosen

sub-sample consisted of 10 percent of the participants whose results were within 5 percent

of the requested dose, one-half of the participants whose results differed from the re- quested dose by between 5 and 10 percent, and all participants whose results differed by more than 10 percent. A total of 143 participants were involved in this re-study,

b. Check on Influence of Incomplete Participation

It was furthermore desired to assess the influence upon the statistical survey results

of the fact that certain licensees had declined to participate or had not responded to the

initial invitation to participate. For this purpose, a randomly selected sample of 10 percent of this group was contacted and 13 of them were persuaded to participate at the end of

the survey. It was impossible to determine exactly how many of the licensees who did not

respond to the initial inquiries were no longer using a cobalt-60 therapy unit. However,

during the selection of participants from the non-respondents, it was found that one-third,

or 7 of the 21 licensees selected, were no longer doing cobalt-60 therapy. If this ratio

is applied to the entire group of non-respondents, then the 13 licensees who were persuaded

to participate in the last round represent eight percent of the licensees who had not re- sponded or declined to participate in the original survey.

3. DESCRIPTION OF DOSIMETRY SYSTEM

3.1 Choice of Detector

A mailable dosimetry system should be suitable for storing irradiation information and

should be mechanically rugged and thermally stable. For these reasons, thermoluminescence

(TL) detectors are generally chosen for mailed systems. They do exhibit a certain amount

of fading of the stored information, but the fading characteristics are well known. Also,

since ours was to be a long-term project covering a large number of facilities with relatively few dosimeters, it was necessary for the chosen detector to withstand repeated use.

-4-
Thus, the TL material chosen had to have a relatively small dependence of TL response on thermal and irradiation history. For this reason, CaF-iMn was selected. The commercially

available dosimeters consisted of two pieces of pressed crystalline CaF

2 :Mn in contact with

a metallic heater strip, enclosed in a quartz bulb with a neutral atmosphere. During shipment and irradiation, these bulbs were encased in black polystyrene blocks, as shown in

figure 2. One bulb each was placed into one of the halves of a block, each half having

dimensions 7.6 cm x 5.0 cm x 1.0 cm. The halves were then screwed together with Nylon

screws. The plastic was of sufficient thickness for maximum electron buildup in polystyrene

at the bulb surface in a cobalt-60 gamma ray beam. Black plastic was chosen since CaF

2 :Mn

is sensitive to prolonged exposure to visible light. The orientation of the bulb with

respect to the incident beam was fixed by bending one of the bulb pins. This also fixed the

orientation during readout, which was found to be necessary for good reproducibility. The

orientation chosen was such that neither of the TL pieces in the bulb was shielded from the

gamma ray beam by the heater strip, and that luminescence emission was not blocked by the

heater strip during readout.

Initially 280 bulbs were used for the survey. They were divided into two batches.

The first batch, purchased earlier, consisted of 88 dosimeters numbered from 3 through 91.

The second batch of 192 was numbered from 101 through 292. The two batches differed from

each other mainly in their average response, the former batch being more sensitive than the

latter batch. In order to increase the number of teletherapy units surveyed per mailing

cycle an additional set of 100 bulbs was purchased during the course of the survey. This

third batch was assigned numbers from 301 through 400. Its sensitivity was about midway

between the two earlier batches.

3.2 Readout Technique

The dosimeters were read out in a commercial assembly which included a bulb-heating

unit using resistive heating of the heater strip, and a cooled photomultiplier for converting light output to electric current. A current integrator was used to determine the

total TL signal, which was transferred automatically to paper tape upon completion of readout. For flexibility in the selection of readout parameters, the entire assembly was con- trolled by an electronic timer in a circuit designed at NBS to permit independent variation

of total heating and current-integration times.

Since the TL dosimetry system employed relied on annealing of the CaF

2 :Mn material

solely during readout, a readout technique was chosen that enabled one to read dosimeters

irradiated over the entire contemplated dose range without appreciable interference from

incandescence or from release of residual trapped electrons. Yet, to prolong the life of

the dosimeters, heating power was kept at a minimum. An annealing time of 21 seconds was

used throughout. For readout of dosimeters mailed to the participants and of all associated

calibration dosimeters which received exposures of 100 roentgens or more, the heater current was 6.0 amperes, which produced close to the highest permitted thermal load to the

dosimeter. For the three further readouts required during each cycle, a heater current of

5.5 amperes was sufficient. For details on calibration and readout see section 4.1.

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Initially, signal-integration times of 14 and 16 seconds for the 6- and 5.5-ampere readouts,

respectively, were used since they were found to produce maximum discrimination against

incandescence, while including most of the glow peaks. However, these integration times

were not sufficient for the readout of the most recently acquired batch of TL dosimeters,

whose main glow peaks lay at slightly higher temperatures than those of the older batches.

Thus, the integration times for all the dosimeters were changed to 16 and 19.5 seconds for

the 6- and 5.5-ampere readouts, respectively. With these longer integration times, it was

still possible to discriminate successfully against incandescence. Figure 3 shows typical

glow curves, with the beginning and the end of the current-integration periods noted.

