NBS TECHNICAL NOTE

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

A Thermoluminescence Dosimetry

System for Use in a Survey

of High-Energy Bremsstrahlung

Dosimetry
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A Thermoluminescence Dosimetry System

for Use in a Survey of Higli-Energy

Bremsstrahlung Dosimetry

Margarete Ehrlich

Christopher G. Scares

MATTONAL BtmSAtl

OF fftXVOAKDB

LIBRARY

MAY 1 3 1980

Quoo

no. 11/9

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

Sponsored by Medical Physics Branch Bureau of Radiological Health

Rockville, MD 20852

.^< °^^o.

4r

%.

Vau ov *

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued May 1980
National Bureau of Standards Technical Note 1119

Nat. Bur. Stand. (U.S.), Tech. Note 1119, 47 pages (May 1980) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

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

Price $2.50

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

Page

1. Historical Review of Work Performed 1

2. The Selected TLD System 4

2.1 Dosimeter Handling . . . , 4

2.2 Annealing 5

2.3 Calibration Procedure 5

2.4 Timing of Irradiation and Readout Sequences 7

2.5 Reader Characteristics 7

3. Data Retrieval and Handling System 8

4. Results of Experiments 8

4.1 Dependence of LiF (TLD-100) Sensitivity on Photon

Energy in the Range from about 10 keV to about 1 MeV . . . 8

4.2 Fading of LiF (TLD-100) Response Under Laboratory

Conditions 14

4.3 Influence of Size of Lucite Phantom on Dosimeter

Response for the Spectra of Interest 14

4.4 Influence on LiF (TLD-100) Response at a Given Phantom Depth

in the Presence of Dosimeters in Several Other Depths . . 18

4.5 LiF (TLD-100) Response at a 10-cm Depth in the Lucite

Phantom for Irradiation with Cobalt-60 Gamma Radiation

and with High-Energy and Low-Energy Bremsstrahlung ... 18

4.5.1 LiF (TLD-100) response as a function of irradiation

level 20

4.5.2 LiF (TLD-100) response at a given irradiation level

to high-energy bremsstrahlung relative to that to

cobalt-60 gamma radiation 23

4.6 Relative Depth Dose in Lucite Phantom Derived from

LiF (TLD-100) Response 23

4.7 Dependence of LiF (TLD-100) Sensitivity on Irradiation

History 27

5. Results of Pilot Comparison Study 29

6. Analysis of the Magnitude of the Uncertainty Expected for the

Planned Survey for Individually and Batch-Calibrated Dosimeters . 32

6.1 Procedure 32

6.2 Reproducibility of Dosimeter Response 33

6.2.1 Procedure for characterizing reproducibility by

batch (dosimeter identity not maintained) .... 33

iii
Contents, continued

Page

6.2.2 NBS experience with batch characterization of

reproducibility of dosimeter response 33

6.2.3 Procedure for characterizing the reproducibility of

individual dosimeters (identity maintained) ... 34

6.2.4 NBS experience with characteristics of the reproducibility

of individual dosimeter response 34

6.3 Estimate of the Uncertainty in the Interpretation of Survey

Doses, Based on a Statistical Analysis 34

6.4 Check on Uncertainty Estimates by an In-House Simulation

of One Survey Cycle 37

7. Recommendations 39

8. Acknowledgement 40

9. References 41

Figures in text

Tables in text

IV
A Thermo! uminescence Dosimetry System for Use in

a Survey of High-Energy Bremsstrahlung Dosimetry

Margarete Ehrlich and Christopher G. Soares

This is the final report covering the work performed under an interagency agreement between the National Bureau of Standards and

the Bureau of Radiological Health during the fiscal years 1978 and

1979. A thermol uminescence dosimetry system suited for a survey of

high-energy bremsstrahlung in U. S. radiation-therapy departments

was selected and calibrated. The experiments leading to the

choice of the recommended operational characteristics, including

dosimeter handling, annealing and readout, dosimeter stability in

the contemplated mode of operation, dosimeter response over the

photon-energy range to be covered, irradiation geometry and irradiation level are treated in detail. Results are reported of a pilot study involving the shipment of a typical survey assembly (a

plastic phantom loaded with a set of dosimeters) for irradiation in one U. S. therapy department and the overall uncertainty of the

proposed survey procedure is discussed.

Key words: Calibration; dosimetry; high-energy bremsstrahlung;

operational characteristics; radiation therapy; recommendations;

selection of thermol uminescence dosimetry system; survey; thermo1 uminescence; United States.

1. HISTORICAL REVIEW OF WORK PERFORMED

In early June 1977, the Bureau of Radiological Health, Division of

Electronic Products (BRH), started negotiating with the National Bureau of

Standards, Center for Radiation Research (NBS), the establishment of a contract to design, evaluate and calibrate a mailable dosimeter system

suited for a survey of the dosimetry in bremsstrahlung beams used in the

United States' radiation-therapy departments. It was agreed that, initially,

beams with maximum bremsstrahlung energies between 4 ancT 10 MeV were to be

considered and that, if possible, the survey system was to be similar to the one used by the BRH for other ongoing dosimetry surveys. The system was to

be capable of dose interpretation with an uncertainty of less than 5 percent.

In anticipation of the formal Interagency Agreement for this work, NBS

proceeded with the procurement of a hot-nitrogen thermol uminescence dosimetry

(TLD) reader (Harshaw Model 2000D)* similar to the readers employed by BRH

*Commercial product identification does not imply a recommendation or endorsement by NBS, nor does it imply that NBS considers the identified products to

be the best available for the purpose.

-1-
and with studies Ca) to ascertain wKether tKere existed a coMnercially

available thermoluminescence CTL) material with fewer drawbacks tfian the LiF

CTLD-100) material now in use by BRH; and [b] to determine whether it would

be feasible to dispense with furnace annealing of the dosimeters between use

cycles. These studies were started prior to the installation of the hotnitrogen reader which was ordered in June 1977 and arrived tn February 1978,

and were then continued and concluded after reader installation. Based on the

results of these studies, ^ we concluded that industry had not as yet

solved all the problems with hygroscopy and fading of LipB.OyCMn) which

otherwise has certain advantages over LiF as a thermoluminescence phosphor.

Therefore, we agreed to focus all further work on LiF CTLD-100) which is in use at BRH for several other surveys.

At the time the Interagency Agreement was formally established CMay 1978), we were in the process of devising a semi-automated data-collection and handling system, of carrying out a long-term fading study and of deciding on the

phantom types to construct for future tests. A few months later, during the

annual summer meeting of the AAPM, an information exchange between the AAPM

Therapy Committee, BRH and NBS took place. NBS had suggested this meeting in

order to ensure that the AAPM Therapy Committee was informed at an early

planning stage of the survey envisaged by BRH. As a result of the information

exchange, the chairman of the Therapy Committee assigned a liaison committee

under N. Suntharalingam's chairmanship to assist with technical and administrative problems that might arise in connection with the NBS project and the

subsequent BRH survey. The liaison comnittee met with interested BRH and NBS

staff members in Washington in October 1978 and made valuable technical

suggestions. It was decided that, in the future, the liaison conmittee would

be apprised of major developments mainly by informal telephone conversations

and also would be sent copies of major reports and papers on the subject.

During the last four months of the initial year of the Interagency

Agreement, the tests of a water and a Lucite phantom were completed. Based

on the results of this study and following the suggestions of the AAPM

liaison committee, it was decided to do all further work with the Lucite

phantom which, although resulting in a more expensive mailing kit, would give

a higher degree of reproducibility than a mailing kit similar to that used by

the lAEA.^^^ (Note that the IAEA holder is loaded with dosimeters

-2-
by the participants, who then insert it into their own water vat.) During

this same period, the data-processing system was essentially completed and

immediately put to use in the evaluation of a large body of information that

had accumulated on the reproducibility of dose interpretation from dosimeter

response obtained with the chosen system under controlled conditions both

with batch- type and with individual dosimeter calibration. This study was completed early during the second year of tlie Interagency Agreement and led

to a decision on the minimum number of dosimeters required per data point for

* obtaining the desired accuracy level for the dosimetry survey. Also,

mainly because of the uncertainties added in each additional calibration

step, NBS suggested that BRH eliminate the intermediate step of a 100-keV

bremsstrahlung calibration from their survey plans.

