ELSEVIER Radiotherapy and Oncology 36 (1995) 143-152

R ADIDTHERAPY

aO~~~~~~~

TLD postal dose intercomparison for megavoltage units in Poland

J. Iiewska*, R. Gajewski, B. Gwiazdowska, M. Kania, J. Rostkowska Medical Physics Department. Cancer Centre. ul. Wawelska IS, M-973 Warsaw, Poland

Received 28 November 1994; revision received 7 April 1995; accepted 11 July 1995

The aim of the TLD pilot study was to investigate and to reduce the uncertainties involved in the measurements of absorbed dose and to improve the consistency in dose determination in the regional radiotherapy centres in Poland. The intercomparison was organized by the SSDL. It covered absorbed dose measurements under reference conditions for Co-60, high energy X-rays and electron beams. LiF powder type MT-N was used for the irradiations and read with the Harshaw TLD reader model 2OOOB/2OOOC. The TLD system was set up and an analysis of the factors influencing the accuracy of absorbed dose measurements with TL-detectors was performed to evaluate and minimize the measurement uncertainty. A fading not exceeding 2% in 12 weeks was found. The relative energy correction factor did not exceed 3% for X-rays in the range 4- 15 MV, and 4% for electron beams between 6 and 20 MeV. A total of 34 beams was checked. Deviation of f 3.5% stated and evaluated dose was considered acceptable for photons and f 5% for electron beams. The results for Co-60, high energy X-rays and electron beams showed that there were two, three and no centres, respectively, beyond acceptance levels. The sources of errors for all deviations out of this range were thoroughly investigated, discussed and corrected, however two deviations remained unexplained. The pilot study resulted in an im- provement of the accuracy and consistency of dosimetry in Poland.

Keywords: Radiotherapy; Quality Assurance; TLD intercomparison

1. IntroductioIl

There are 52 megavoltage facilities operating at the present time in Poland, in 18 regional oncological cen- tres, delivering radiation therapy to cancer patients: 28 cobalt units and 24 linear accelerators. In these centres and in many other small hospitals and medical schools there are about 70 low energy and orthovoltage X-ray therapy machines. In Poland (population approx. 40 million), the approximate number of new oncology pa- tients, which require radiotherapy, either curative or palliative, reaches about 50 000 per year [28]. It is generally accepted [9,25] that f 5% uncertainty in dose delivery to the irradiated volume is a safe limit causing no severe consequences due to the treatment. Due to the complexity of procedures involved in radiotherapy, from the beam dosimetry, patient data acquisition and treatment planning, to the irradiation of the patient, the development and application of relevant quality assur-

* Corresponding author.

ante (QA) and quality control (QC) programmes seems to be a key factor in reducing overall uncertainty associ- ated with subsequent steps of the radiotherapy chain. There exist many national and international recommen- dations [9,11,15,19,21,25] concerning quality control of high energy machines, and it is rather obvious that car- rying out well established QC procedures in every-day hospital life is an essential condition for reducing some errors in the performance of radiotherapy machines. The uniformity and consistency of the basic dosimetry among different centres is another fundamental factor, which must be considered in contemporary radio- therapy.

Until the late 1980s in Polish regional cancer centres, various dosimetry protocols were in use: ICRU-23 [lo], NACP [20], AAPM [l] and, most recently, IAEA [8]. In 1988, the Polish Secondary Standard Dosimetry Labo- ratory (SSDL) at the Cancer Centre in Warsaw was ap- proved as a member of the IAEA SSDL network and took some responsibilities for carrying out a pro- gramme, the aim of which was to investigate and to

0167-8140/95/$09.50 0 1995 Elsevier Science Ireland Ltd. All rights reserved SSDI 0167-8140(95)01604-F

144 J. Izewska et al. /Radiotherapy and Oncology 36 (1995) 143- I52

reduce the uncertainties involved in the measurement of absorbed dose and to improve dose determination con- sistency in the regional radiotherapy centres. Starting from 1991, most of the Polish centres switched to a na- tional code of practice of absorbed dose determination [4] which is based on TRS-277 IAEA protocol (81, and is extended to determination of absorbed dose to water based on plastic phantom measurements, as well as giv- ing coefficients for ionization chambers manufactured in Poland. A few countries [3,7,12,13,16,18,22,24,26,27] and, at the international level, a few organisations (IAEA [22,23], EORTC [6] and, recently, the EC net- work [2]) have established dosimetry intercomparison for radiotherapy centres. In Poland, the TLD postal dose intercomparison pilot study was organized by the SSDL in 1990-1993 and it covered intercomparison of beam outputs of Co-60 units, high energy X-rays and electron beams. Since the SSDL’s main interest was to study the accuracy and consistency of the basic dosimetry, in every subsequent year one cobalt beam, one high energy X-ray or electron beam per centre was checked. All centres participated in the TLD intercom- parison voluntarily and showed an interest in continua- tion of the project.