4. DOSE INTERPRETATION FROM TL RESPONSE

4.1 Method

Prior to their initial use in the program, all dosimeters were administered a series of

ten identical cobalt-60 gamma ray exposures between 3 and 5 roentgens and were read out by

the method described below. The results of these successive readings yielded initial values

of individual response per unit exposure (sensitivity) and indications of the extent of re- producibility of dosimeter response. The relative standard deviation of repeated individual

dosimeter readings was found to range from about 0.1 to 2 percent, with an average of 0.8

percent.

During each mailing cycle, each dosimeter was read four times. The first readout

yielded the raw dosimeter response to the irradiation administered by the participant (or

by NBS if that dosimeter had been kept at NBS for use in the calibration procedure). The

three subsequent readouts were for correction purposes. Five days after the first readout,

each dosimeter was read a second time, for a determination of its residual response level.

Then on the next two days, each dosimeter was exposed to the same low level (3 to 5 R) and

read out approximately 4 hours later, for a determination of its individual sensitivity.

The data from each of the four readouts were fed into a Univac 1108 computer, and were

analyzed by a program developed for the purpose. This program used an algorithm involving

the determination of absorbed dose received by a dosimeter from dosimeter response, corrected for (a) relative sensitivity of the dosimeters to cobalt-60 gamma radiation, (b)

change in photomultiplier sensitivity during readout, and (c) fading. The following is a discussion of these three corrections:

(a) The sensitivity of each dosimeter was checked between successive mailings. This

was necessary since sensitivity was found to change with repeated use, as is illustrated

in figure 4. A correction for the difference in response of individual dosimeters was determined by exposing each dosimeter to the same low level (3 to 5 R) of cobalt-60 gamma

radiation, with the time intervals between irradiation and readout adjusted to eliminate

the need for fading corrections. The average of the responses after two such exposures,

performed on successive days, formed the basis of the sensitivity correction for each

dosimeter.* A furt her correction was required for each dosimeter because of the presence of

* It might have been preferable to use as the basis for the sensitivity correction the mean

of the two response values before and after a given irradiation-and-readout cycle. This

would have tended to average out the fluctuations in the sensitivity changes recorded

between successive irradiation and readout cycles (see fig. 4).

-7-
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Figure 3. Typical Glow Curves for the Three Dosimeter Batches. The

continuous lines represent the response to a 3-5 R exposure, while the

clashed line (drawn with the ordinate multiplied by four) represents

the sum of the residual glow curve from a previous 300-R exposure, and

incandescence. The current-integration period started at time zero and

extended to 19.5 seconds.

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-9-
residual thermoluminescence, the magnitude of which depended upon the absorbed dose re- ceived during the mailing cycle. This residual'TL was measured in a second readout of each

dosimeter five days after the first readout. For the irradiation and readout sequence em- ployed, this correction amounted to between 0.5 and 1.5 percent of the response to a 3 to 5 roentgen exposure.

(b) A correction for change of photomultiplier sensitivity during readout was necessary. To convert the TL signal to an electric current for integration, a bialkali-dynode

type photomultiplier was used at 620 volts. This voltage led to current signals of about

10 microamperes for the TL response to 300 rads. Under these operating conditions, the

photomultiplier gain was found to increase monotonically by almost 4 percent during the

roughly 3 hours required for the readout of about 270 dosimeters, of which about 200 re- ceived ^ 300 rads. A suitable correction for this increase was obtained with the aid of

readings taken at regular intervals during the readout sequence of the constant light source built into the reader. This gain change was probably due to insufficient cooling of

the photomultiplier at high signal levels, since an overhaul of the cooling fan motor decreased the effect considerably. After this overhaul was performed, the gain change during

readout was never larger than 2 percent.

(c) During the three years of the survey, fading characteristics of the dosimeters

were monitored at regular intervals. Initially the reduction of the TL signal with an in- crease in the delay between irradiation and readout ("fading") was found to be about 4 percent over the decade between 20 and 200 hours, while it was only about 2.5 percent over the

subsequent 500 hours. In the course of the survey, it was noticed that the first dosimeter

batch (lowest numbers) was fading to a lesser extent than the other batches, and that it was therefore necessary to apply two different fading corrections, one for the first batch

and one for the other two batches, which were found to exhibit the same degree of fading.

An example of the fading behavior is shown in figure 5, where TL response relative to that

obtained in a readout 24 hours after irradiation is plotted against time in hours between

irradiation and readout. From the figure it is seen that the correction for fading applied

to the readings of the dosimeters exposed by the participants varied between 3 and 7 percent

for the usually encountered 170-to-700-hour delay between irradiation and readout. The

time and date of irradiation. supplied by the participants on page 1 of the information form

(table 6) gave us the data necessary for the application of the appropriate fading correction.

The corrected dosimeter response was obtained by applying each of the above corrections

to the raw response data. Implicit in this procedure is the assumption that relative

dosimeter sensitivity is independent of irradiation level. Absorbed dose was derived from

the corrected response with the aid of an absolute dosimeter calibration carried out by

administering known cobalt-60 gamma ray exposures to a group of dosimeters in a geometry

identical to that used by the participants. Several dozen dosimeters were employed, with

the absorbed dose delivered being computed from exposure using method 3 (table 8) and

parameters from table 9. These calibration dosimeters were read out along with the

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dosimeter returned by the participants, and were handled identically in subsequent readouts

and irradiations.

