IAEA-CN THE ESTRO-EQUAL RESULTS FOR PHOTON AND ELECTRON BEAMS CHECKS IN EUROPEAN RADIOTHERAPY BEAMS*

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THE ESTRO-EQUAL RESULTS FOR PHOTON AND ELECTRON BEAMS CHECKS IN EUROPEAN RADIOTHERAPY BEAMS* H. Ferreira, A. Dutreix, A. Bridier, D. Marre, J. Chavaudra, H.

1 THE ESTRO-EQUAL RESULTS FOR PHOTON AND ELECTRON BEAMS CHECKS IN EUROPEAN RADIOTHERAPY BEAMS* H. Ferreira, A. Dutreix, A. Bridier, D. Marre, J. Chavaudra, H. Svensson ESTRO-EQUAL Measuring Laboratory, Service de Physique, Institut Gustave-Roussy, Villejuif, France Abstract XA Background and purpose: European Society for Therapeutic Radiology and Oncology (ESTRO) has set up a Quality Assurance network for radiotherapy (EQUAL) carrying out dosimetry audit. Some of the work is done in cooperation with the IAEA. The network deals with measurements performed with mailed TLD irradiated in reference and non-reference conditions, for on-axis points in photons and electrons beams. Material and methods: The LiF DTL937 (Philitech, France) was used and read with the PCL3 automatic reader (FIMEL-PTW). The participating centres irradiate the TLD capsules to an absorbed dose of 2 Gy determined with the Treatment Planning System used in clinical routine. Results: Statistical data from the participating centres on their radiotherapy structure such as number of machines and beams qualities available, dosimetry protocols and equipment in use were analysed. 23 European and 2 Mediterranean Basin countries participated. Photons beam audit: 282 centres and 757 beams have been checked; 11% 60 Co beams and 89 % of X-ray beams. Compared to the EQUAL reference dosimetry 1.4 % of the reference beam output dose values and 3% of the percentage depth doses are outside the tolerance level (deviation >±5%). The standard deviation for the reference beam output is 1.8 %. Five percent of the rectangular field dose checks and 4 % the wedge transmission factors had deviations >±5%. The analysis of the global results shows deviations >±5% in at least 1 point for 133 out of the 757 beams, mainly for large and rectangular fields and for wedged beams. At least 45 of these centres had one "real dosimetric" problem in one or more parameters, which corresponds to 7% of the checked beams. Electron beam audit: 97 centres and 277 beams have been checked. 1.0 % of the reference beam output values (field size 10 cm x 10 cm) and about 2 % of the beam output for the others field sizes (15 cm x 20 cm, and 7 cm x 7 cm) are outside tolerance level (deviation >+5%). The standard deviation for the reference beam output is 2.1% and for other field sizes about 2.4%. There are about three times more deviations outside the optimal level (> 3%) with electron beams than with photon beams. This indicates that improvements on the electron beam calibration and on the TPS algorithms for the electron beams are needed. Conclusions: The EQUAL programme shows good results for the reference beam output for photon and electron beams, which facilitate exchange of experiences between centres. The largest deviations are for non-reference geometries, which might result in some dose variations between patients in a centre treated with the same prescribed dose but with different set-ups. Keywords: Radiotherapy, Quality Assurance network, Dosimetry intercomparison, TLD Introduction ESTRO (European Society for Therapeutic Radiology and Oncology) has a Quality Assurance network (EQUAL) to check the dose on axis in reference and non-reference conditions for external radiotherapy for photon and electron beams. The external audits covered by the network are based on measurements made with mailed thermoluminescent dosimeters (TLD). The radiation-induced morbidity is related to the quality of radiotherapy treatment delivered. The EQUAL programme is therefore one of the tasks of the MORQA project funded by a EU contract (Reduction of Radiation Morbidity Through Q.A. of Dosimetry and Evaluation of Morbidity). The goal is to improve the radiotherapy in the EU countries and in particular * Supported by the Europe Against Cancer Programme of the European Union. 625