During the first months of the second year of the Interagency Agreement,

studies were completed also on minimum phantom size and on the feasibility of

irradiating dosimeters simultaneously at several phantom depths. Then, in

March 1979, a pilot study involving only one participant was initiated. The

selected phantom, loaded at four depths with the minimum number of required

dosimeters, was shipped to Massachusetts General Hospital for irradiation

with 10-MV bremsstrahlung and returned to NBS for an evaluation of dosimeter

response (TL signal) in terms of cobalt-60 absorbed dose to water, using most

of the correction and calibration factors to be employed in the final survey

(i.e., the corrections for individual dosimeter response and for fading and

the cobalt-60 absorbed-dose calibration factor). A report on this and other

studies carried out under this Agreement was included in a paper presented in

late March, 1979 at the IAEA meeting of the Advisory Group on High Energy

Photon and Electron Dosimetry. The report is contained in a paper that will

be published in an IAEA document on the proceedings of this meeting. A copy

of the preprint of this paper has been made available to BRH. A copy also

was made available to the chairman of the AAPM liaison committee.

*

In the summer of 1979, a report on this phase of our work was prepared

after inclusion of further information gathered during our small pilot study

(see second paragraph above) and consultation with the NBS Center for Sta-"

tistical Engineering. The report was made available to BRH and to the

chairman of the AAPM liaison committee and is encompassed by this Final

Report.

-3-
In a follow-up discussion between NBS and BRK in March 1979, BRH decided

that the participants would be asked to cojnpute the dose delivered for a depth of 10 cm in water rather than for tPie 5-cm depth at which many of them

obtain their calibrations. In April 19.79, as a result of discussions at the

IAEA meeting in which Robert Morton, Chief of the BRH Therapy Branch, participated, it was agreed that BRH, in their instructions to the participants,

should ask them to administer to the Lucite phantom, at a depth of

10 cm, the dose computed for a depth of 10 cm in water, i.e., carry out the

irradiations a£ i_f the phantom were water rather than Lucite. As a result, we decided to determine high-energy bremsstrahlung absorbed dose at a depth

of 10 cm in Lucite. We also decided to carry out the main bulk of the highenergy bremsstrahlung calibrations at the IQO-rad level. This removes the

need for additional annealing procedures to eliminate interference from

spurious signals, possibly associated with deeper, hard-to-empty electron

traps.

Finally, during the summer and fall of 1979, NBS determined dosimeter

response in the agreed geometry to 4-, 6-, and IQ-MV clinical bremsstrahlung

beams relative to that to cobalt-60 gaima radiation. The comparison at 6 MV

between the absorbed-dose computation from ion-chamber response and C^

values to that from absorbed-dose calorimetry was initiated, but so far has

not been completed. During this period, NBS also assisted BRH in a calibration of the BRH batch of LiF (TLD-100) in the NBS cobalt-60 and 100-kV

facilities, and — toward the end of September 1979 -- started on the cobalt-60

gamma-ray calibration of five BRH ion chambers in terms of exposure and

absorbed dose. In the following sections, the final results of the NBS

studies on the selected TLD system are presented in detail and recommendations

are made regarding the BRH survey procedures.

2. THE SELECTED TLD SYSTEM

Since the TLD system used for most of the studies is similar to the systems available at BRH,^ ' no physical description is given here. The remarks

are restricted to a description of the dosimeter handling, annealing, calibration and readout procedures.

2.1 Dosimeter Handling

The LiF (TLD-100) dosimeters were never exposed to direct sunlight or any other illumination containing a large ultraviolet component. A vacuum

pickup was used for dosimeter transfer.

-4-
2.2 Annealing

Between experiments, the Lif CTLD-100) dosimeters were annealed in

blocks of 100 on a stainless-steel tray shown in figure 1. Pre-irradiation

annealing consisted of first placing the loaded trays for 1 hour in a 2-KW

muffle furnace maintained at a temperature of 400 C; then allowing them to

cool for 30 minutes on a Transite (asbestos) surface and subsequently placing

them for 1 hour in a CENCO laboratory oven jnaintained at 100°C. After another

cooling cycle on the Transite sheet, the dosimeters were ready for the next

irradiation. Immediately preceding readout, the dosiineters were loaded into

the Harshaw disc- type readout holders and the holders were placed in the

100 C CENCO oven for another 10 minutes of heating, followed again by cooling

on Transite.

2.3 Calibration Procedure

Characterization of dosimeter response (TL signal) can be done either on

a batch or an individual basis. Both involve reproducible reference irradiations. For all experiments described, these irradiations were performed in

a cobalt-60 gamma-ray beam with the dosimeter under an equilibrium thickness

of 5 mm of Lucite. Exposures were administered for a fixed length of time

(90 s), and were in the 4-to-6 R range. If dosimeter identity is maintained

during this procedure, it is possible to correct the subsequent response of

each dosimeter to its reference response and hence eliminate the uncertainty

due to differences in sensitivity between individual dosimeters. We performed

this type of individual calibration between any two successive uses of the

dosimeters. The sensitivity assigned to each dosimeter in a particular use cycle was the average of the sensitivities obtained in the preceding and the

following calibration cycle.

An alternate approach is to select a batch of dosimeters such that the

dosimeter sensitivity falls within a certain range of values. The average

of this group of sensitivity values is then used to characterize the batch

for comparisons between experiments. We found that when the histories of all

dosimeters in a batch were similar, the dispersion of dosimeter sensitivities

(as given by the standard deviation from the mean) reproduced well over many

*

In this Report, dosimeter response (TL signal) at a given irradiation level

(or per unit of the irradiation quantity) for brevity's sake is also referred

to as "sensitivity", regardless of whether the irradiation quantity is ex- posure or absorbed dose.

-5-
ANNEALING TRAY

Fig. 1 Stainless-Steel Annealing Tray. Each square cutout holds 25 LiF (TLD-100)

dosimeters.

-6-
use cycles. In general, the batch-calibration approach leads to less precise

results, hi)weyer, since, in practice, it is not feasible to select a dosimeter

batch with inittal sensitiytties differing from each other by less than ±3 to

±5 percent and dosimeter histories are not necessarily similar, a condition

that may lead to significant changes in sensitivity. (See also Section 4.7

of this Report.

1

2.4 Ttroing of Irradiation and Readout Sequences

For most of the experiments described, irradiations were made in the

afternoon, with readout the following morning, i.e., after approximately 16

to 18 hours. This allowed for extended irradiation periods (up to as much as

2 hours) without any significant difference in fading of dosimeter response

between Irradiation and readout.

2.5 Reader Characteristics

All readouts were performed with the hot-nitrogen reader, Harshaw Model

2000D, at a gas temperature of 300°C (,±10°C) and a nitrogen flow rate of 4.0

C±0.3) £/min. This was the maximum flow rate attainable with this reader at

this temperature. Some time after we had started our experiments, Harshaw

modified the heater design, increasing the diameter of the gas-inlet tube to

the reader chamber. This had the effect of increasing the effective heating

volume and resulted in faster heating of the dosimeters and reduced dependence

of dosimeter response upon dosimeter orientation and position in the gas

stream. Experiments with the original heater indicated better reproducibility

of responses and glow curves with the dosimeters oriented parallel to the

direction of gas flow. This effect all but disappeared with the newer design

and, in fact, after installation of the new heater, we determined that there

was a slight enhancement in response (< 1%) with the dosimeters oriented 45°

with respect to the flow direction. However, we continued to perform all

readouts with the parallel orientation. Also, proper alignment of the vacuum

pick-up used to raise the dosimeters into the gas flow was quite critical

with the original heater design, but not nearly as critical with the later

design.

Another consequence of the new heater design was an increase in photomultiplier (PMT) dark current. This is probably due to the larger heating

volume and insufficient PMT cooling. However, at the irradiation levels em- ployed, dark current was sufficiently small compared to the TL signal so that

no dark-current correction was required.

-7-
One disappointing feature of the Harshaw hot-nitrogen reader was tlie

lack of a reliable and accurate reference-light source for niQnitortng PMT

stability. The light source furnished -- a green li.^h^t-emttting diode operated

at 2 V --is temperature dependent. Since the reader's temperature-stabilization

system (water circulation in the block adjotntng the PMT] is not sufficiently

effective to prevent temperature fluctuations in the light source, the light source is essentially useless. Another difficulty resulting from the relatively poor design of the temperature-stabtllzatton system is the easy clogging

of the sjTiall water-circulation tubes. This was a continuing problem until we filtered the water supply in order to remove inorganic contaminants.

3. DATA RETRIEVAL AND HANDLING SYSTEM

Current from the PMT was fed to a fixed-scale current integrator where

it was digitized for display. The analog signal was also monitored with an analog plotter for recording glow curves. The Integral in digital form initially was converted to serial Ascll code for printout with a standard teletypewriter. This necessitated paper- tape storage of the data, for which

retrieval for analysis is rather slow. To remedy this, an off-the-shelf

microcomputer was obtained and programmed to accept and display the serial

data and to store them on 5" floppy disks. This system allows compact storage

of a large amount of data (from five to forty thousand readings per disk)

with quick and easy retrieval for analysis. Thus only simple programs were

required for such tasks as Individual calibration corrections and statistical

analyses. Figure 2 gives a schematic representation of the final arrangement

used.