4.2 Uncertainties in NBS Method of Determining Absorbed Dose from Dosimeter Response

A valid assessment by NBS of the participants' ability to deliver a prescribed absorbed

dose necessitated a knowledge of the uncertainty in the NBS procedure for determining absorbed dose from dosimeter response. In table 10, values are given for the systematic and

random uncertainties inherent in the NBS procedure for determining absorbed dose. The systematic uncertainties, stemming from the use of dosimeter-calibration and fading curves, were obtained from the least-squares fits of these curves. The individual systematic uncer- tainties were taken to be three times the average standard deviations of the predicted

values in the ranges of interest. The total systematic uncertainty then was derived as the

sum of the individual uncertainties listed.

The total random uncertainty was determined by considering both the limits in the re- producibility of the response of any one dosimeter (obtained prior to the start of the survey, see sec. 4.1), and the variation in the response from dosimeter to dosimeter. The

limit in the reproducibility of the response was taken to be three times the relative standard deviation, S, of the readings of a dosimeter after repeated irradiations and readouts

(2.4 %). This same reproducibility limit also is reflected in the correction factor applied to the reading of an individual dosimeter, to take into account the variation in re- sponse from dosimeter to dosimeter. The total random uncertainty in the determination of

absorbed dose from dosimeter response then was computed as the square root of the sum of

the squares of these two uncertainties (3.4 %).

The corresponding random uncertainty of the average of the absorbed-dose values obtained from the readings of five dosimeters then was obtained by division by V 5, finally

leading to an estimate of about 4 percent (or ± 12 rad at the 300-rad level) for the algebraic sum of the uncertainty due to the total systematic and random errors. This is taken

to be the total uncertainty of the NBS dose interpretation from the average response of

five dosimeters. The hard-to-assess uncertainties associated with the generally accepted

values of the parameters and constants used in the computation of absorbed dose from exposure were not included.

5. DISCUSSION OF SURVEY RESULTS

5.1. Dose Interpretation from Response of Irradiated Dosimeters

Figure 6 shows the distribution of the differences between the absorbed dose of 300

rads to be delivered by the participants and the NBS dose interpretations from the average

of the responses of the five dosimeters they irradiated. The total number of dosimeter sets

involved was 906, of which 751 or 83 percent yielded dose interpretations with ± 5 percent

of the requested dose of 300 rads in water. In view of the error analysis presented in the

previous section, differences of 5 percent or more (in either direction) in the average dose

interpretation must be considered significant. Seventeen percent or 155 sets yielded dose

interpretations differing by more than 5 percent from the requested dose, with 34 of these

(4%) differing by more than 10 percent. This distribution can be replaced by a Gaussian

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-13-
envelope with a width, a, of 3.8 percent. There is a positive bias in the data, the average

deviation for all sets shown being +0.7 percent. We believe that the reason for this bias

is that most participants used the conversion factor from exposure to absorbed dose in soft

tissue (.957) rather than that to absorbed dose in water (.965) which was used at NBS in

the dosimeter calibrations. (For the purposes of this study, we did not make a distinction

between absorbed dose in soft tissue and water, see table 5). The results for the participants who were selected for a re-survey are shown in figure 7. There was a definite improvement in the group for which the dose delivered initially differed from 300 rads by more than 10 percent. The survey results of the 143 selected participants show essentially

the same performance breakdown as the group as a whole, i.e., 83 percent within 5 percent,

12 percent between 5 and 10 percent, and 5 percent with deviations outside of 10 percent.

This is interesting because this group contained most of the participants who had performed poorly, but only 10 percent of those who had performed well.

A histogram of the results for the sample of initial non-participants who participated

at the end of the survey is shown in figure 8. Although the statistics are poor, they

indicate the same performance breakdown as for the other groups surveyed. Hence there is reason to believe that the fact that some licensees did not respond to the initial com- munications or refused to participate did not detract from the generality of our results.

5.2 Sources of Error in Delivered Dose

Only through laboratory visits is it possible to isolate all sources of error in

performance. The NBS study did not involve any laboratory visits. However, from the

information forms returned with the dosimeters, it was often possible to separate errors

in calculation of the absorbed-dose rate from errors in dose delivery. Distinct from both

of these were discrepancies which arose because participants misread or misunderstood the

instructions. Thus, in the course of the survey, we received dosimeters which had been

irradiated to 30, 200, 900 or 100 rads. The latter value occurred more than once, undoubtedly because of confusion caused by the concurrent Nuclear Regulatory Commission TLD survey. This type of misunderstanding of the instructions accounts for the discrepancies

greater than 25 percent shown in figure 6. We were able to ascertain that further errors resulted when participants failed to place the dosimeters on the shipping carton for

irradiation. This generally resulted in larger dose interpretations than expected because

of an increase in the back-scatter contribution to the dosimeter response. Also, irradiating dosimeter blocks upside-down sometimes led to small deviations (< 4%).