2. Material and methods

2.1. TLD system

Lithium fluoride thermoluminescent virgin powder type MT-N (LiF: Mg, Ti - natural abundance, doped with magnesium and titanium) of Polish production was used for the intercomparison. Powder was encapsulated in waterproof perspex containers (4 mm inner diameter, 20 mm inner length and 0.5 mm wall thickness) in por- tions of about 280 mg, which were large enough to ob- tain about 15-18 independent readings. Powder from the same manufacturing batch was always selected and sifted before encapsulation to obtain reproducible mass of the aliquots from the reader dispenser. Irradiated powder was dispensed onto the platinum planchette by the dispenser attached to the Harshaw TLD reader model 2OOOC, which coupled with picoammeter 2000 B was used for all TLD readings. During all readouts the glow curves were recorded in order to eliminate possible errors due to the temperature shift of the reader.

Capsules were irradiated in a water phantom at the reference depth using a holder stand, as shown in Fig. 1. In photon beams, capsules were inserted into one of two lower holes at a depth of 5 or 10 cm (10 x 10 cm, SSD set up) depending on beam quality. The water level was adjusted precisely to the top of the holder and the axis of the beam aligned with the holder axis. For elec- tron irradiations the capsules were inserted into the upper hole, positioned 1 cm from the top of the holder. The water level was adjusted with the help of the dis-

dice indrcator \

TLD capsules

TLD holder /

r

?

z:

1

0 Ln

7

Fig. I. Schematic drawing of the TLD holder stand designed for Polish TLD intercomparisons. The holder should be put into the water tank for the irradiations. The TLD capsules should be inserted into one of three. openings: for photon beams at a depth 5 or 10 cm (depending on the beam quality), the water level is aligned with the top of the holder, the distance indicator is not used; for electrons TLDs should be put into the top opening and the water level adjusted with the use of the distance indicator to get TLDs at the d-, the distance indica- tor is removed during the irradiations.

tance indicator to have TLD at the depth of the maxi- mum dose for the particular electron beam. The distance indicator was removed from the holder before TLD ir- radiations. The holder was not designed for X-ray quali- ty index checks, since Dz,,lDlo is rather insensitive to small dose distribution changes and it allows to detect only large deviations of Dz,-JDlo. For this reason the quality index was not chosen for the intercomparisons.

The determination of absorbed dose to water by the SSDL was based on the TRS-277 IAEA protocol [8] and dose was measured using the secondary standard ioniza- tion chamber type 2561 certified by the Primary Stan- dard Dosimetry Laboratory in Warsaw.

J. Izewska et al. /Radiotherapy and Oncology 36 (1995) 143-152 145

The absorbed dose to water, D (Gy), at the location of TLD was calculated from the TL signal, R, registered by the TLD reader using the following formula

D = R X &I X &in X &ad X &n (1)

where: R is the TLD reading normalized to the mass of aliquot of the powder and corrected for the reader’s daily fluctuations, &,t (Gy/pC) is the calibration factor of the TLD system; J&i is determined for 2 Gy from Co-60 beam, Ktin is the dose response linearity correc- tion factor, Kfad is the fading correction, and Z&., is the X-ray energy response correction.

The calibration factor J&i was calculated as the ratio of dose to TL-signal for the dose to water equal to 2 Gy at the centre of the TLD capsule. The irradiations were done with Co-60, Theratron 780C. The capsules were in- serted into the holder and placed in water phantom at a depth of 5 cm, 10 x 10 cm, SSD = 80 cm. Before TLD irradiations Co-60 beam output was checked with the ionization chamber type 2571 used as a local standard. The TLD calibration factor was determined directly before mailing the samples to the participating centres and was not corrected for the daily reader fluctuations. Only the readings of the samples were corrected for the reader’s sensitivity changes, which were examined a few times during each reading session with the sample ir- radiated to the standard dose of 2 Gy. The stability of the sensitivity of the reader depends on its electronics, optics and the reflectivity of the planchette. Apart from standard samples readings, the constancy checks of the reader stability were also done between the subsequent readings using the internal reference light source.

The dose response linearity factor K,in corrects the TL-response as a function of dose delivered to the powder. To determine the linearity factor, TLD samples were irradiated with Co-60 beam at standard conditions with different doses in the range of 1.5-2.5 Gy.