4. RESULTS OF EXPERIMENTS

4.1 Dependence of LIF (TLD-100) Sensitivity on Photon Energy

in the Range from about 10 keV to about 1 MeV

Because of the relatively heavy filtration in the accelerator beams , it

is not expected that there will be an appreciable contribution of low-energy

photons in the primary bremsstrahlung beam. This Is borne out by the

*The 10-MV bremsstrahlung beam from a Clinac 18 is filtered with about 2 cm

of tungsten, while the 4-MV bremsstrahlung beam from a Clinac 4 has a leadantimony flattening filter of a maximum thickness of 1 cm. (4)

-8-
TLD

READER

5 ^ANALOG CURRENT 1 CURRENT

INTEGRATOR

ANALOG

PLOTTER

BCDuDIGITAL SIGNAL

BCD TO ASCII

CONVERTER

IaSCII SIGNAL

FLOPPY DISK —MICROCOMPUTER —^

PRINTER

+

CRT

DISPLAY

Fig. 2 Schematic Representation of Data-Acquisition System

-9-
(5)

spectrometry results of L. B. Levy et al ^ who find a low-energy cutpff at

about 0.4 MeV in the spectruni of the beajn from an 8-MV Mevatron, having, among

other filtration, a tungsten-copper flattening filter of more tKan 1 cm in

thickness in the primary beam. However, as the beam enters the phantom,

considerable buildup of a low-energy component is expected. In fact, according to the calculations first carried out by Spencer and Fano , a spectral

peak located between 50 and 100 keV (regardless of source energy), gradually

grows in importance as the depth of penetration Into the phantom is increased.

For the resulting photon spectra at a depth of 10 or 15 cm in the phantom

(15 cm of water representing over 1 mean-free path for a 1-MeV photon and

about one-half of one mean-free path for a 4-MeV photon) the effect on the

response of the LiF (TLD-100) of the contribution to absorbed dose from the

photons in the vicinity of the low-energy peak may not be negligible, particularly for a 4-MV accelerator. (Note that, for a 1-MeV photon, the con- tribution to absorbed dose at a depth of 1 mean-free path amounts to about 9

percent.

)

Because of the presence in the bremsstrahlung beam of a photon component

in the energy region in which dosimeter response was not expected to be

independent of photon energy, a study was carried out with batch-calibrated

dosimeters annealed during readout in the Harshaw hot-nitrogen reader, having

sensitivities lying within a ±2 percent band. The radiation spectra were

those of the heavily filtered NBS low-energy standard bremsstrahlung beams ^

'

and of the NBS standardized cesium-137 and cobalt-60 gamma-ray sources.

Identical irradiations were administered to four samples each at three different exposure levels between about 120 and 800 mR, for which supralinearity

proved to be negligibly small over the range of photon energies employed. For

all but the cobalt-60 and cesium-137 gaima irradiations, the samples were

irradiated bare, supported on a Bakelite strip. For the cobalt-60 and cesium137 irradiations, an electron-equilibrium layer of about 5 mm of Lucite was

used over the dosimeters.

Figure 3 shows average dosimeter sensitivity (response per unit exposure)

as a function of effective photon energy , all relative to the sensitivity to

cobalt-60 gamma-ray photons. There is a relatively large increase in dosimeter

*Effective energy is here defined as the energy of a monoenergetic photon

beam associated with the total (narrow-beam) attenuation coefficient obtained

from the attentuation curve of the photon beam in question in a suitable

absorber (for example, Al or Cu), extrapolated to zero absorber thickness.

-10-
O

SH +J

(O fO JC E -M o

s- o

»«- 4->

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4-> T3 C CU

•I- 4-» 4J

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-11
* sensitivity at low-photon energies auDOunting to about 50 percent at 34 keV.

As a result of the increase in sensitivity at low photon energies of the LiF

dosimeters, the depth-dose data that will be obtained in the survey may not

exactly correspond to depth-dose data as measured with ton chambers. This

will be of no significance in the envisaged survey, in which the participants

will be asked to deliver an absorbed dose to water at one prescribed depth in

the Luctte phantom. Nevertheless, in order to investigate the limitations of

the present system, we also computed response for a given irradiation level

as a function of depth in water from the response as a function of depth in

Lucite. The response as a function of depth in Lucite was measured by maintaining the distance between the source and the detector at 1 meter and

changing the depth of the dosimeters inside the phantom. For all irradiations,

the beam cross section in the plane of the dosimeters was 10 cm x 10 cm. Dosimeter readings were related to absorbed dose to water at the same location,

utilizing source-standardization data derived from absorbed-dose calorimetry

in the same beam for the sdme field size and distance. The depth in water

^^water^ equivalent to a given depth in Lucite (t|_^J^^•^g) was computed as

^water " \ucite ^^^^^Lucite ^ ^^^^^water ' ^^^

where (Z/A) stands for the average of the quotient of atomic number and atomic

weight here taken for Lucite and water, respectively, as indicated by the subscripts. The depth, t + , computed in this way is the depth at which the

percent depth dose in water is the same as the percent depth dose at the

(Q)

depth, t. . , in Lucite.^ ^ Figure 4 shows the results. The dosimeter

readings were arbitrarily fitted at a depth of 5 cm, but could be fitted at

any other depth, just as well. The results indicate that, at least for the

fin spectrum emitted by the Co gamma- ray source used for this experiment, the

difference in slope of the response-versus-depth and the depth-dose curves is

relatively small and results in a slight increase in response per unit of

absorbed dose with increasing phantom depth (i.e., for lower photon energies).

*This was one of the reasons why we investigated the feasibility of replacing

the LiF dosimeter by a LioBy,0-, dosimeter. Note that H. K. Pendurkar et al at

iQ) 2 4 7 C.E.N. , Mol , Belgium^ \ report a 50-percent higher sensitivity of LiF (both

TLD-100 and microrods) for irradiation to about 30-keV photons than for irradiation with 1-MeV photons. On the other hand, they report the corresponding

increase for Li^B-O., microrods to be less than 15 percent.

-12-
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S- q: (O +-> -E ro a ^ <U fO o s s- +->

•+-> fO E •-- ro 3 Q.

(O E O > CU E O CU 2: E QJ -r- Ui -O

fO O T3 -)-> O O 1— fO fO >^ ro

+-> -o -M ro O X S- -O 03 S- +-> CU VO fO CU T3 1 ro

CO 1 E T3 +-> ro O -!-> O E fO CU E CU Q 1— fO •!— CO E >

fO O +-> T3 O ro S- T3 J3 1— CO to -O CD 3

<U O S- 1 O

J2 CJ) CO CU S- -E O S- fO -E •!- +J CD CU O -E 3 +J Q. 1 CO

CO -l-> CO CU +J O

J3 •!- CU -M S_ T3 r— -O <: S N E (U ro 1 •I- CU +-> U ja T3

•< E CO CO CU •!- O CU E O CU E S- O JD O •- T3 S- •- +J s- CO 4-> 1— O. CO CU cu o •1- fO CU CU O E E 00

s- •>- •.- s- -a -r- fO JD

ro T3 4- S- CO fO CL to CO CU O

E S- CU S- E r— CU (U o s- -E fo •!- <a ^ J= O 1-t 1— Xi E O +J -M

"dC7)

3SN0dS3H ail io *po; '3soa a3aaos9v

-13-
4.2 Fading of Lif (TLD-100) Resppnse Unde;' Lahofcitory Conditions

Tfiis experijnent was carried out for storage in the dark, at temperatures

between 18^ and ZtPq. and at relative hurDidities of less than 45 percent.

Tests over a wide ran^e of temperatures and relative humidities were not

considered necessary since fading for the annealing methods employed is known

to be relatively small for any ambient condition and possible effects due to

variations in ambient conditions thus would be within the measurement uncertainty. Furthermore, BRH has employed the system for postal surveys for

several years, without experiencing any difficulties due to variations in

ambient conditions.

Figure 5 shows the results of the study, which covers storage periods

between irradiation and readout in the range from about 3 to 24 days at three

cobalt-60 gamma irradiation levels (10, 100 and 200 rad). The response data were arbitrarily normalized to the three-day response points. Since the

amount of fading was shown not to be a function of irradiation level, a least-squares fit regression line Csee formula in fig. 5) was fitted to all

the data points obtained for the period from three to 24 days between irradiation and readout. Over this period, fading is seen to amount to about 2H. percent.

4.3 Influence of Size of Lucite Phantom on Dosimeter Response

for the Spectra of Interest

In order to determine if it would be feasible to use for the survey a phantom smaller than the standard phantom cube with sides of 30 cm in length,

a study was carried out with smaller cubes. Cubes with sides 15 cm and 20 cm

in length loaded at three and four depths, respectively, were irradiated with

cobalt-60 gamma radiation and with 10-MV bremsstrahlung. Figure 6 illustrates

the loading of one of the phantoms. The results of the experiments are shown

in table 1. Listed is the response for a fixed irradiation level averaged

over the values for nine dosimeters irradiated simultaneously at the particular

depths in the two phantoms and radiation beams. The field size was 10 cm x 10 cm

in the plane of each measurement. The data reveal a slight trend for responses

in the larger phantom to be somewhat higher than those in the smaller phantom,

for irradiation with either radiation beam. Since this trend, if significant.

