Performance characteristics of some cylindrical ion chamber dosimeters in Megavoltage (MV) photon beam according to TRS-398 dosimetry protocol

Abstract

The current study aims to characterize and compare the correction factors of some widely employed cylindrical ion chambers for Megavoltage (MV) photon dosimetry following the recommendations of the IAEA TRS-398 dosimetry protocol. Four different types of cylindrical ion chambers including Farmer (TM30013), Semiflex 0.125 cm3 (TM31010), Semiflex 0.3 cm3 (TM31013), and PinPoint (TM31016) were employed, and correction factors of polarity (kpol), ion recombination (kS), and quality conversion factor (kQ,Q0) were determined at 6 and 15 MV photon energy produced by a Varian Trilogy clinical LINAC. Measurements were performed inside an automatic MP3-M water phantom at the reference condition of 100 cm SSD (source to surface distance), 10 cm measurement depth, and 10 × 10 cm2 field size at the phantom surface. The maximum kpol values belonged to the TM31010 chamber, while the minimum values were observed for TM30013 one. The maximum and minimum kS values were also related to the TM31016 and TM30013 chambers, respectively. The obtained kS values following the TRS-398 dosimetry protocol were in accordance with those measured by Jaffe-plot analysis. The type of employed ion chamber had a minimal impact on kQ,Q0 value, so that the maximum difference between the measured quality conversion factors with different ion chambers was about 0.3% and 0.05% 6 and 15 MV photon energy, respectively. Based on the obtained results, it can be concluded that the Farmer chamber shows the best performance characteristics for absolute dosimetry purposes. Nevertheless, all considered chambers fulfilled the requested criteria for absolute dosimetry in high energy photon fields.