The TL-readings should also be corrected for fading with correction factor Kfad, if the irradiation of the samples was done at a different time than the TLDs used for the determination of the system calibration factor. The long-term fading for MT-N powder was determined from the readings taken over 12 weeks, at l-week inter- vals. A dose of 2 Gy was delivered to each of 13 TLD capsules the same day. The first reading was done one week after irradiation.

For the energy response study, the capsules were ir- radiated with 2 Gy in a water phantom under the refer- ence conditions, with X-rays in the energy range 4-15 MV, and electron beams in the range 6-20 MeV. For each energy beam, three capsules were irradiated. As a reference, one capsule was irradiated with a Co-60 beam. Samples taken from the same LiF batch were con- sidered to have consistent calibration factor, dose re- sponse linearity, fading and energy characteristics.

Other factors, which might influence the accuracy of determination of absorbed dose from TLD readings have also been investigated. One of them is the statistical distribution of TLD readings. Twenty TLD capsules providing 300 readings were irradiated to the same dose of 2 Gy from Co-60 and read in the same conditions within the same interval between irradiation and readout.

The combined uncertainty in dose calculation from TLD measurements comprises the uncertainty of dose determination by ionization chamber measurements and the TLD system itself. The factors &I, Klin, Kfad and K,, used in Eq. (1) for the calculation of absorbed dose from the TLD reading R are all determined experimen- tally, or evaluated from experimentally determined functions, and all these factors have corresponding ran- dom errors.

Since absorbed dose to water based on ion chamber measurements was determined using IAEA TRS 277 formalism [8], the contribution to the combined uncer- tainty originating from the IAEA Code of Practice inter- action coefficients, as well as the calibration of secondary standard, were disregarded due to the system- atic nature. The SSDL applied IAEA TRS 277 [8] values for the uncertainties of absorbed dose at the reference point, which is, for Co-60 0.5%, and for high energy X- rays and electron beams 1.0%. The uncertainty of the fading correction was estimated from the standard error of the least square regression coefficient - slope of the fading function [ 171. The uncertainty of the energy cor- rection factor is the combined uncertainty of the ion chamber dose determination for Co-60, high energy X- ray or electron beams and TLD reading uncertainty. The total uncertainty of the TLD system is defined as square root of the sum of variances of individual coeffi- cients, which is valid under an assumption that the uncertainties of the different coefficients are indepen- dent and follow a Gaussian distribution.

2.2. TLD intercomparisons

In the three subsequent years, three separate inter- comparisons were performed: (A) Co-60 intercom- parison in 1991; (B) high energy X-rays in 1992; and (C) high energy electrons in 1993.

Each participant of the study was identified by a code number to keep all results confidential. For each inter- comparison radiotherapy centres were provided with:

l a copy of national dosimetry recommendations [4]; l an information sheet describing the method of irradi-

ation of TL-detectors; l a data sheet to enter the specifications of the therapy

machine, measuring instrument, and also the method used for the absorbed dose to water determination, and the details concerning TLD capsules’ irradiation;

146 J. Izewska er al. /Radiotherapy and Oncology 36 (1995) 143-152

l the perspex holder stand, shown in Fig. 1, in which TLD capsules were placed for irradiation;

l three waterproof perspex capsules filled with lithium fluoride powder for irradiations;

l one perspex capsule (4 mm inner diameter, 15 mm inner length and 0.5 mm wall thickness) tilled with the same LiF powder serving as a background record.

The participants were requested to irradiate the cap- sules in sequence to the absorbed dose of 2 Gy and to check the beam output with their dosimetry system before the irradiation of TLDs. The intercomparison was scheduled so that the irradiations in all participating centres were done at nearly the same time (within 1 week). At this time, the SSDL irradiated one reference sample per centre with 2 Gy from Co-60.

For each capsule the mean reading value and the stan- dard error of the mean (SD) were determined. The aver- age reading value of three capsules (evaluated absorbed dose) for each participant was calculated. The devia- tions A of reported and measured absorbed dose for each participant were calculated according to the for- mula recommended by the IAEA [8]:

Dp - ljSsm , lw, A= - 0 DSSDL

(2)

where: &sDL is average absorbed dose determined by the SSDL; and D, is dose reported by the participant.

The quotient of the mean reading of each capsule to the average reading from three capsules (measured dose/average dose) for each participant was also determined.