Note that this finding agrees with earlier depth-dose measurements with ion

chambers in graphite phantoms made by J. Pruitt and S. Domen of our laboratory, who found a difference of 0.7 percent at a 10-cm depth.

-14-
ojxi

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if) enUJ

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1 fO TD -o S- ro Q)

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to i- <NJ <L) JD O Q.

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s- o oix:

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•1— O S- o Q O I— MOl #« • r> QJ

<U <U r- 3

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I-.* s- a >» o 0) -O fO

li- JD ro -O

S- S- 1 eu O 1 <u

(/) to O 0) c -Q O S- o 03 1— x:

O- 4J

co M0) o •• cu Qi W JC

+J O) -l-> S- •1- s- 0) C fO o

+-> 3 3 4-> OJ cr E s- oo -o

OJ CU tn Q. "> N

o r"— 'r— Q CU CUi—

W > fO

<+- E cu E O O r— C

Q. O

cr> to -O E

£ CU fO

•1

S- S_ O) -o 1 &- fO to O CU U- •!-— S

LT)

cr>

-15-
Fig. 6 Exploded View of the 15-cm x 15-cm x 15-cm Lucite Phantom. Cutouts just

deep enough to accommodate on layer of dosimeters were provided for nine

dosimeters at nominal depths of 2.5 cm, 5 cm and 10 cm. Larger phantoms

were obtained by adding Lucite plates to all surfaces. When the phantom

depth was 20 cm, a cutout at a nominal depth of 15 cm could be provided,

as well. In this view, the cutouts are shown on an enlarged scale.

-16-
<D

tocO

Dco<U -!-> cn C

<u i- F

<D <1J 4-> s- O) 3 E to

•r- fO W OJ O F Q ^- C o o

CU a» c N eo

•1— ^^

oo a. E c O

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03

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to

CU -0 • • • • •r—to -C +1 +1 +1 +1 cn+J E C -r- CM ^ LO CM

=3 S CT> CO OJ r— 00 VO LO ^ E CM • * • <a f—

S- •4-> 4-> C

to (0

to J^

— E Q-

<U

S- (J JD -r- J2 vo •^ •=d- > 3 to S CU 1 T3 • • • UJ C •!- on 1— •!- to ^ +1 +1 +1 o E

Q_

a 00 yD CVJ 00 a^ <T> 00

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q; ^» • cd CD

q:UJ

1—

UJ

2:

>—1

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CU UJ 0

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UJ to +J E +1 +1 +1 +1 > >,T- < (T3 S <T> r— >=d- i_ cri U3 E c\J CTi 00 to -*

fO • • • • g +-' S E

fO fO CD-C

Q.

to1 •^"

4-> J2

r— 13 CO CO "^

(O to J2 CU c -a • • -1- -r-

</) +1 +1 +1 E

r^ CO r^

LO CT> LO CO LO

n^ • • •

j=

4-> Q. E

O) -0 4-> LO 1

03 E CVJ LO LO

fO -c ^— ^— c Q.

"r~E E• r— ^

-17-
would be negligible in the envisaged survey, we decided to dispense with

tests employing a phantom cube with 30-cm side length and tests employing 4-

and 6-MV bremsstrahlung and concluded that it would be safe to use a phantom

cube with sides 15 cm in length for the survey.

4.4 Influence on LiF (TLD-100) Response at a Given Phantom Depth

in the Presence of Dosimeters in Several Other Depths

BRH had expressed an interest in designing the survey in such a way as

to obtain some information on depth-dose data from the various participating

institutions. If possible, this information should be obtained from the

results of one single irradiation of the phantom, loaded with dosimeters at

different depths along the central phantom axis. In order to ascertain the

feasibility of such a procedure, we investigated whether simultaneous phantoin

loading at different depths influences dosimeter response to the different

radiation beams under consideration. Table 2 gives the results of this investigation. Shown is average response of nine dosimeters at a nominal depth of

10 cm in the phantom, in the presence and in the absence of dosimeters at

other depths, and the relative standard deviations of these averages. Because

of a BRH decision to use phantom cubes with 20-cm sides in their survey, we

did not obtain data with all types of radiation for the other phantom sizes.

The data shown in the table reveal a slight trend of questionable significance (i.e., a trend within the limits of the reproducibility of the

experiment) for the presence of dosimeters at other depths to decrease the

response of the dosimeters at the 10-cm depth -- at least for cobalt-60 gamma

radiation and 4-rw bremsstrahlung. Nevertheless, it is safe to conclude that

the BRH plan to load the mailing phantoms in several depths beyond the peak

of the depth-dose curve is entirely feasible. In fact, the work reported in

Sections 4.5 and 4.6 of this Report was performed in a cubic phantom with

20-cm sides, loaded with dosimeters at nominal depths of 2.5 cm, 5 cm, 10 cm and 15 cm. The field size was 10 cm x 10 cm at a depth of 10 cm. 4.5 LiF (TLD-100) Response at a 10-cm Depth in the Lucite Phantom

for Irradiation with Cobalt-60 Gamma Radiation

and with High-Energy and Low-Energy Bremsstrahlung

The irradiations were performed with the NBS standardized cobalt-60

gamma-ray source and with 100-kV bremsstrahlung, the latter because BRH had

initially intended to use 100-kV bremsstrahlung for their day-to-day calibrations. Irradiations with high-energy bremsstrahlung were done with accelerators both at the Radiation Oncology Department of the National Cancer Institute

-18-
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-19-
and at the Radiation Thejrapy Departroent of George Washington University Hospital.

At the National Cancer Institute, tKe two Siejuens Meyatrons were used at 6 MV

and one of them also at 10 My after its conversion to a Mevatron-XII. At

George Washington University Hospital, we used the Clinac-4 and the Clinac-18,

at 4 MV and 10 MV, respectively. The source-to-detector distance was 100 cm

for all but the 4-W high-energy bremsstrahlung beam, where it was 80 cm. Absorbed dose for the high-energy bremsstrahlung irradiations was obtained by detennining initially the reading of a NBS-calibrated Farmer chamber

and later the reading of a special NBS graphite chamber at a 10-cm depth in a water phantom cube with sides 30 cm in length and converting the reading to

absorbed dose to water by multiplying with the appropriate C^ factor as given

ri 0) in ICRU Report 24^ , where it is designated by the symbol F. (The factor is

0.94 for 4 and 6 MV and 0.93 for 10 MV.) At 6 MV, the absorbed dose obtained

in th_is way also was compared with the absorbed-dose value deduced from

calorimetry in graphite via readings of the NBS graphite ion chamber in the

water phantom, for future direct conversion to absorbed dose to water. For

the cobalt-60 irradiations, absorbed dose to water was obtained by using

results from earlier measurements at the same location with the standard graphite calorimeter.

4.5.1 LiF (TLD-100) response as a function of irradiation level . Figure 7 shows typical data obtained for the average response of sets of nine dosimeters

irradiated with high-energy bremsstrahlung and with cobalt-60 gamma radiation at a nominal depth of 10 cm, as a function of absorbed dose to water

at the same depth. Dosimeter response is seen to be supralinear, even at

relatively low dose levels. Quantitatively, this supralinearity is shown

better in figure 8, where response/absorbed dose [sensitivity) is plotted as a function of absorbed dose, normalized for all irradiation conditions at the zero absorbed-dose level. Sensitivity is seen to increase essentially linearly

with absorbed dose, the increase amounting to about 4 percent of the initial

sensitivity at the 100-rad level. The difference in the slope of the regression lines obtained by least-squares fitting for the different photon spectra

is only marginally significant since, in addition to the uncertainty in the

fits indicated in figure 8 one must consider an experimental uncertainty

-20-
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100 200 300 400

ABSORBED DOSE . rod

Fig. 7 Typical Dosimeter Response as a Function of Absorbed Dose. Dashed line - linear relationship between response and absorbed dose.

-21-
RELATIVE RESPONSE /ABSORBED DOSE

o QJ

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V> H- 1+ H- •+ >

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c/) ro 2 o

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of at least 1 percent. As a consequence, no supralinearity correction is re- quired for a comparison between high-energy bremsstrahlung response of the

dosimeters and response to cobalt-60 gamma radiation, at least not at irradiation levels in the vicinity of 100 rad.