2 reduce unnecessary radiation induced complications. The present paper reports the third year's experience on photon and electron dosimetry checks Material and methods From 1998, the EQUAL Network provided dosimetry checks of Co-60 and X-ray beams (Ferreira et al, 2000, Marre et al, 2000) and from December 1999 also of electron beams. Results The figure 1 shows the number of participating centres (checked, and scheduled to be checked) and the total number of centres per country. About 50% of the eight hundred European centres have applied showing the great interest for external dosimetry audit. There are in some countries national or international programmes (IAEA/WHO) on dosimetry audit which is the reason for some of the low numbers (Thwaites et al 1992, Nisbet and Thwaites 1997). Figure 1: Percentage of centres per country that have applied to participate in EQUAL programme (data in October 2000), compared to the total number of radiotherapy centres in each country. Dosimetric results and observed deviations: Photon beams The ratios between the measured dose value Qm and that stated by the participating centre Qs were calculated for all checked parameters: reference beam output (RBO), percentage depth doses (PDD), beam output variation (BOV open and BOV Wedged) and wedge transmission factor (WTF). Definitions are given by Ferreira et al (2000). The global results for the photon beam checks are shown in the Table 1. The number of beams with deviations in each of the intervals used in the EQUAL protocol ( 3%, >3 to 5%, >5% to 10% and >10%) are reported for RBO, PDD, BOV and the WTF. A total of 6700 parameters have been checked. The values in standard characters contain all checked beams, including the beams with deviations due to a possible set-up error during the TLD irradiation. The values in bold-italic characters are the results of checked beams in an interval minus the beams with possible set-up errors in the 1st check. Set-up errors are assumed when the MU (monitor units) has not been modified between the 1 st and the 2 n d check but the deviation of the recheck measurement is inside the tolerance level (<+5 %). The global result analysis shows deviations >±5 % in at least 1 point for 133 out of the 757 beams. In these 133 rechecked beams, 88 beams had some deviations >±5 % due to set-up errors and 45 beams had "real dosimetric" problems. The set-up errors are considered as being 626

3 due to the incorrect SSD or SDD, an error on the irradiation depth, the wedge orientation or other obvious mistake during the TLD irradiation. For the reference conditions (RBO), 95 % of the results are in the optimal level of deviations (Qm/Qs < ± 3 %), 4 % are in the level between ± 3 % and ± 5 %, and only 1.4% are outside the tolerance level (Qm/Qs >± 5 %). For the percentage depth doses 90% of the checked beams are in the optimal level, 7 % have deviations between ± 3 % and ± 5 %, and 3% of the checked beams are outside the tolerance level (table 1). For the beam output variation using open beams 84%, and using wedged beams 87%, are in the optimal level, 4% (open beams) and 3% (wedged beams) are outside the tolerance level (table 1). For wedge transmission factors 81% of the results are in the optimal level and 4% are outside the tolerance level. The "dosimetric" problems concern 45 beams out of the 133 beams rechecked. A total of 7% (45/669 beams) of the checked photon beams have at least one checked parameter with "dosimetric" problems. The largest number of set-up errors is for wedged beams. Probably, this is so also in clinical practice. Table 1: Number of photons beams with deviation in different intervals. The values in standard characters contain all checked beams, including the beams with deviations due to a set-up error in the TLD irradiation. The values in bold-italic characters concern the results of checked beams minus the beams with set-up errors in the 1st check when the MU has not been modified between the 1 st and the 2nd checks. Parameters checked Reference Beam Output Percentage Depth Dose Beam Output Variation Open Beams Beam Output Variation Wedged Beams Wedge Transmission Factor < 3% 676 (90%) 637 (95%) 603 (87%) 548 (90%) 599 (84%) 529 (84%) 558 (85%) 511 (87%) 520 (78%) 475 (81%) Deviation Levels (Qm/Qs) >3 to < 5% 54 (7%) 25 (4%) 62 (9%) 45 (7%) 82(11%) 78 (12%) 73(11%) 60 (10%) 99 (15%) 85 (15%) >5to<10% 17 (2%) 5 (1%) 16 (2%) 26 (4%) 21 (3%) 14 (2%) 22 (3%) 19 (3%) >10% 10 (1%) 2 (<0.4%) 4(1%) 9 (1%) 3(1%) 15 (2%) 3(1%) 24 (4%) 5(1%) Total of Beams Dosimetric results and observed deviations: Electron Beams Since November 1999 till end of September 2000, 97 centres, out of 267 centres that have applied, and 287 beams qualities have been checked. Table 2 contains all checked electron beams, minus the beams with set-up errors in the 1st check and not yet followed up (287 beams minus 10 beams). As for the photon beam checks, the analyse of the overall results for the electron beams checks is based on the results excluding beams with set-up errors during the TLD irradiation. 86% of the reference beam output values were in the optimal level of deviation (Qm/Qs< ± 3 %), 13 % of the beams had a deviations between ± 3 % and ± 5 % and only 1 % were outside the tolerance level (Qm/Qs >± 5 %).As can be seen from Table 2 the largest deviations were for the largest field size 15 cm x 20 cm. 627