A relatively large number of errors concerned delivery of the prescribed dose. The

most common one involved improper setting of the mechanical timer by one minute in either

the positive or negative direction. This error was identified 15 times in the course of

the approximately 4500 doses that were delivered, indicating that the timer systems in

general use may be in need of improvement.

We later learned that other delivery errors had resulted from irradiation at improper

distances, transposition of digits in treatment times, attenuators not accounted for, dosimeters irradiated twice by mistake, and errors caused by timer and other equipment

-14-
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-17-
malfunctions. There even was a discrepancy that was traced by the participant to an error

in the computer output upon which his dose-rate chart had been based. Most of the above errors could also occur during patient irradiation. It should be noted however that the

exposure protocol for the dosimeters did not completely reflect the usual clinical setup

and therefore a greater incidence of human error was to be expected, especially for individuals who were participating in the survey for the first time.

Of interest also were the methods and factors used by the participants to derive

absorbed-dose rate in water from the calibration data. Most participants calibrate in terms

of exposure rate in air and use one of the first three methods shown in table 8 to convert

(?)

to absorbed-dose rate. Examination of some of the early results ' indicated that a majority of the errors made in this conversion stemmed from a failure to apply one or more of the

necessary factors, or from the use of incorrect factors. A full analysis of the results

correlated with calibration method is being done by the Bureau of Radiological Health.

6. COMPARISON WITH RESULTS OF OTHERS

In the United States, the only other comprehensive survey somewhat comparable to that

carried out by NBS was initiated on an emergency basis by the Nuclear Regulatory Commission

(NRC) at a time when the NBS study was more than half completed. In this study, for which

the portion comprising mailing of TL dosimeters was patterned after the NBS survey, all U.S.

teletherapy users who could not show satisfactory results from a participation in another

recognized independent national survey program (such as, e.g., the NBS-BRH voluntary survey

here described) were made to participate. A total of 592 units was checked, of which about

64 percent were found to come within 5 percent of the requested dose.* Inasmuch as the

results of this study showed a considerable bias in the direction toward delivery of lower

doses than those requested, while no such bias was found in our study, it cannot be ruled

out that this bias had its origin in the dosimetry system that had to be assembled rather

hurriedly. Thus, the overall performance conceivably was better than indicated by the

results. In states in which the NRC has inspection responsibilities, the mailed study was followed up by direct inspection and measurements, through which almost all the discrepancies could be satisfactorily resolved.

Preliminary results obtained at the Radiological Physics Center in Houston, in which

all but three of the 230 units surveyed by mail were found to have delivered within 5 per-

(4) cent of the requested dose, v ' cannot be considered to have been representative of perfor- mance throughout this country. These results were obtained during the first eight months

* The result quoted here is for both the states in which the NRC inspects the facilities

of all their licensees ("non-agreement states") and for the states in which the state health

departments have this function ("agreement states"). Both in the NBS and the NRC studies

there was no significant difference in the performance of teletherapy users in the two

categories of states. The NRC results quoted here are for both categories combined. The

data for the non-agreement states were reported by Dicey et alJ 3 ^ The data for the

agreement states are not as yet generally available.

-18-
of a continuing survey of institutions involved in certain interinstitutional clinical

trials. The institutions had been visited by physicists from the Center prior to being

surveyed by mail—although in some instances not for some time. Earlier results obtained

by this Center in personal visits to 174 institutions involved in clinical trials showed

that of the 352 teletherapy units checked and of the 768 associated tumor-dose prescription

(5) protocols reviewed 88 percent were within 5 percent of the prescribed tumor dose. K ' In our own mailed TL dosimetry study, we found that 83 percent of the 906 units surveyed delivered doses to a water phantom that were within 5 percent of the requested dose. If one assumes that institutions involved in clinical trials are representative of all institutions

administering teletherapy, this result would indicate that it is possible to obtain information from mailed TL dosimetry studies that correlates with that obtained in actual visits

to the institutions.

Results obtained from a cross section of national institutions by a secondary-standard

dosimetry laboratory are also available from India. ' There, 19 of the 32 teletherapy

units (59%) came within 5 percent of the requested dose, and four had results differing

by more than 10 percent. A continuing international survey is being conducted by the

International Atomic Energy Agency in conjunction with the World Health Organization.

The results for the years 1970 - 1975 * ' covering 417 measurements (including follow-ups

on some of the participants) showed about 63 percent of the delivered doses to be within

5 percent of the requested dose and 15 percent to differ by more than 10 percent.

7. ACKNOWLEDGEMENT

The authors wish to express their appreciation to Mr. J. M. Cameron, chief, Office of

Measurement Services (now retired) and to Dr. H. H. Ku, chief, Statistical Engineering

Division, for their assistance with the analysis of the uncertainties in the NBS method

for determining absorbed dose from dosimeter response.

19-
8. REFERENCES

1. Shukovsky, L. J., Dose, Time, Volume Relationships in Squamous Cell Carcinoma of the

Supraglottic Larynx, Am. J. Roentgenol., Radium Therapy and Nucl. Med. 108, 27-29 (1970)

fin 2. Ehrlich, M. , Welter, G. L., Nationwide Survey of Co Teletherapy Dosimetry, J. Res.