4.5.2 LiF (TLD-100) response at a given irradiation level to high-energy

bremsstrahlung relative to that to cobalt-60 gamma radiation . Table 3

shows two series of values for the quotient, Q^ , which is defined as (dosimeter response in Lucite to high-energy bremsstrahlung) / (dosimeter

response in Lucite to cobalt-60 gamma radiation for identical levels of

absorbed-dose to water). The first series is based on experiments performed

over a period of several months. For the high-energy bremsstrahlung, absorbed

dose to water at a 10-cm depth in a water phantom was determined from readings

of Farmer ionization chambers, with a random reading uncertainty of about

0.3 percent. The irradiations for the second series were all completed within

a period of two days and all readouts were done in one sitting. For the highenergy bremsstrahlung, absorbed dose to water was determined from readings of

a special NBS graphite ionization chamber, with a random reading uncertainty

of about 0.1 percent. Just as could be expected from earlier studies with

(13) various configurations of LiF TLD material,^ ' no significant trend is

observed as a function of initial beam energy and the difference from unity of

any one of the quotients is within the total uncertainty of "^2 percent. (See

uncertainty given for Qp in table 6.)

4.6 Relative Depth Dose in Lucite Phantom

Derived from LiF (TLD-100) Response

Figure 9 shows representative depth-dose data for depths of 2.5 cm, 5 cm,

10 cm and 15 cm in Lucite obtained in the study described in Section 4.5.

Inasmuch as the participants in the BRH survey will be asked to determine

absorbed dose to water at a depth of 10 cm in the phantom, all data are plotted

relative to the dose at this depth. Also, in order to facilitate the use of

these results, the coefficients of the least-squares fits of all the depth-dose

data obtained (including those plotted in fig. 9) are shown in table 4. These

coefficients have attached to them an uncertainty of about 1^ percent beyond the

It is not too surprising that the difference is not large, even in the light

of the known dependence of supralinearity on LET^ ' ' since it may be expected

that at the 10-cm depth at which the dosimeters were irradiated, the spectra

had become much more similar. See also Section 4.1 of this Report.

-23-
Table 3. Quotient, Dosimeter Response in Lucite per Unit of Absorbed Dose

to Water at a IQ-cm Depth in the Lucite Phantom

Radiation Source Nominal

Peak Energy

Quoti

Series 1 ent^^^Series 2

Varian Clinac-4

(George Washington U. Hosp.)

4 MeV 1.01^ 0.99^

Siemens Mevatron VI

^^^ (National Cancer Institute)

6 MeV 1.02^

Siemens Mevatron VI

(National Cancer Institute)

6 MeV 1.01"^ 1.00^

Varian Clinac-18

(George Washington U. Hosp.)

10 MeV 0.99^ 0.97^

Siemens Mevatron XII ^

^ (National Cancer Institute)

10 MeV 0.97^ 1.00°

Avera ge:

1 004 ^^'^^^

'•^^^ -0.027

99^

""^-^^

(a) Before conversion to Mevatron XII (see 10-MeV points obtained at the

National Cancer Institute).

(b) After conversion from Mevatron VI.

(c) See Section 6 for the computation of the total uncertainty in this quotient,

-24-
a0)N

rr—

.« «o E ^ O E +J O c c:

(O ^ 0) D- s-0) (U S 4-> •1- (O • O -M^^

r3 (O wi _J -a so

OJ 0) s_ x: in s4-> c re T3>

C

•1- -o O)

QJ 4-J ^ J3 -rt-> S- O

QO 3

OJ to _1 Eu

M- -1- O O) J= E f O

•rO

X +-> • 1— 1- O O) C to *4- Q_ 3 C O

U- o LU Q..C O Ol CL

to d: o) fO TJ

S- S- Ol (O

OJ +->

+-> OJ +-> fO E n3 3 -r-tO O o o cu

+->

Q E• f— QJ E re

lO

O +->

O S- -Q Q H- O

-D -a o;

o) OJ to J3 4-> O

s- 13 jr O Q.+J

i/i E

X3

O

O

cC

O +->

3Soa a3ayosav 3Aiivn3d

-25-
Table 4. Regression Lines, Dose Interpretation from Dosimeter Response in

Lucite as a Function of Depth in the Lucite Phantom

y = ap, + a, X + a, x

Type of

Radiation ^0 ^1 H

Cobalt-60

Gamma Radiation

1.887 ± 0.004 -0.103 ± 0.001 0.00147

±0.00007

4-MV bremsstr.

(Clinac 4)

1.973 ± 0.006 -0.121 ± 0.002 0.00232

±0.00010

6-MV bremsstr.^ '^^ (Mevatron VI)

1.747 ± 0.005 -0.088 ± 0.001 0.00136

±0.00008

6-MV bremsstr.

(Mevatron VI)

1.785 ± 0.008 -0.095 ± 0.002 0.00164

±0.00013

10-MV bremsstr.^^^

(Mevatron XII)

1.615 ± 0.006 -0.071 ± 0.002 0.00093

±0.00010

10-MV bremsstr.

(Clinac 18)

1.635 ± 0.004 -0.074 ± 0.001 0.00109

±0.00007

(a) Absorbed dose was set equal to unity at a depth of 10 cm in Lucite.

(b) Before conversion to Mevatron XII.

(c) After conversion from Mevatron VI.

-26-
uncertainty indicated in the table. This means that the differences between

the curves obtained at any one nominal bremsstrahlung energy are not significant. On the other hand, the fitting parameters for the curves obtained at

the different nominal bremsstrahlung energies show that these curves are significantly different. As a consequence, one must conclude that the method

may be sufficiently accurate for obtaining some information on a participant's

gross beam energy from survey data at three or four different phantom depths,

but that it will be difficult to detect small differences among linear

accelerators of the same nominal energy. It may be noted that plotted relative

to the dose at a depth of 10 cm, the curves obtained for the NBS cobalt-60

gamma-ray beam and for the 4-MV bremsstrahlung beam cannot be distinguished

from each other. This agrees with results we obtained from depth-dose curves measured by others.

4.7 Dependence of LiF (TLD-100) Sensitivity on Irradiation History

The experiments described so far in Section 4 were all carried out with

the same set of 100 dosimeters. During these experiments, we observed that

dosimeter response to the constant test irradiations of ^5 R of cobalt-60

gamma radiation decreased monotonically with dosimeter use at the testirradiation levels. When an experiment involving irradiation levels of ^100

rad or more was carried out between any two test irradiations, this pattern

was interrupted, the glow curves for the test irradiations following the ex- periment reflecting electron release from traps deeper than those filled at

the lower irradiation levels, a release that tended to disappear only after

several annealing and use cycles. Figure 10 shows the resulting behavior of

the 100 dosimeters throughout 35 irradiation and annealing cycles. The data

points indicate the average response of these dosimeters to the constant test

irradiation. Where experiments involving the irradiation of some of the

dosimeters followed by readout and annealing of all the dosimeters were performed between any two test irradiations, the types and levels of the

intervening irradiations are indicated.

* (14)

For the spectrum of the cobalt-60 source employed, see Ehrlich et al ^ . While the percent depth dose at a 10-cm depth is around 4 ± 0.5 percent

(15) lower for cobalt-60 gamma radiation than for the 4-MV bremsstrahlung,^ ' the depth doses relative to the depth dose at a 10-cm depth agree with each

other to within V-^ percent over the range of depths from about 3 to at least

12 cm in water.

-27-
RELATIVE AVG. RESPONSE

to

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3 c-l- S

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=5

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VI -s -s O -s -s

=! OJ QJ

fD D. a.

_J. —I, O a> Q)

-+i <-*- r+ ^> —J. r+ o O

ZT 3 Z3 n>

(/) .J ni

fD CO

00 Q--a (-1- o O

VI O -^ rl-'<

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f-l- a.

ft) • > -5 <

(/I fD —1 -s CU '< CU T a CQ

fD fDtofD —

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13 OJ fD Q. :3 CO —

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fD fD Q00

00

CD CD

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CD O

CD

m

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o

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ro

1 r 1 r

0-300 rod Co-60

O

- h

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CD

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^

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1 Co-60 -

10-300 rod 10MV,Co-60-

0-239 rod lOOkV

0-320 rod 100kV,Co-60

0-400 rod Co-60

-O 1

0-213 rod 4MV,Co-60

0-166 rod Co-60

0-146 rod 10MV,Co-60

0-203 rod 6MV

J L

0-100 rod 4,6,10MV,Co-60

1 J I I I I I L J L

-28-
The solid straight line represents a least-squares fit to the test-data

points, indicating an overall loss in dosimeter sensitivity of '^^9 percent

after the 35 irradiation and annealing cycles. The dashed line is a fit to

the first six data points for test cycles done in succession without intervening experiment runs. The decrease in sensitivity is seen to be

larger, amounting to almost 2 percent after these six cycles, i.e., to 'U).3

percent per test cycle.