4 Table 2: Number of deviations for EQUAL electron beam outputs checks with different field sizes and SSDs. The results concern the electron beams checked minus the beams with set-up errors in the 1 st check. Reference Beam Output 10 cm x 10 cm 15 cmx20 cm 7 cm x 7 cm 10 cm x 10 cm Total (SSD > 105 cm) TLD used land All TLDs <3% 239 (86%) 212(81%) 241 (83%) 116(79%) 788 (83%) Deviation Levels (Qm/Qs) >3 to < 5% >5to< 10% 35 (13%) 3 (1%) 46 (17%) 6 (2%) 41 (15%) 4 (2%) 29 (20%) 1 (1%) 151 (16%) 14 (1%) Total of >10% beams The standard deviation for the beam output values in reference conditions (10 cm x 10 cm, at SSD 100) is 2.1% with a mean value of for Qm/Qs. The beam output for the 15 cm x 20 cm and 7 cm x 7 cm field size has a standard deviation of 2.4 % and 2.3 % respectively with a mean value of Conclusion The EQUAL network has carried out a total of 757 photon and 277 electron beam checks during the last three years. A total of 398 centres have applied to participate in the EQUAL programme (about 50 % of European Radiotherapy centres). The EQUAL programme for electron beams were initiated in November 1999 and 268 centres have already applied to participate. In the first year of the programme mostly large centres applied. Efforts were therefore made to stimulate small centres. This has resulted in a good participation also of centres with one or two radiation machines. For the photon beams in reference geometry the mean ratio of measured to stated doses is with a SD of 1.8 % and for electrons the mean value is with a SD of 2.1 %. About 7 % of the checked beams have dosimetric problems, which could have a real influence on the patient cancer treatment. Beams with dosimetric problems have been followed up and the errors have been corrected and confirmed by the rechecked measurements or on site visit performed by the EQUAL physicists. The results for the electron beam checks are very good, 98% of the beam outputs are within the tolerance level (Qm/Qs < 5%).For electron beams the number of deviations outside the optimal level are about three times that for photon beams. This difference can be explained by less accurate TPS algorithms and a larger uncertainty in the ionization chamber dosimetry procedure than for photon beams. The EQUAL programme shows that a consistent radiotherapy dosimetry for photon and electron beams can be achieved in Europe. However, a number of dosimetric problems in photons beams (7 % of the beams checked) for non-reference conditions have been discovered. Improvements on the Quality Assurance programmes by some of the centres and on the TPS algorithms are measures that could reduce the number of deviations. 628

5 Acknowledgements IAEA-CN They are very grateful to the physicists of the European centres for their participation and cooperation to the EQUAL network. The present work would not have been possible without the strong support of the ESTRO Committee for Quality Assurance and the financial help of the EU Programme "Europe Against Cancer". References [1] Marre D, Ferreira I H, Bridier A, Bjoreland A, Svensson H, Dutreix A and Chavaudra J 2000 Energy correction factors of LiF powder TLDs irradiated in high energy electron beams and applied to mailed dosimetry for Quality Assurance Networks, Phys. Med. Biol. 44, [2] Ferreira I H, Dutreix A, Bridier A, Chavaudra J and Svensson H 2000 The ESTRO- QUALity assurance network (EQUAL) Radiot. & Oncol [3] Thwaites, D.I., Williams, J.R, Aird, E.G., Klevenhagen, S.C. and Williams, P.C. A dosimetric intercomparison of megavoltage photon beams in UK radiotherapy centres. Phys. Med. Biol. 37: , [4] Nisbet, A., and Thwaites, D.I., A dosimetric intercomparison of electron beams in UK radiotherapy centres. Phys. Med. Biol. 42: , [5] Izewska, J. and Andreo, P. the IAEA/WHO postal programme for radiotherapy hospital, Radiother. Oncol ,

TLD as a tool for remote verification of output for radiotherapy beams: 25 years of experience

IAEA-CN-96-82 TLD as a tool for remote verification of output for radiotherapy beams: 25 years of experience J. Francisco Aguirre, Ramesh C. Tailor, Geoffrey S. Ibbott, Marilyn Stovall and William F. Hanson