Nat. Bur. Stand. (U.S.), 80 A, 663-668 (1976).

3. Dicey, B. B., Moe, H. J., and Tongue, T. M. , Nationwide Evaluation of Teletherapy

Calibrations: A Project Report presented at the Annual winter meeting of the American

Association of Physicists in Medicine, Chicago, IL (1977); accepted for publication in

Medical Physics.

4. Hanson, W. F., Radiological Physics Center of the University of Texas System Cancer

Center, Houston, Texas, private communication.

5. Shalek, R. J., Kennedy, P., et al, Quality Assurance for Measurements in Therapy,

Proceedings of the Symposium on Measurements for the Safe Use of Radiation, NBS SP 456,

111-118 (1976); for sale by the Superintendent of Documents, U.S. Government Printing

Office, Washington, D.C. 20402

6. Sundararao, I. S., Naik, S. B.,et al , Evaluation of Dosimetry Accuracy and Uniformity

for Co Radiation Therapy, Proceedings of the International Symposium on National and

International Standardization of Radiation Dosimetry, Atlanta, GA (1977); to be published by the International Atomic Energy Agency, Vienna, Austria.

7. Eisenlohr, H. H., and Jayaraman, S., IAEA-WH0 Cobalt-60 Teletherapy Dosimetry Service

Using Marked Dosimeters. A Survey of Results Obtained during 1970-75, Phys. Med. Biol.

22, 18-28 (1977).

-20-
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

Table 1

Initial Inquiry Letter

As a part of a program of testing the National Measurement System, the

National Bureau of Standards, in cooperation with the Bureau of Radiological Health, is undertaking a study to determine whether the NBS methods

for making dosimetry standards available are adequate or need to be improved.

For this purpose, we are conducting a test of the accuracy with which a prescribed dose of cobalt-60 gamma radiation is delivered to a specified point

in a water phantom. This test will involve radiotherapy departments throughout the United States.

You are among those selected on a random basis for participation in this

study, which will be performed at no cost to the participants. If you are willing to participate, several thermoluminescence dosimeters will be

mailed to you for irradiation and subsequent return to us for evaluation.

We will then inform you of our dose interpretation for the readings on the

dosimeters, which is capable of revealing discrepancies of five percent or

more. While our study cannot be considered in any way to take the place of

a regular calibration of your unit, we believe that the information gained

will be of value to you and will provide some compensation for your efforts.

Please indicate on the enclosed form whether or not you are willing to

participate and, if you are, to whom all future correspondence should be

directed, including the report of the results. Return the completed form

in the enclosed stamped and addressed envelope. The performance of the

individual participants in this study will not be disclosed by us to anyone,

and only anonymous statistical compilations of the results will be reported

to the Bureau of Radiological Health or otherwise made public.

Sincerely,

R. Loevinger

Chief, Dosimetry Section

Center for Radiation Research

NOTE: A SELF-ADDRESSED STAMPED ENVELOPE IS ENCLOSED.

-21-
Table 2

Participation Questionnaire

1 am 60 willing to participate in the NBS Co teletherapy dosimetryI am not

assurance study. (Please strike out the phrase that does not apply.)

60

I have Co teletherapy units.

how many

Name and address for all future communications (please print or type)

Name

Organization

Street Address:

Telephone No. (Area Code)

(Date)

(Signature)

Note: If there are any times during the next few months when you would

be unable to participate because of vacations or the like, please

indicate.

-22-
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D.C. 20234

Table 3

Follow-up Inquiry Letter

We have not received your answer to our inquiry regarding your interest in cooperating in a cobalt-60 dosimetry survey.

We are anxious to include in our survey a large fraction of the

groups that we initially approached. We had selected these groups

on a random basis from among all those owning cobalt-60 teletherapy

equipment. If a large number of those approached were not to

participate, the validity of the conclusions drawn from our survey

results would suffer.

In case you meant to participate but happened not to return the

first questionnaire, please mail the one enclosed with this note,

if possible some time before For your con- venience, we also are sending you a self-addressed envelope that

needs no stamp.

Sincerely,

Margarete Ehrlich, Physicist

Dosimetry Section

Center for Radiation Research

Enclosures

-23-
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

Table 4

Announcement of Dosimeter Mailing

Thank you for letting us know that you are willing to participate in our comparison of dose calibrations of 60<3 O gamma-ray therapy units.

We should like to reemphasize that this study will be carried out free

of charge, that it is confidential, and that only anonymous statistical

compilations of the results will be made public, but that each participant will be notified of his own results.

Co

On or about one set each of six dosimeters per 60

teletherapy unii. will be mailed to you. The dosimeters consist of

CaF2:Mn the rmo luminescence material in glass bulbs, contained in black

plastic blocks. Five of the six dosimeters are for irradiation, one

is a control that should be kept away from radiation at all times.

Please do not open the black blocks. The bulbs inside are lightsensitive and fragile. Along with the dosimeters, we are going to

mail you

instructions;

an exposure-information form;

a return-address label requiring no postage; and

a copy of this letter.