3. Results

3.1. TLD system

The example of fluctuations of the Harshaw reader registered from the internal reference light source is shown in Fig. 2. In Fig. 2a the histogram of 100 measurements of the internal light signal is shown. The readings are normally distributed (x2 = 7.59, OL = 0.5) with a standard deviation of (I = 0.2%. In Fig. 2b the daily fluctations measured within 8 h are plotted. Each point represents the average of 10 readings. A small increase of the reading values in time was noted, but the maximum change was less than 0.4% which was not con- sidered to be significant.

The histogram representing statistical distribution of 300 independent TLD readings (20 capsules irradiated to 2 Gy with Co-60 beam) is shown in Fig. 3. The TL-response follows again a Gaussian distribution (x2 = 12.5, a! = 0.25) with a mean value of 1.92 PC (K&t = 1.0417 Gy/&) and standard deviation of a sin-

200.0 3

0 199.0

5

Fl98.0

I-- ~/+/----

- - -w ----_- _

-E

f 1970

‘950 1,,,,,,.,,,.,..,.,,,,,,,,,,,,.,,,,,,,,.,,

0 1 2 3 ~i-~our]~ 6 7 8 Time

Fig. 2a. The histogram of 100 measuCements of the internal light signal of the Harshaw reader. The readings are normally distributed (x2 = 7.59, Q = 0.5) with the standard deviation of u = 0.2%. Fig. 2b. The example of daily fluctuation of the Harshaw reader registered from the internal reference light source. Each point of the plot represents the average of 10 readings. An insignificant increase of the readings in time does not exceed two standard deviations.

gle reading, (I = 2.4%. Standard deviation of the mean (SD) for a single TLD capsule does not exceed 0.6% for Co-60 with its average value of 0.5%. For X-ray beams the deviation remains almost the same, but for electrons, due to non-uniform dose distribution within the capsule originated from the scattering from the walls of the perspex holder, the maximum standard deviation of the mean is 1.1% and its average value equals 0.7% (data from 36 capsules).

The linearity correction for the dose within 2 f 0.5 Gy is &in = 1 (i 0.005) The fading of the TL-signal of the MT-N powder, shown in Fig. 4, did not exceed 2% per 12 weeks. SSDL corrected the readings for the fading individually for each mailing, by irradiating reference capsules in the same week as the participants. The resulting uncertainty of the fading correction was found to be less than 0.5%. Table 1 shows the relative energy correction factors for LiF powder in perspex cap- sules irradiated with X-rays and electrons in water under

J. Izewska et al. /Radiotherapy and Oncology 36 (1995) 143-152 141

0 ,"""'"I""""'I"'"""I""'"" 1.78 1.83 1.88 1.93 1.98 2.03

Reading [PC]

Fig. 3. The histogram of 300 independent TLD readings taken from 20 capsules irradiated to 2 Gy with a Co-60 beam. The TL-response follows the Gaussian distribution (x2 = 12.5, Q = 0.25) with a mean value of 1.92 CC (Qt = 1.0417 Gy/&) and standard deviation of a single reading u = 2.4%.

the reference conditions. Assuming that the uncertainty of ion chamber dose determination for Co-60 is 0.5% and for X-rays 1% (IAEA [8], Table XXIII), and uncer- tainty of TL-reading is 0.5% for photons, the combined uncertainty of the energy correction for X-rays is 1.3% SD. For electrons, due to larger uncertainty in dose

10 4

0 1 2 3 4 5 0 7 0 9 10 11 12

Time intfzrval [weeks]

Fig. 4. The fading of TL-signal of the MT-N powder. TLD capsules were irradiated the same day and read in subsequent 12 weeks. The readings were corrected for the daily fluctuations of the reader sensi- tivity, checked with freshly irradiated reference samples. The fading does not exceed 2% per 12 weeks.

Table 1 The relative energy correction factors for LiF powder in perspex cap sules irradiated in water under reference conditions

Photons Electrons

Nominal TPRZO,s Correction Energy Correction energy factor factor WV)

1.25 - 1 4.0 0.64 1.01 9.0 0.72 1.02 15.0 0.76 1.03

6sEs9MeV 1.04 IO s E s I8MeV 1.03 20 MeV 1.02

reproducibility of 2.4%, which comprises dose deter- mination as well as geometry and TL-reading uncer- tainty, the energy correction is evaluated with an uncer- tainty of 2.5%.

The total uncertainty of the TLD system was estimated to be of about 1 .O% for Co-60 and about 1.7% for high energy X-rays and about 2.7% for electrons. Table 2 summarises the contribution of the individual uncertainties to the total uncertainty of the Warsaw SSDL TLD system. Based on these uncertainties, and to find a compromise for photon beams, an acceptance level of 3.5% has been chosen for both Co-60 and X-ray intercomparisons (22SD) and 5% (approx. 2SD) for electrons. All deviations detected during the intercom- parison being beyond the acceptance level were carefully investigated and the reasons for errors examined and discussed with the participants.

