Energy correction factor for plane parallel ion-chamber and it’s use in clinical photon beam dosimetry

Abstract

This study was an attempt to find out the energy response correction factor for plane Parallel plate ion chamber considering its perturbation factor and use it in clinical photon beam dosimetry. In dosimetry protocol TRS-398, experimental values of energy correction factor are available for a particular chamber which can be compared with theoretical calculation considering perturbation factors. However, for Plane parallel chamber, energy correction factor are not available as they are not recommended for photon beam due to the relatively large uncertainty in the wall perturbation factor. Brag-gray theory and Spencer-Attix water/air stopping-power ratios, Sw, air, of beam qualities Q and Qo, are employed in this study to investigate the energy correction factors for Plane parallel chamber considering the perturbation factor of the specific chamber in high energy photon beams. Using TRS-398 and Spencer Attix data available in TRS-277 protocol, values of perturbation factor are calculated with high statistical precision for Plane parallel chamber for both 6 and 15 MV photon beams. The dependence of perturbation factor on the beam quality is also studied. The variations of beam quality index for “PPC-40 Roos” from “Exradin A19” were very much in agreement (0.39% and 0.29%) for 6 and 15 MV respectively. This indicates that the responses of both the chambers are within acceptable limit of 1 %. Energy correction factors “Kqqo” for PPC-40 which was determined theoretically and experimentally shows that the variation between theoretical and experimental for PPC-40 was very less i.e. 0.5% and 0.08% for 6 and 15 MV respectively. With precise calculation of perturbation factor, energy correction factors of PPC-40 for higher beam energy can be determined theoretically and compared with experimental values, showed a good result which is within 0.5%. So, Plane parallel chamber can be recommended for dosimetry of high energy photon beams after considering specific chamber perturbation factor.