Experimental determination of beam quality correction factors in clinical high-energy photon and electron beams
Date
2011-02-25
Authors
Katumba, Moses Fredrick
Journal Title
Journal ISSN
Volume Title
Publisher
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.