Table 2 Combined uncertainties of the individual coefficients to the absorbed dose to water determined from TLD readings

Coefficient Uncertainty (%)

Ion chamber dose determination: co-60 0.5 X-rays 1.0 Electrons 1.0

Capsule reading Calibration coefftcient Linearity correction Fading correction

co.7 0.7 0.5 0.5

Energy correction: X-rays Electrons

1.3 2.5

Total Co-60 1.0 Total X-rays 1.7 Total electrons 2.7

148 J. Izewska et al. /Radiotherapy and Oncology 36 (1995) 143-152

Table 3 Data concerning detertnination of absorbed dose to water for mailed TLD intercomparison for Co-60 beams

Beamcude number 9001 9002 9003 9004 9005 9007 9008 9009 9011 9012 9013 Model of cobalt Philips Theratron Gamma- Alcyon II Gamma- Theratron Theratron Gamma- Theratron Gamma- Gamma-

unit 780C tron s tron 3 780 780 tron S 780 tron S tron S Year of installa- 82 86 78f90 83 7of90 86 85 80187 88 78184 78190

tion/source replacement

Type of electro- Ionex Ionex Ionex Farmer Ionex Ionex Farmer Farmer Fartner PTW Ionex meter 2500/3 250013 DM 2590 2570/lB 2500/3 250013 2570A 2570A 2570/1B DL4/DI4 2570/l

Type of ion NE NE 2581 NE NE 2571 NE 2571 NE 2571 NE 2581 NE 2571 NE2581 PTW NE 2571 chamber 2505/3A 2505l3A 23331

Measured/average 0.999 1.008 1.006 1.004 1.007 1.002 1.000 1.001 0.994 1.000 0.996 dose, TLD 1.002 0.997 0.996 0.999 0.995 0.997 1.000 1.002 1.004 1.001 1.006

0.999 0.996 0.998 0.997 0.999 1.001 1.000 0.998 1.002 0.999 0.998 Deviation A (%) 0.9 -1.5 1.0 0.4 -0.7 -0.5 -5.0 12.1 0.4 2.5 1.5

3.2. TLD intercomparisons

Out of 12 Polish regional centres performing Co-60 and -13 accelerator radiotherapy, the cooperation of most centres was obtained (11 centres for Co-60, 11 for X-rays and 12 for electron beams). Tables 3-5 present selected data gathered from the centres concerning their radiotherapy units, dosimetry systems and the deter- mination of absorbed dose to water by the ionometric method and TLD for Co-60, high energy X-rays and electron beam intercomparison, respectively.

Ionization chambers (0.6 cc) manufactured by the Nuclear Enterprises type 2571 and 2581, connected to Ionex 2500/3 or Farmer 2570 dosimeters, are in common use in Poland. A review of the information comprised in the data sheets, confirmed that most of the radiotherapy centres use dosimeters calibrated in the preceding 3 years; two centres used non-valid SSDL calibration cer- tificates. Fortunately, the deviations of the beams in

these two centres were found to be within acceptance limits. Most participants determined absorbed dose to water in the reference point using a water phantom, one participant uses a perspex phantom for beam output calibrations for photons and a polystyrene phantom for electrons.

A majority of the participants, with the exception of one centre, have declared their intention to follow the TRS-277 IAEA [S] recommendations, however a few mistakes and inconsistencies revealed by the data sheets prove that, sometimes, an understanding of the employed protocol is not full or some minor mistakes in interpretation are made. All these mistakes and misinterpretations were carefully discussed with the par- ticipants, and explained and corrected.

Table 6 summarizes the results of our study in the context of other recent TLD intercomparisons perform- ed by different national and international institutions [2,5-7,18,22,23]. To compare the results of this work

Table 4 Data concerning determination of absorbed dose to water for mailed TLD intercomparison for photon beams

Beam code number 9201 9202 9203 9204 9205 9207 9208 9209 9210 9211 9212 Model of Satume Satume Neptun Neptun Neptun Neptun Neptun Neptun Neptun Neptun Neptun

accelerator II+ 43 lop lop lop lop lop lop lop lop lop Nominal X-ray 10 15 9 9 9 9 9 9 9 9 9

beam energy WV)