In order to study the behavior of the dosimeters after irradiation at

levels higher than the 'v5-R test level, the change in the response to the test

irradiation is plotted in figure 11 for the average readings on a group of

nine dosimeters irradiated simultaneously. The points again are for the

response to the test irradiations. Where experiment cycles were interspersed,

the irradiation types and levels are indicated. Note that a 400-rad irradiation causes a sensitivity increase of "^ percent, followed by a decrease in

sensitivity of 'v^S percent per subsequent test cycle. At the 100-rad level,

the increase, on the average, just offsets the decrease expected for any two

successive test irradiations.

5. RESULTS OF PILOT COMPARISON STUDY

The Radiation Physics Department of the Massachusetts General Hospital

(MGH) was selected for this pilot study because it had been a participant a few

weeks earlier in an IAEA pilot comparison study with the Harvard School of

Medicine. (This study also is covered in the IAEA report on the proceedings

of the March 1979 meeting referred to in Section 1 of this Report.) At the

time of the NBS-MGH study, the decision had not been made as yet to ask for

delivery of 100 rad to water at a lO-cm depth in Lucite. For the NBS-MGH

study, we asked MGH to administer to our phantom (a Lucite cube with 20-cm side

length, loaded with 9 dosimeters each in the depths of 2.5 cm, 5 cm, 10 cm, and

15 cm) an absorbed dose from their 10-MV bremsstrahlung beam (Clinac-18) cor- responding to 200 rad to water. The dose was to be delivered at a depth in

Lucite of 5 cm, using a 10 cm X 10 cm field size at this depth. We computed

the absorbed dose to water from a cobalt-60 source that corresponded to the

response in Lucite measured with the dosimeters that had been irradiated at MGH

in the 10-MV bremsstrahlung beam. We arrived at the absorbed dose to Lucite

(D. .. ) from the absorbed dose to water as measured by the NBS standard

-29-
RELATIVE AVG. RESPONSE

CO

(£3

r+ Q- 03

13 rD a -•• r+ O

ZS CD u^

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DJ o 3 —

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00 _J.

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-> Cu —1. -5 z: <

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d; Q

r+ 00 S .^. r+ _j. O C-+ 13 -5 rr O -> 1—

3 QJ T

CL ^

OJ —1. OJ —J Cu CL —

c-t- —1. —1. Q;

tX> O <-+ 13 _j. CL O

o _j. 13 CO 00

—J. n: 3 -a -J. CD —

00 n- o (-1- CD r+ o

-^ <-!- ->

00 fD '< Q- • QJ • T

fD J= — < —1 •< (T> 3 X3 T

Cl CD QJ

—1 00 CQ n ro O) O)

c^- :3 T

n> CL ro Q. 00 -a

ro o

< :3 CD 00

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o

CD

(Ji

CO

CD

CD

>

z:

m

>

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200 rod lOMV

200 rod Co-60

239 rod lOOkV

100 rod Co-60

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100 rod Co-60

100 rod 10MV

69 rod 6MV

100 rod 10MV

J 1 I I 1 » » J I I L

-30-
absorbed-dose calorimeter as

\ucne [^o'L'^^^*^) ^Lucite] = °„ater kt^^ter) t

] y

^ucite

j^,

•- -• L J water

where the y^ symbols stand for the mass attenuation coefficients for the incident spectrum for Lucite and water; y ^^^^^ is the quotient of the average

^"water

mass energy absorption coefficient for Lucite and for water, taken at a depth

in water, t^^^g^ = 5 cm, and the corresponding depth, t, .^g, computed by

eq (1). Since, for our cobalt-60 gamma -radiation source, 5 cm of a low-Z

material represents only a fraction of one mean-free path and y is essen- tially constant over the energy range represented, y values for the incident — Lucite ^^ beam were used in the computation of y "^ ^^^ ^"water

A similar computation was performed for the 10-MV bremsstrahlung beam.

Equation (2) was again used to determine the relationship between absorbed dose

to Lucite at a 5-cm depth in Lucite and absorbed dose to water at the corresLuci te ponding depth in water. However, since y changes by about 2 to 3

water_ lucite

percent between 1 and 10 MeV, an average value, y , was computed

^"water

under the assumption of a triangular bremsstrahlung spectral distribution filtered by 1.9 cm of tungsten, which, according to Varian Associates, is the

filtration in a 10-MV Clinac-18 beam.^ ' The resulting value for the interpretation of the dosimeter readings in Lucite at 10 MV in terms of cobalt-60

ganma-ray absorbed dose to water was 199.6 rad, while the value quoted by MGH

was 201.8 rad. This led to a value of 1.011 for the quotient of the dose

assigned by MGH and the NBS dose interpretation in terms of NBS cobalt-60

gamma rays. The corresponding quotient in the MGH - IAEA comparison was quoted to us by the IAEA and the Harvard Medical School as having been 1.014.

This difference is well within the uncertainty limits of the IAEA and the NBS

pilot studies.

Another result concerned the slope of the dosimeter response-versus-depth

curve obtained with the 10-MV Clinac-18 at MGH, which was about 1 percent

smaller than the slope we obtained at 10 MV both with the Mevatron XII at NCI

and the Clinac-18 at GWU Hospital. Considering that the geometries were not

quite comparable (10 cm x 10 cm field size at a 10-cm depth at NCI and at GWU

Hospital, at a 5-cm depth at MGH), this agreement is also satisfactory.

-31-
6. ANALYSIS OF THE MAGNITUDE OF THE UNCERTAINTY EXPECTED FOR THE PLANNED

SURVEY FOR INDIVIDUALLY AND BATCH-CALIBRATED DOSIMETERS

The objective of the study reported h^ere was to detennrne th.e number of

TL dositneters necessary per survey point to achieve a given limit of random

uncertainty associated with the dosimeter handling, preparation and readout

procedures agreed upon by BRH and NBS. The determination of the uncertainty

was done by means of a statistical analysts based on the experimentally

determined reproducibility of the TLD readings and was checked in part By means of the simulated survey discussed in Section 6.4 -- in both instances

considering the case of individually calibrated and of batch-calibrated

dosimeters. Considered were the uncertainties in the dose interpretation

arising from each of the steps in the survey procedure, with the exception of

the systematic uncertainties, common to surveyor and surveyed alike, which

are inherent in the cobalt-60 gamma-ray calibration of the ion chamber and in

the factor C, , both used in the determination of absorbed dose to water in

the high-energy bremsstrahlung beam.

6.1 Procedure

It is envisioned that the survey protocol will require that the participants be furnished a Lucite phantom loaded with LiF TLD-100 dosimeters from a batch in which TLD sensitivity had been characterized either for each individual dosimeter or for the batch as a whole. The participants will be asked

to deliver a prescribed absorbed dose to water (say, 100 rad) to a point at a 10-cm depth in the phantom, computing the dose as if the phantom material had

been water. At the same time, TL dosimeters from the same batch will be

irradiated by BRH with cobalt-60 gamma radiation from one of the standardized

NBS sources at suitable calibration levels, in a similar phantom. BRH then

will compute the high-energy bremsstrahlung absorbed dose to water delivered

by the participant (D„^^. . ) at a 10-cm depth in the water phantom from the

parti c

average dosimeter response of the dosimeters irradiated by the participant

(r^,^. . ) at a 10-cm depth in the Lucite phantom as parxi c

%artic. = '^partic. ^^/^^BRH ^Co' ^^)

where i^/^)^^^ is the quotient of absorbed dose over TLD response at a 10-cm

depth in the Lucite phantom, as obtained from the current BRH cobalt-60

gamma-ray calibration curve, and Qp is defined in section 4.5.2. Even though

-32-
we found Qp to be unity within the total uncertainty of our experiment (see

sec. 4.5,2), the uncertainty inherent in the survey procedure has to contain

the uncertainty associated with Op . Also, in practice it may become necessary to add a fading-correction factor to eq C3) if the BRH calibration

irradiations with the MBS cobalt-60 gamma-ray source are not going to he

administered at approximately the same time as the high-energy bremsstrahlung

irradiations are administered at the participating laboratories.

6.2 Reproducibility of Dosimeter Response

6.2.1 Procedure for characterizing reproducibility by batch (dosimeter

identity not maintained ). All available dosimeters were annealed and then irradiated identically and read out. A sub-group ("batch") was chosen such that

the TL response of the included dosimeters could be characterized by a preselected standard deviation ($) about the mean. This batch then was used in

the survey procedure, without regard to dosimeter identity. Work at BRH has

shown that batch characterization in terms of S^, once established, is mainb

tained well through many experiment cycles at diagnostic irradiation levels.

This was confirmed by our studies. However, we also demonstrated a temporary

increase in sensitivity spread after a portion of the batch was used at therapy

levels. (See the spread in the response to the sequence of test irradiations

in figure 10, after the experiment involving a 400-rad irradiation.)

