Comparison of high energy electron dose with parallel plan and cylindrical ion chamber according to technical report series 398 calibration protocol

The purpose of this study is; According to the TRS 398 calibration protocol, parallel plate and cylindrical ion chambers used for dose calibration of high energetic electrons are used to examine the effect of doping according to the type of energy used and ion chamber. The TRS 398 dose calibration protocol recommends using a cylindrical planar ion chamber or parallel planar ion chamber at 10 MeV or above and at electron energies and a parallel plan ion chamber at energies below 10 MeV. In our study, 5 different electron energies with 4, 6, 9, 12 and 15 MeV energies were used in the linear accelerator and calibration processes were performed for these electron energies. Dosimetric controls with water phantom were also started after the measurements of all mechanical controls of the device were made before the dose calibration procedures were started. According to the TRS 398 dose calibration protocol, in the reference conditions, separate water phantom scans for each electron energy are made with a parallel plan ion chamber and the percent depth dose curves to be used in the calculation are obtained. Then, to obtain the data to be used for the calculation of the correction factors included in the protocol, cylindrical and parallel plan ion chambers were irradiated and required readings were taken. Then, by using the cylindrical ion chamber, cross calibration for the electron with the highest energy is performed to the parallel plan ion chamber and the calibration factor for the parallel plan ion chamber is calculated. With this data and other data obtained, absorptive dose data in water for each electron in each energy value was calculated for both parallel plan and cylindrical ion chamber.Finally, the differences between these calculated doses were evaluated. In this study, the dose difference in the measurements made with the cylindrical and parallel planar ion chambers used in the calibrations of electrons is %-2.61 for 4 MeV, %-0.90 for 6 MeV, %-0.40 for 9 MeV, %-0.15 for 12 MeV and %0 for 15 MeV, and this difference is less in electrons with higher energies, whereas it is found to be higher in electrons with lower energies. At the end of the study, it was found that the use of a cylindrical or parallel planar ion chamber for electron energies of 10 MeV and above and a parallel plan ion chamber for electrons of less than 10 MeV are suitable for calibration of dosimetric measurements with high energy electrons as suggested in TRS 398 protocol. Key words: Dose calibration; TRS398; Cross Calibration; Cylindrical-Parallel Plan Ion Chamber