TPRZO/l 0 0.74 0.73 0.709 0.73 0.716 0.731 0.70 JlON20 1.52 1.61 1.607 1.62 1.602 1.64 1.626 Type of electro- Fanner Farmer Farmer Ionex Ionex PTW Ionex Fartner Ionex Ionex Farmer

meter 250313 257Of 1 B 25023 250013 250013 D14-775 1 2500/3 2570llB 250013 250013 257011 Type of ion NE NE 2571 NE NE 2571 NE 2571 PTW NE 2571 NE2581 NE2581 NE 2571 NE 2581

chamber 250513A 2505/3A 23331 Measured/average I .OOO 1.008 1.004 0.994 0.998 0.997 1.022 0.993 1.005 0.993 0.986

dose, TLD 1.005 0.982 1.008 1.002 0.991 1.012 0.990 1.026 0.994 1.001 1.013 0.995 1.011 0.988 1.004 1.012 0.992 0.988 0.981 1.002 1.007 I.@30

Deviation A [“‘I 0.8 -2.6 -7.2 0.9 -6.4 -0.6 4.1 -3.5 -I -0.6 -3.2

Tabl

e 5

Dat

a co

ncer

ning

det

erm

inat

ion

of a

bsor

bed

dose

to w

ater

for

mai

led

TLD

in

terc

ompa

rison

for

ele

ctro

n be

ams

Beam

code

num

ber

9301

93

02

9303

93

04

9305

93

06

9307

93

09

9310

93

11

9312

93

13

Mod

el o

f ac

cele

rato

r N

omin

al e

lect

ron

beam

en

ergy

(MeV

) Av

erag

e en

ergy

at

the

phan

tom

sur

face

EO

W

V)

Dep

th o

f do

se m

axi-

mum

(a)

Type

of e

lect

rom

eter

Type

of

ion

cham

ber

Mea

sure

d/av

erag

e do

se, T

LD

Dev

iatio

n A

(%)

Satu

me

II+

Satu

me

43

Nep

tun

IOp

Nep

tun

1Op

Nep

tun

IOp

Nep

tun

IOp

Nep

tun

1Op

Nep

tun

1Op

Satu

me

II+

Nep

tun

IOp

Nep

tun

1Op

Nep

tun

1Op

9 10

.5

10

10

10

8 10

10

15

10

8

8

8.6

9.79

8.

85

8.90

9.

20

6.85

8.

70

9.00

13

.3

9.3

7.27

7.

25

2.2

2.4

2.3

2.35

2.

4 1.

7 2.

2 2.

2 3.

0 2.

2 1.

8 1.

7

Ione

x Fa

rmer

Io

nex

2500

/3 Io

nex

2500

13 Io

nex

2500

13 lo

nex

2500

13 Fa

rmer

Fa

rmer

Io

nex

2500

13 Io

nex

2500

13 Fa

rmer

Io

nex

2500

13

Dos

emas

ter

2570

/l 25

7011

25

70llB

25

7011

N

E 25

71

NE

2534

N

E 25

05J3

A N

E 25

71

NE

2571

N

E 25

81

NE

2581

N

E 25

81

NE

2581

N

E 25

71

NE

2571

N

E 25

81

0.99

0.

96

- 1.

03

1.02

1.

04

0.93

1.

00

0.99

0.

99

1.05

1.

02

1.03

1.

02

1.04

0.

97

1.02

1.

00

1.04

0.

98

1.00

1.

03

0.99

1.

02

0.98

1.

02

0.96

I.0

0 0.

98

0.97

1.

03

1.02

1.

01

0.98

0.

97

0.97

I.0

-2

.5

-0.5

-4

.9

-2.1

-2

.9

2.0

-3.8

1.

8 2.

0 0.

0 4.

7

150 J. Izewska et al. /Radiotherapy and Oncology 36 (1995) 143-152

Table 6 Results of a few recent TLD photon and electron beam intercom- parisons in reference conditions

Reference Study Number Mean Standard of beams deviation

Dutreix at al. PI

Hanson at al. 151

Hansson at al. I61

Huntley at al. (71

Kiyak at al. 1181

Penchev at al. WI

Svensson at al. 1231

This work

EC 91-93 photons

RPC 84-92 photons electrons

EORTC 87-92 photons

Australia 92 photons

Turkey 89-91 CO-60

Bulgaria 75-92 co-60

IAEA 69-87 co-60

Poland 91-93 photons electrons

125 1198

8895 5215

357

30

45

173

1945

22 12

0.970 0.095 0.985a 0.025a

1.000 0.024 0.993 0.032

1.007

0.993

1.025

0.986

0.998

1.004 1.004

I.040

0.033

0.048

0.033

0.067

0.038 0.027

BExcluding deviations > 12%. The mean values and the standard deviations of the distributions of measured-to-stated doses are reported.

with other studies, we recalculated mean and standard deviation of the distribution of ratios of measured-to- stated doses.