6.2.2 NBS experience with batch characterization of reproducibility of

dosimeter response . Starting with a group of 200 dosimeters with a spread of

about 12 percent between the responses of individual dosimeters to a constant

exposure, it was possible to select a group of about 100 for which the value

of Sg was about 2 percent. Obviously the value of Sg not only depends on the

variability of the response among the dosimeters but also on the rigor of

Strictly, the standard deviation from the average of the responses in the

batch, Sp, is given by

^

2 , . 2,.^i'2

Sg = (S, + Sj /n)

where Sn, is the standard deviation due to the variability among the dosimeters

S. the standard deviation reflecting the reproducibility of individual dosimeter readings (see sec. 6.2.3) and n the number of dosimeters from which S^

and Sd were determined.^ ^ For the dosimeters used here, the second term

proved negligible for values of n of the order of 10 or more.

-33-
the selection procedure. In practice, there is some lower bound on S^. With

D the initial group of dosimeters used at NBS, 2 percent was relatively easy to

obtain while 1.5 percent probably represented the practical limit for batch

selection on the basis of a single set of readings. More rigorous procedures

are possible, but the reproducibility of a smaller value for $„ from cycle to

D cycle would be questionable. For a 100-dosimeter batch selected by NBS from

the initially normal population of 200, the value of Sn = 2 percent reproduced

D well over twelve test cycles. The selected population was again essentially

normal

6.2.3 Procedure for characterizing the reproducibility of individual

dosimeters (identity maintained) . All available dosimeters were annealed and

then irradiated identically and read out. This procedure was repeated several

times, in order to obtain a statistically valid average TL response for a given irradiation level. Since dosimeter response for a given irradiation

level decreased from one to the next cycle (see fig. 10) and reader characteristics may not be identical from day to day, successive readings on a single chip cannot be used to directly determine a value for Sj, the uncer- tainty associated with an individual chip reading. Instead, S, was determined

indirectly from an examination of quotients of two successive readings on the

rel I same chip. An average of such quotients is then characterized by Sj =-^2 Sj.

6.2.4 NBS experience with characteristics of the reproducibility of

individual dosimeter response . In order to arrive at a value for S, for 200

dosimeters, NBS irradiated and read them out in nine consecutive cycles. A

rel value for Sj of somewhat less than 1 percent was obtained, leading to a value of about 0.7 percent for Sj, or about one-half of the best feasible

values for Sg. Thus, the increase in time and effort involved in maintaining

dosimeter identity pays off in a considerable decrease in the uncertainty in

the characterization of dosimeter response. There is evidence that, with the newer heater model in the hot-nitrogen reader, it is possible to achieve even further improvement.

6.3 Estimate of the Uncertainty in the Interpretation of Survey Doses,

Based on a Statistical Analysis

The analysis involves assigning uncertainty limits to the individual

steps in the computation of absorbed dose from the average of identically

irradiated survey dosimeters (see eq [3]). Tables 5 and 6 give a review of

-34-
!-> 01

s- in

0) c

(> o

c Q. —

)

m

0) oct

4-> 1-0) p t-> a) 0)

>-> b: m

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lO O E>

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value,

%

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Systematic

Uncertaint

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positioning

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of

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source)

timing

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(high-energy

bremsstr.

source)

uncertainty

in

ionization

measurement

(high-energy

bremsstr.

source)!^)

•esponse

ized

individu- ally lO «-r> odd O 00 lO r^dd

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a< vo <a- dad

1

value,

%

dosimeter

t

character

by

batch CSJ O^ ID

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rtalnty^^^

Computec

number

of

dosimeters O T3 "^ d^ CM in o

nc

Ol lO in

1— CM

IIc

o

^"in o o CM in

n

LU c

n O

c

Random

Unce

r

computation

esponse

characterized

individually

o

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0) m

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01 >/) j-i-i

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Formulae

fo

dos1meter-r

by

batch o

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ro i>-

o

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+

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c

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LD <_>

c

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•>- <-> <o t-

•tJ rJ. 1-

01 U3 U 10 C 1- 3 I.

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I/) 01 c c -o m

a> o 3 1.

tn O. r— o

o tn u L. JC U C L. o 1. •-• ai

i i

0) T-

-35-
Table 6. Computation of Total Uncertainties (a)

quantity

uncertaint

random

y, %

systematic ^^'

row sum, %

algebraic summed

sum in quadrature

[0.6^ + Z.O^t'

= 2.1

0.9

0.2

1.4

2.3

2.1

2.1

1.7

grand totals: 4.4% 2.7%

(a) The values indicated are for the chosen or recommended numbers of dosimeters

for the determination of each quantity, as indicated by the footnotes in

Table 3.1.

(b) The individual parts of each systematic uncertainty were summed algebraically,

36-
the. results, computed on the basis of the values for Sg and S, discussed in

Section 6,2. Random uncertainties were computed as three times the standard

deviations from the average of the dosi~meter readings, on the hasts of several

choices for the number of dosi)neters employed in each phase. Where required,

the total standard deviations corresponding to multi-component expressions

were obtained by compounding in quadrature the standard deviations of each

part of these expressions. The uncertainty in the quotient, Qp , was con- sidered to enter into eq (3) as a systematic uncertainty. Numbers of dosimeters and mode of dosimeter-response characterization are shown in the footnotes of table 5 and the final results for the chosen numbers of dosimeters

and modes of response characterization are given in table 6.

6.4 Check on Uncertainty Estimates

by an In-House Simulation of One Survey Cycle

This study was undertaken in order to check the validity of the statistical model used for the assignment of uncertainties in the previous section.

For this purpose, a typical survey cycle was mimicked almost in its entirety

by an in-house experiment. Since suitable high-energy bremsstrahlung is not

available at NBS, cobalt-60 gamma radiation was used for the dosimeter irradiations simulating the "unknown" irradiations of the survey. Since the

"unknown" irradiations actually were administered simultaneously with the

known cobalt-60 calibration irradiations, the systematic uncertainties due to

phantom positioning and timing were eliminated from the estimate of the un- certainty in the dose interpretation. Also, the uncertainty in Qp was excluded.

Eighty-one dosimeters that had been prepared in the usual way were irradiated in a Lucite phantom to known absorbed-dose levels of cobalt-60 gamma

radiation. Some of these dosimeters then were assigned the role of calibration dosimeters, while the remaining dosimeters were considered to have

received "unknown" doses. Nine irradiation levels were used, with absorbed

doses to water ranging from 100 to 300 rad, distributed symmetrically about

200 rad, which was supposed to correspond to the requested irradiation level.

Nine dosimeters were used per irradiation level.

All dosimeters were read out in close succession and calibration curves were prepared, both on the basis of individual response characterization and

of response characterization by batch. Table 7 shows the results for two

-37-
Table 7. Comparison, Uncertainty of Results of Simulated Survey and Computed

Uncertainty Estimates

Number of

"unknown

points (a) points ^^^ for

fit

Standard

deviation

percent

Spread, difference between /^\

dose interpretation on "unknowns

and "true" dose,

percent

Batch Characterization of Reproducibility (Simulated Survey)

2.1

1.7

- 1.8% to + 1.1%

- 2.3% to + 1.5%

Computed Uncertainty Estimates: 2.9% (algebr. sum)

2.3% (summed in quadrature)

Individual Characterization of Reproducibility (Simulated Survey)

0.8

1.5

- 0.45% to + 0.16%

- 0.99% to + 0.92%

Computed Uncertainty Estimates: 1.8% (algebr. sum)

1.3% (summed in quadrature)

(a) 9 dosimeters per point

(b) From regression line

-38-
different assignments, of the irradiated dosimeters to the "calibration" and

"unknown-dose" groups. Also shpwn are corresponding results of the uncer- tainty esti^nates based on the simplified statistical model. In all cases,

the actual deviations between reported and assigned C'true") doses are seen

to have been well within the uncertainty estimates. It is to be expected

that, for a larger sample of "unknowns", the difference between actual

deviations and computed estimates would have been smaller and that some of

the deviations in fact would have been larger than the estimated uncertainties.

7. RECOMMENDATIONS

It is recommended that BKH proceed during the survey much as NBS proceeded during the various calibration and test phases, eliminating as much as possible intermediate steps that could increase the already relatively high

estimated overall uncertainty for the dose interpretation. Specifically, we have the following recommendations:

(a) Prior to the start of the survey, serious consideration should be

given to replacing batch characterization of dosimeter response by individual

dosimeter-response characterization in all phases of the survey, and to

replacing the original heater in the hot-nitrogen TLD reader by the improved

heater now sold by the Harshaw Chemical Company, if this has not been done

already.

(b) A Lucite cube with a 20-cm side length should be used as a phantom.

(c) Nine dosimeters should be positioned next to each other near the

phantom mid line, at a 10-cm depth in the phantom and, if desired, also at

several other depths ^ 5 cm.