Please carry out the irradiations within three workdays of the receipt

of the dosimeters. Follow the instructions in detail and return all

dosimeters and the completed exposure-information form in the original

mailing carton. Also, note that for the study to yield realistic results

the dosimeter irradiations should be performed under conditions duplicating as closely as possible the routine patient irradiations and the

individuals carrying out the irradiations should be those routinely

involved in patient set-up and treatment.

We may not be able to evaluate the doses received by dosimeters

reaching us after

Sincerely,

Uv>U^\A,<X(_,

Of!

Margarets Ehrlich, Physicist

Dosimetry Section

Center for Radiation Research

telephone: (301) 921-2366 or 2361

-24-
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

Table 5

Page 1 of 3 pages

Instructions for Irradiating the Dosimeters

1- Please carry out the irradiations as soon as possible after the

dosimeters arrive, if possible mailing them back within three working

days.

2- When you are ready for the exposures, remove all six black plastic

dosimeter blccks from the mailing carton. (Five of these are for

irradiation, one is a control.) Take the mailing carton and one block

at a time into the treatment room. Turn the carton on its side, place

it on top of your treatment table, and center the block, screw heads up, on the carton.

3- Use yo«r normal treatment distance unless it is smaller than 50 cm, in which case use a distance of 50 cm. If you normally measure your

distance to the surface ("SSD Technique") ; follow the instructions in

3.1. If you normally measure distance to the tumor ("SAO Technique"),

follow the instructions in 3.2.

Please note that while the exposures are not to be made with the dosimeter

blocks in a phantom, the absorbed-dose rate and the exposure time should

be computed as if. the dosimeters (located in the plane of the crack in

the block) were at a depth of 1 cm in a large water phantom.

Also note that for the study to yield realistic results the dosimeter

irradiations should be performed under conditions duplicating as closely

as possible the routine patient irradiations and the individuals carrying

out the irradiations should be those routinely involved in patient set-up

and treatment.

PLEASE DO NOT ADD AMY PHANTOM MATERIAL.

-25-
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

Table 5» continued

Page 2 of 3 pages

Instructions for Irradiation the Dosimeters, cont,

3 . 1 SSD Technique (See Sketch 3 .1)

a- Measure the source-surface distance to the proximal surface

of the block.

b-Set the field size at the measured distance to 10 cm x 10 cm. c- Set the timer to deliver 300 rads calculated at a 1 cm depth

in water (soft tissue*) and expose.

d- Proceed with step 4.

SKETCH 3.1, SSD Technique

SKETCH 3.1, SSD Technique

SSD to

surface of block

dosimeter block

deliver 300 rads

here U 1 cm

mailing carton

*For this study, absorbed dose in water and in soft tissue will be

assumed to be the same.

-26-
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

Table 5, continued

Page 3 of 3 Pages

Instructions for Irradiating the Dosimeters, cont .

3.2 SAD Technique (See Sketch 3.2) a- Measure the source-axis (or source- tumor) distance to the center

of the block (where the crack is) , i.e., 1 cm below the surface.

b- Set the field size at the measured distance to 10 cm x 10 cm.

c- Set the timer to deliver 300 rads calculated at a 1 cm depth in

water (soft tissue*) and expose.

d- Proceed with step 4.

SKETCH 3 . 2 , SAD Technique

r^

SAD to

center of block

dosimeter block

deliver 300 rads

here lllHl "2 1 cm

mailing carton

4- Irradiate five of the six blocks identically,

unexposed to serve as a control.

One block must remain

5- Fill out the Irradiation-Information Form completely( three pages).

If possible, have your physicist complete pages 2 and 3« The information

on these pages would enable U3 to determine how abasrbed-dose rate and

exposure time for the irradiation of the NBS dosimeters were obtained,

6- Place all six dosimeter blocks and the exposure-information form back

into the mailing carton and return the carton to us by mail, using the

enclosed address label which requires no postage

*For this study, absorbed dose in water and in soft tissue will be assumed

to be the same. _27_
Table 6 Pa8e X of 3 Pa8es

IRRADIATION INFORMATION FORM

( TO BE RETURNED WITH THE DOSIMETERS )

I. Please supply all pertinent information on this page; then proceed to

pages 2 and 3. The information on this page is essential to the interpretation of the dosimeter readings; therefore this page should be returned

with the dosimeters. Pages 2 and 3 may be returned at a later date.

Date of irradiation:

Time of day of irradiation:

Distance to dosimeter-block surface, SSD = cm or

Distance to point at which 300 rads delivered, SAD = cm.

Field size at specified distance cm x cm.

Irradiation time (actual time, not corrected timer setting): min

sec.

Dosimeters exposed by

(name, title)

* * *

This page was filled out by

(name, title)

on

(date)

PLEASE DC NOT FORGET TO RETURN THE PACKAGE CERTIFIED MAIL

AND TO OBTAIN AND SAFE-KEEP THE CERTIFIED KAIL NUMBER

-28-
Table 6, continued Page 2 of 3 pages

II. Please supply all pertinent information on this and the next page, if possible.

If there is a significant difference between our dose interpretation and the dose

that you believe you delivered to the dosimeters, the information on these pages

will enable us to determine if this difference is due tc calculation error.