4. Diion

4.1. Results of the intercomparisons

A. TLD intercomparison for Co-60 beams As can be seen from Table 3, the centres did their ir-

radiations with Co-60 beams produced by Theratrons (four beams), Gammatrons (five beams), Alcyon (one beam) and Philips (one beam). The reported percentage depth dose (PDD) at 5 cm for Co-60 ranged from 77.2 to 78.9%, which is interesting, since some participants employ published isodose data for Co-60, and some use data measured by the department.

The deviations A of the quoted absorbed dose from the average of the SSDL evaluations were within f 3.5% acceptance level for nine participants out of eleven. For two centres, differences in stated to evaluated dose became more serious (-5% and 12.1%). These cases were thoroughly discussed with the participants, and the sources of errors traced during on-site visits. During the first visit it was explained that -5% deviation was caus- ed by a few small errors in geometry and calculation, which is additive. The reason for the 12.1% discrepancy was due to calculation mistake: wrong chamber factor,

exposure factor N, instead of absorbed dose factor No was used. In both cases the mistakes were made during intercomparison and the outputs used for patient ir- radiations were not influenced. In the latter case, how- ever, the physicist did not compare the output used for patient treatments with that reported in the TLD data sheet.

The average quotient of the measured dose for a sin- gle capsule to an average of three capsules was found to be 0.3% for all participants, which shows very good reproducibility of TLD irradiations and measurements.

B. High energy X-ray intercomparison For high energy X-ray checks (Table 4), the par-

ticipating centres typically used 9 MV beam from Nep- tuns 1OP (9 beams), however Satume II+ and Satume 43 beams (10 and 15 MV) were also checked.

The deviations A of the quoted absorbed dose from the average of the SSDL evaluations were within f 3.5% acceptance level for eight participants out of eleven (see Table 4). In one case value of the quoted dose differed from SSDL value by 4.1%. In two cases, large errors were detected (-6.4 and -7.2%): both were caused, ac- cording to the participants, by geometry errors during TLD irradiation; 4.1% deviation remained unexplained. All three beams were immediately remeasured by the participants, and the outputs used for patients agreed with rechecked and reported values, which suggests that patients were probably not at risk. The reason for the bad TLD results may be thus related to insufficient care taken during TLD irradiations.

Review of the data sheets revealed a few inconsisten- cies in the values of the displacement of the effective point of measurement, in the applied perturbation cor- rection factor, and in the stopping power ratio water-to- air - which were incorrectly read from Fig. 14 and Table XVIII of the TRS-277 recommendations [8]. Cal- culation errors were also made in the dose delivered to capsules. These factors only slightly (< 1%) influenced the value of the dose calculated as compared to the dose reported by the participants.

As was calculated from Table 4, the average deviation of the dose from the single capsule from the average of three capsules was 0.9%.

C. Intercomparison for electron beams Most of the participants in the electron project (Table

5) irradiated their capsules with nominal beam energy of 8-10 MeV using Neptuns 1OP accelerator. Only one centre irradiated TLDs with the energy of 15 MeV.

From the review of data sheets it appeared that four centres made small mistakes (< 1%) and one centre made a large error (19%) in the chamber perturbation correction factor. The physicist wrongly used for chamber radius a value for the diameter, and ex- trapolated perturbation corrections from Table XI of

J. Izewska et al. /Radiotherapy and Oncology 36 (1995) 143-152 151

TRS-277 [8] and reached an extremely large correction. There were also minor mistakes in the values of the stop- ping power water-to-air ratio. A few inconsistencies in values of the displacement of the effective point of mea- surement appeared. Calculation mistakes were also made, however most of these mistakes were small (< 1%) and did not significantly influence a value of the absorbed dose delivered to the capsule. In one case seri- ous errors were made; but combined with a 19% error of perturbation factor they luckily cancelled out! One may expect, that in this centre, the patient treatment may be potentially affected.

It can be seen from Table 5 that the deviations A of the quoted absorbed dose from the average of SSDL measurements were all within iS%. It was observed that the standard error of the mean for each capsule is larger for electrons (0.4-l. 1%) than that for X-rays (0.3-0.6%). The reason may have been inhomogeneous dose distribution within the capsule due to scattered electrons from the holder walls.

