Determination of absorbed dose in water below 10 MeV electron beams using a cylindrical ionization chamber

초록

The IAEA Technical Reports Series No. 398 (TRS-398) recommends to use only plane-parallel ionization chambers for beam output calibration for electron beams below 10 MeV. According to the protocol, it is necessary to do a cross calibration at a higher electron beam energy using an ionization chamber calibrated at PSDL (Primary Standard Dosimetry Laboratories) for a reference beam quality Q0. However, the cross calibration is a complicated process and involves several uncertain factors. In this study, the beam quality correction factor was calculated, without cross calibration, for the use of a cylindrical ionization chamber below 10 MeV electron beams. BEAMnrc was used to simulate an Eldorado 8 radiotherapy unit and a linear accelerator (Varian Clinac 2100C) at 6 MeV, 9 MeV and 12 MeV. As a result, the phase-space data files containing the information of particles were generated. EGSPP was used to calculate the calibration factors in terms of absorbed dose to water for 6 MeV, 9 MeV and 12 MeV by using the phase-space data file. EGSPP was also used to calculate the calibration factors in terms of absorbed dose to water for the Co-60 therapy source, a reference beam quality of SSDL (Secondary Standard Dosimetry Laboratory). The beam quality correction factors, calculated with these two calibration factors in terms of absorbed dose to water, were applied to the cylindrical ionization chamber to determine the absorbed dose to water. Then, the absorbed dose to water was compared with that of plane-parallel ionization chamber under the same energy and conditions (6 MeV, 9 MeV and 12 MeV). Our result shows that these two results agree within 1-3%, which suggests that it is possible to use a cylindrical ionization chamber below 10 MeV electron beams.