1- Type of machine mount (circle one ) : Vertical Isocentric

2- Calibration

The unit was last calibrated (fill in whichever is applicable)

2.1 in terms of exposure rate (no phantom) for a field size of cm x cm. at a distance of cm; exposure rate: R/min on (date);

2.2 in terms of exposure rate in a large phantom for a field size of cm x

cm, at a SSD of cm, at a depth of cm; exposure rate : R/min

on (date) ; 2.3 in terms of absorbed-dose rate in an equilibrium mass of water

("nu.niph.antorn") for a field size of cm x cm, at a distance

of cm to the center of the miniphantom; absorbed-dose rate: rad/min

on (date) ; 2.4 in terms of absorbed-dose rate in a large phantom1

for a field size of cm x cm, at a SSD of cm, at a depth

of cm; absorbed-dose rate: rad/min on (date).

2.5 other (specify):

3- Absorbed-Dose Computation

3.1 Factors used

Please note: Not all of the factors listed can be used in any one calculation. Do not specify values for factors that you did not use. 3.1.1 Source-decay correction for time elapsed between calibration

and irradiation date:

Please specify phantom material (check one or write in)

Water Polystyrene Lucite Other:

2 For 60 Co radiation, a sphere of water of 0.5cm radius is an equilibrium mass, i.e.^

just large enough for electronic equilibrium.

-29-
Table 6, continued Page 3 of 3 pages

3.1 Factors used, continued

3.1.2 Exposure-to-absorbed-dose conversion factors

F factor:

f factor:

CA

:

attenuation correction:

(cap correction, A„, A ) C eq

displacement correction:

otbers:

3.1.3 Inverse-square correction from cui to cm:

3.1.4 Tissue/air ratio:

3.1.5 (Back) scatter factor, i.e., buildup factor due to scatter in

large phantom:

3.1.6 Percent depth dose:

3.1.7 Others (explain):

3 . 2 Result of computation of absorbed -dose rate for use in irradiation of the

set of NBS dosimeter blocks :

Absorbed-dose rate at 1 cm depth in large water phantom: rad/min.

* * *

Pages 2 and 3 of this form filled out by on_ (name, title) (date)

-30-
Table 7

Text of Report Sent to Participants

ACCURACY WITH WHICH A PRESCRIBED ABSORBED DOSE

OF COBALT-60 GAMMA RADIATION IS DELIVERED TO A SPECIFIED POINT IN WATER

Report on the Performance of the

Participants that were sent Dosimeters in

by

Margarete Ehrlich and Christopher Soares

Dosimetry Section, Center for Radiation Research

National Bureau of Standards

This study was undertaken under Interagency Agreement FDA-IAG 74-41 (0),

Modification No. 1, between the National Bureau of Standards and the Food

and Drug Administration, Bureau of Radiological Health.

31-
Table 7, continued

Attached are the results and some pertinent background data for the

survey involving participants ( sets). The following

information is supplied.

Table la* - A list of the major methods in use for computing absorbeddose rate at a depth of 1 cm in a water phantom from the calibration data

of a cobalt-60 gamma-ray source referred to the date of irradiation. NBS

used eq. (3) in the computation of absorbed-dose rate from exposure parameters for the NBS cobalt-60 calibration source. For checking the participants' computed doses, NBS used the numerical values listed in Table lb

for the pertinent parameters and constants.

Table lb *- A list of symbols used in Table la and of values for pertinent

parameters and constants.

Table 2 - This table contains the following information:

(a) A list of the individual absorbed-dose interpretations obtained for

all irradiated dosimeters, the average dose interpretation for each participant, and the percent difference, A, between the NBS dose interpretation

and the dose to be delivered (300 rads in water, unless otherwise indicated).

Note that the red arrow points to the results of the participant to whom

this report is being mailed .

Absorbed dose was computed from the readings of the dosimeters irradiated

by the participants after suitable corrections for differences in individual

dosimeter sensitivity, and for fading. Differences A outside the NBS overall

error limits (see below) are an indication of wrong source calibration or of

wrong delivery parameters (such as, faulty positioning, wrong field size,

wrong timer setting, faulty timer) or of improper computation (such as wrong

equations, wrong values for the parameters and constants, errors in arithmetic)

In this paper shown as tables 8 and 9.

32-
Table 7, continued

(b) A list of the doses computed by NBS from the information supplied

by the participants, and of • the methods according to which the computations

were carried out. A discrepancy between the computed dose and the dose to

be delivered (300 rads in water, unless otherwise indicated) is an indication of improper computation.

Figure 1 - This is a histogram of the percent difference, A, between

the average NBS dose interpretation obtained from all the individual interpretations for a participant, and the dose to be delivered (300 rads).

Details on the dosimetry system used and on the method of dose evaluation

will be published in the future, along with a review of typical survey

results. At this point, it suffices to state the following results of our

error analysis:

1 - The bound for the total systematic error of our method is estimated

to be ^2% percent.