The average deviation of the dose from a single cap- sule to an average of three capsules, calculated from Table 5, was 2.4%, which indicated that the reproducibility of the TL-signal of samples irradiated with electrons had a larger spread than TLDs irradiated with photons (0.3% for Co-60,0.9% for high energy X- rays). Since it is rather unlikely that the stability of all checked linacs (mainly Neptuns 1OP) is generally worse for electron beams than for photons, the larger spread of electron beam data may be because of the larger uncertainty in the reproducibility of irradiation geometry.

4.2, Comparison with other TLD studies

The requirement of l 5% accuracy in the delivery of absorbed dose to a target volume in a patient, as it is generally accepted [9,25], presupposes that the uncer- tainty in the dose determination in a reference point in a phantom is smaller. The EORTC in Europe and RPC in the USA [5,14] set a tolerance level of *3% for the uncertainty of the calibration of the therapy machines as an acceptable practice in radiotherapy. Using mailed TLdosimetry with its limited precision due to uncer- tainties incorporated into the method (see Section 3.1), we set acceptance limits of *3.5% for photons and f 5% for electrons. Our method does not allow the satis- factory identification of small deviations to check if the beams meet the criterion of *3%, however the accep- tance levels in this work remain consistent with other TLD networks with acceptance limits between 3 and 5% [5-71.

As can be seen from Table 6, the mean value of the ratio of measured-to-stated doses is, in this study, 1 JO4 for photons and 1.004 for electrons, indicating, that no large systematic error was detected, even if the statistics

is based only on 22 checks for photons and 12 checks for electron beams. The mean of measured-to-stated doses ranges from 0.970 for the EC network (including devia- tions > 12%) to 1.025 for Turkey intercomparisons. The standard deviation of the results in our study is SD = 0.038 for photons and SD = 0.027 for electrons, whereas for other intercomparisons it ranges from 0.024 (RPC) to 0.095 (EC network, including deviations > 12%) for photons and 1.032 (SD) for electrons (RPC). As was reported by Dutreix et al. [2], excluding devia- tions larger than 12% (approximately 5% of the results), the mean and standard deviation of the distribution become 0.985 and SD = 0.025, respectively. In the IAEA Co-60 intercomparison, which covers the SSDL network and many radiotherapy departments mainly in developing countries, deviations exceeding 30% (ap- proximately 1% of results) have also been excluded from the analysis. The IAEA standard deviations taken only for European countries, as was reported by Svensson [23], vary from 0.019 to 0.083 for different countries. The EORTC intercomparison covers only selected radiotherapy centres, which take part in clinical trials and for which lower deviations might have been ex- pected, whereas national intercomparisons are organiz- ed for all radiotherapy centres willing to participate. The figures from this work compare well with the statistics of other intercomparisons, even if the number of beams checked in our study is small. In this pilot study we focused on the protocol compliance only, omitting clini- cal aspects of the dosimetry, since our goal was to test the consistency of the basic dosimetry in Poland as a first step of the larger QA programme. Conclusive veri- fication of absorbed dose actually delivered to a patient during radiation treatments may be performed by in vivo dosimetry.

5. c0nc1usI0ns

(1) The intercomparison pilot study has made it possi- ble to test the consistency of high energy photon and electron beam dosimetry in regional radiotherapy cen- tres in Poland.

(2) The TLD intercomparison has shown that the high energy photon and electron beam dosimetry in most centres in Poland remains within acceptable limits.

(3) Out of 34 checks of Co-60, high energy X-rays and electron beams (see Tables 3-5), in 29 cases the devia- tion of the absorbed dose stated by the participants compared with values measured by the SSDL did not exceed the acceptance level of *3.5% for photons and *5% for electrons.

(4) The standard error of the mean for a single TLD capsule, which is less than 0.6% for X-rays and less than 1.1% for electrons showed that the precision of the pro- cedure developed by the SSDL was satisfactory.

(5) It follows from the experience acquired in the pilot

152 J. Izewska et al. /Radiotherapy and Oncology 36 (1995) 143-152

study, that there is a permanent need for contact with physicists from regional centres to discuss the dosimetry problems they face in professional life, and also that the support they receive is fully appreciated. All participants are in favour of continuation of the project.

(6) The pilot study confirmed that there is a need for further clinical dosimetry training of physicists em- ployed in oncological centres performing radiotherapy.

Acknowledgements

The authors wish to acknowledge Danuta Milkowska for her help with the TLD readings as well as other col- leagues from the Medical Physics Department of the Cancer Centre in Warsaw, who were temporarily involv- ed in the TLD intercomparisons. This work was partial- ly supported by Research Contracts 6013/RB and 6013/RB/Rl awarded by the IAEA.

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