(d) The participants should be asked to administer 100 rad to water at a 10-cm depth in the phantom, using a 10-cm x 10-cm field at this depth.

(e) At least one test cycle consisting of irradiation of all dosimeters

in the batch at a constant level below 10 R followed by readout about one day

after irradiation should be inserted between any two survey cycles.

(f) For each survey cycle, the relationship between dosimeter response

and absorbed dose to water should be obtained with cobalt-60 gamma radiation

from a standardized NBS source, covering absorbed doses to water at least over

the range from 50 to 300 rad. This relationship should be determined from

the response of sets of dosimeters irradiated at approximately the same time as the participants administer their survey irradiations, and read

-39-
out along with the dosimeters irradiated by the participants.

(g) Regardless of whether batch or individual calibration is used, the

survey-dosimeter response should be adjusted to the average of the dosimeter

response during the test cycles immediately preceding and following the

survey cycle.

(h) The average adjusted response of the dosimeters irradiated at any

one depth in the phantom should be evaluated in terms of absorbed dose to

water, utilizing the current cobalt-60 gamma-ray calibration data discussed

in (f).

(i) The relationship between absorbed dose to water and depth in Lucite,

normalized to a depth of 10 cm, should be obtained from the data computed in

(h), for a later study of slope as a function of nominal peak bremsstrahlung

energy.

(j) At least twice during the survey BRH should provide NBS with a phantom loaded with dosimeters for irradiation with cobalt-60 gamma radiation

at a level unknown to BRH at roughly the same time as the irradiations are administered by the current set of participants, for evaluation along with the

participants' dosimeters, for the purpose of measurement assurance.

8. ACKNOWLEDGEMENT

We wish to thank the staff of the Radiation Oncology Branch of the

National Cancer Institute (especially Drs. L. R. Kirkland and T. N. Padikal)

and of the George Washington University Hospital's Radiation Therapy Department (Dr. C. C. Ling) for giving us access to their linear accelerators and

for assisting us with obtaining the TL response data at 4, 6, and 10 MV. We

also wish to thank Dr. A. L. Boyer of the Radiation Physics Division, Massachusetts General Hospital, for irradiating our phantom loaded with TL dosimeters in his 10-MV bremsstrahlung beam. Special thanks are due also to

Dr. H. H. Ku, Chief, Statistical Engineering Division, NBS Center for Applied

Mathematics, for his assistance with the analysis of the Magnitude of the

uncertainties that will enter into the BRH survey of high-energy bremsstrahlung therapy dosimetry and to Dr. J. S. Pruitt of our own Division for

carrying out all ion-chamber measurements in the high-energy bremsstrahlung

beams.

-40-
9. REFERENCES

1. Scares, C.G. and Ehrlich, M., Concerning Li^B-O^ Thermoluminescence

Dosimeters, Med. Phys. 6, 312 (1979).

2. See, e.g., Bjarngard, B.E., Ruden, R.-I. et a]_. Evaluation of the 1977

IAEA Pilot Study of Postal Dose Intercomparison (TLD) for Orthovoltage

X-Ray Therapy, Proceedings of a Symposium on National and International

Standardization of Radiation Dosimetry, Atlanta, December 1977, Vol. I,

319, International Atomic Energy Agency, Vienna (1978).

3. Ohlhaber, T.R., Design of the BENT Dosimetry System and its Application

to a Nationwide Mammography Quality Assurance Program, Symposium on Biological Effects, Imaging Techniques, and Dosimetry of Ionizing

Radiation, Bureau of Radiological Health, 1979 (to be published).

4. LaRiviere, P., Varian Associates, private communication.

5. Levy, L.B., Waggener, R.G., Wright, A.E., Measurement of Primary Bremsstrahlung Spectrum from an 8-MeV Linear Accelerator, Med. Phys. 3_, 173 (1976).

6. Spencer, L.V. and Fano, U., Penetration and Diffusion of X-Rays. Calculation of Spacial Distributions by Polynomial Expansion, J. Research,

NBS, 46^, 446 (1951); for the results referring to a plane-parallel source,

see e.g., Goldstein, H., The Attenuation of Gamma Rays and Neutrons in

Reactor Shields, prepared for the Division of Reactor Development, U.S.

Atomic Energy Commission (1957).

7. Calibration and Related Measurement Services of the National Bureau of

Standards, NBS Special Publication 250 and associated Appendix, U.S.

Government Printing Office (1978-1979).

8. Pendurkar, H.K., Boulanger, R. , Ghoos, L. , e_t al, Energy Response of

Certain Thermoluminescent Dosimeters and their Application to the Dose

Measurement, Proceedings, Third International Conference on Luminescence

Dosimetry, Ris0 Report No. 249, Part III, p. 1089, Danish Atomic Energy

Commission, Research Establishment Ris0 (1971).

9. Spencer, L.V., and Lamkin, J., Slant Penetration of y-Rays in H2O,

National Bureau of Standards Report 5944 (1958).

10. Determination of Absorbed Dose in a Patient Irradiated by Beams of X or Gamma Rays in Radiotherapy Procedures, ICRU Report 24, International

Commission on Radiation Units and Measurements (1976).

-41-
n. Naylor, G.P., Thermoluminescent Phosphors: Variation of Quality Response with Dose, Rhys, in Med. and Biol. ^O, 564 (1965).

12. Ehrlich, M., Thermoluminescence Response of LiF to X- and Gamma Rays; a Study of Rate and Energy Dependence over a Wide Range of Exposures,

Proceedings, First International Congress of Radiation Protection, Rome,

1966; Pergamon Press, NY (1968).

13. Bagne, F., A Comprehensive Study of LiF TL Response to High Energy Photons

and Electrons, Radiology 123_, 753 (1977).

14. Ehrlich, M. , Seltzer, S.M., Bielefeld, M.J., Trombka, J.I., Spectrometry

fin of a Co Gamma-Ray Beam Used for Instrument Calibration, Metrologia 1_2_, 169 (1976).

15. For the cobalt-60 data (100 cm SSD) see, e.g.. Central Axis Depth Dose

Data for Use in Radiotherapy, Cohen, M., Jones, D.E.A., and Greene, D.,

editors, Supplement Number 11, British Journal of Radiology, British

Institute of Radiology, London (1978); and for the 4-MV data (80 cm SSD)

see, e.g., Peterson, M. and Golden, R., Dosimetry of the Varian Clinac-4

Linear Accelerator, Radiology 103 , 675 (1972).

16. Snedecor, G.W. and Cocheran, W.G., in Statistical Methods, Chapter 10,

6th Ed., Iowa State Univ. Press, Ames, lA (1967).

-42-
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. TN 1119

2.Gov*t Acfcesslw) No. W^^^^U^^^^^^^M

4. TITLE AND SUBTITLE

A Thermoluminescence Dosimetry System for Use in a Survey

of High-Energy Bremsstrahlung Dosimetry

S. Publication Date May 1980

1. Ferfemtfttg Orphtzation Code

7. AUTHOR(S)

Margarete Ehrlich and Christopher G. Soares

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

l#. PR>t6ctftasft/Woi* Unit No.

11. Contract/Grant No. FDA 224-78-6002

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS cs^reef. City, state, ZIP) Medical Physics Branch

Bureau of Radiological Health

Rockville, MD 20852

13. Type of Report & Period Covered Final Report - Fiscal

years 1978 and 1979

14. i^Ksisofing Agency Code

iiihiii^iii?^%i^ 15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of mo3t aigrtificant information. If document includes a significant bibliography or

literature survey, mention it here.)

This is the final report covering the work performed under an interagency agreement

between the National Bureau of Standards and the Bureau of Radiological Health

during the fiscal years 1978 and 1979. A thermoluminescence dosimetry system suited

for a survey of high-energy bremsstrahlung in U. S. radiation-therapy departments

was selected and calibrated. The experiments leading to the choice of the

recommended operational characteristics, including dosimeter handling, annealing

and readout, dosimeter stability in the contemplated mode of operation, dosimeter

response over the photon-energy range to be covered, irradiation geometry and

irradiation level are treated in detail. Results are reported of a pilot study

involving the shipment of a typical survey assembly (a plastic phantom loaded with

a set of dosimeters) for irradiation in one U. S. therapy department and the

overall uncertainty of the proposed survey procedure is discussed.

17. 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)

Calibration; dosimetry; high-energy bremsstrahlung; operational characteristics;

radiation therapy; recommendations; selection of thermoluminescence dosimetry

system; survey; thermoluminescence; United States.

18. AVAILABILITY [x3 Unlimited

I I For Official Distribution. Do Not Release to NTIS

pXl Order "M^orn Sup. of Doc, U.S. Government Printing Office, '—^ Washington, D.C. 20402

I I Order From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

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21. NO. OF

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47

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