Experimental determination of beam quality correction factors in clinical high-energy photon and electron beams

Abstract

Background: Recent protocols for the determination of absorbed dose to water in high-energy photon and electron beams are based on air ionisation chambers calibrated in terms of absorbed dose to water in a 60Co gamma ray beam ( 60 , Co D w ). To determine the absorbed dose to water in any other high-energy beam (excluding neutrons), the protocols use chamber dependent beam quality conversion factors. Such factors are published in different protocols but only for a selected number of ionisation chambers used clinically. These beam quality correction factors can alternatively be determined experimentally in the user’s beam qualities for each ionisation chamber. The measurement of beam quality correction factors (kQ for photons and kq,E for electrons) accounts for the actual design of different ionisation chambers. Direct measurement in the user’s beams also helps to minimise the uncertainties inherent in the theoretical determination of beam quality correction factors based on a unified design. Purpose: The purpose of this work was to determine values of the beam quality correction factors in clinical high-energy photon and electron beams for PTW 30013 and PTW 23333 0.6 cm3 ionisation chambers, a PTW 31006 ‘Pinpoint’ ionisation chamber, a PTW 31010 0.125 cm3 ionisation chamber, a PTW 23343 Markus and a PTW 34045 Advanced Markus ionisation chamber. Methods and materials: Siemens Primus linear accelerators were used to generate 6 and 18 MV photon beams and 5, 6, 7, 9, 12, 14, 15, 18 and 21 MeV electron beams. An Equinox Theratron External Beam Therapy System was used to generate the 60Co beam used in this study. The ionisation chambers were all cross-calibrated for 60 , Co D w against the PTW 23333 0.6 cm3 reference ionisation chamber at 5 cm water-equivalent depth in the 60Co beam. The field size at the reference depth was 10 cm x 10 cm. For the same set-up, the absorbed dose to water using the IAEA TRS-398 (IAEA, 2000) was determined using the PTW 23333 0.6 cm3 reference ionisation chamber. The exposure calibration factor ( x) for the PTW 23333 0.6 cm3 reference chamber was then derived by equating the absorbed dose to water calculated from the IAEA TRS-398 protocol to the absorbed dose to water calculated from the AAPM TG-21 (AAPM, 1983) protocol. The cavity-gas calibration factor ( gas) was then determined for the PTW 23333 0.6 cm3 reference ionisation chamber. The cross-calibrated 60 , Co D w for each cylindrical chamber and the absorbed dose to water due to the PTW 23333 0.6 cm3 reference ionisation chamber in the 6 MV and 18 MV photon beams were used to determine kQ for each ionisation chamber at the respective photon energies. The plane-parallel and the cylindrical ionisation chambers were then cross-calibrated for gas in the 21 MeV electron beam. The absorbed dose to water in the electron beams was then calculated from first principles using the AAPM TG-21 worksheets for all of the chambers. The kq,E were then derived for each of the ionisation chambers at each of the electron energies. Results: The measured kQ values as a function of TPR20,10 (the tissue-phantom ratio in water at depths of 20 cm and 10 cm, for a field size of 10 cm x 10 cm and a constant source-chamber distance of 100 cm) for the different ionisation chambers and the published IAEA TRS-398 kQ values for the PTW 30013 0.6 cm3 ionisation chamber are tabulated below:

ominal energy/MV TPR20,10 PTW 23333 PTW 31006 PTW 31010 PTW 30013 PTW 30013 (IAEA TRS-398) 6 0.674 0.991 0.998 0.997 0.993 0.991 18 0.770 0.973 0.973 0.985 0.973 0.972 The measured kq,E values as a function of R50 for the electron beam qualities for the different ionisation chambers and the published IAEA TRS-398 kq,E values for the PTW 23343 Advanced Markus ionisation chamber are tabulated below:

ominal Energy (MeV) R50/ cm PTW 23333 PTW 30013 PTW 31006 PTW 31010 PTW 34045 PTW 23343 PTW 23343 (IAEA TRS-398) 5 2.05 0.890 0.899 1.035 0.877 0.950 0.917 0.925 6 2.40 0.884 0.890 1.023 0.872 0.946 0.915 0.921 7 2.75 0.878 0.884 1.022 0.870 0.946 0.917 0.918 9 3.51 0.868 0.873 1.002 0.862 0.926 0.900 0.913 12 4.68 0.858 0.859 0.988 0.864 0.912 0.891 0.906 14 5.28 0.851 0.854 0.978 0.859 0.904 0.885 0.902 15 5.93 0.851 0.852 0.978 0.851 0.895 0.893 0.899 18 7.30 0.859 0.861 0.986 0.861 0.893 0.899 0.893 21 8.23 0.820 0.824 0.932 0.822 0.847 0.850 0.888 The average observed difference between the measured values and those published in the IAEA TRS-398 protocol was 0.2% for the PTW 30013 0.6 cm3 in the photon beams and 1.2% for the PTW 23343 Markus ionisation chamber in the electron beams. Conclusion: Beam quality correction factors for ionisation chambers can be determined experimentally or confirmed in an end-user’s beam quality.