2 - The bound for the total random error in our dose interpretation

is estimated to be

a - %3 percent for readings on a single dosimeter;

b - ^V-2 percent for the average of readings on five dosimeters.

3 - Therefore, the overall error in the average NBS dose interpretation

for readings on five dosimeters is estimated to be less than 4 percent.

\U\N AJ-oCLl lUnyK

Margarete Ehrlich

Date

A bhristophe'r b>otfres
Table 8

Method for Computing Absorbed-Dose Rate at a

Depth of lcmin a Water Phantom

Method

Number

Source Calibrated for Method for computing D

at 1cm

X at SSD^)

(no phantom)

X at SSD + 0.5cm

(no phantom)

X at SAD

(no phantom)

X at depth d cm

(in phantom)

(a) (X) [SSD/(SSD+0.5)] 2 (f) (A) (BSF)(%DD)

1

(b) (X)[sSD/(SSD+l)] 2 (f)(A)(TAR)

1

cm

cm

(X)(f)(A)(BSF)(%DD)

1 cm

(X)(f)(A)(TAR)

1 cm

D at center of water miniphantom,'

positioned at SSD

(X)(f)(A)(%DD).lcm

/(%DD)

d _cm

(a) (D) [SSD/(SSD+0.5)] 2 (BSF)(%DD)

1

(b) (D) [SSD/(SSD+ 1)] 2 (TAR) cm

cm

D at center of water miniphantom,

positioned at SSD + 0,5 cm

D at center of water ndniphantom,

positioned at SAD

(a) (D)(BSF)(%DD)

1 cm

(b) (D) [(SSD+0.5)/(SSD+ 1)] 2 (TAR)

1 cm

(D) (TAR).

D at depth d cm in phantom

1 cm

(D)(%DD) 1 /(%DD)

1 cm d cm

(1) All distances are in cm.

( 2 > ,M 60 For Co gamma rays, a water sphere of 0.5 cm radius is a "miniphantom",

i.e., a phantom just large enough for establishing electronic equilibriu

at its center.

jquilibrium

-34-
Table 9

Parameters and Constants Entering Into

Absorbed-Dose Computations

Symbol Quantity Value and source,

where applicable

X exposure rate —

D absorbed-dose rate —

SSD source-to-surface distance —

SAD source-to-axis distance —

f (D) /(X) obtained under

water 'air,

electron 0.965,0) ICRU< 2 ) equilibrium conditions

A attenuation correction, cap correction, etc. 0.985, Johns and

Cunningham ()

F (f)(A) —

BSF (back/) scatter factor BJR Supplement ^

%DD percent depth dose BJR Supplement u

TAR tissue/air ratio BJR Supplement^

(1)

(2)

(3)

00

(5)

Vfe use the value for water in our calculations for this survey. For

soft tissue, the corresponding ratio is 0.957#

Table IV. 1, ICRU Report 10d(NBS HB 87), 1963.

Table IX. 1, p. 274, The Physics of Radiology, Third Edition, 1969.

Tables 6.1 through 6.4, BJR Supplement No. 11, 1972.

Table 6.5, BJR Supplement No. 11, 1972.

-35-
Table 10

Uncertainties in NBS Method of Determining

Absorbed Dose from Dosimeter Response

Systematic Uncertainty

NBS exposure calibration of Co-60 Gamma-ray source .... 0.7%

Uncertainty in dosimeter calibration .... 0.7%

Uncertainty in dosimeter fading correction 1 .0%

Total 2.4%

Random Uncertainty

Reproducibility of individual dosimeter response (3S). . 2.4%

Correction for sensitivity variation between dosimeters . 2.4%

Total for corrected individual dosimeter response . 3.4%

Total for average corrected' response of five

dosimeters 1.5%

Total Uncertainty in dose interpretation from average of

five dosimeters «v 4.0%

-36-
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 978

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Nationwide Survey of Cobalt-60 Teletherapy Dosimetry

5. Publication Date August 1978

6. Performing Organization Code

7. AUTHOR(S)

C. G. Soares and M. Ehrlich

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Radioactive Materials Branch

Bureau of Radiological Health

Rockville, Maryland 20852

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Between September 1974 and December 1977 the National Bureau of Standards, in

cooperation with the Bureau of Radiological Health, performed a study of the accuracy

with which a prescribed absorbed dose of cobalt-60 gamma radiation is delivered to a specified point in a water phantom. Approximately two-thirds of the cobalt-60 teletherapy units in the U.S. were surveyed by mail, using a rugged thermoluminescence

dosimetry system. The dose given by participants was evaluated from dosimeter

response, and information supplied by participants was used to check their computations of the dose delivered. In this nationwide study, 83 percent of the units

surveyed yielded dose interpretations within 5 percent of the requested dose, 13 percent yielded differences between 5 and 10 percent, and 4 percent of the dose interpretations differed by more than 10 percent from the dose requested. Sources of

discrepancies are discussed, and the results of this survey are compared with those

of other dosimetry surveys.

'7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Absorbed dose; cobalt-60 gamma radiation; computation check; dose interpretation;

mailings; survey; teletherapy; thermoluminescence dosimeters; water phantom

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