Assessment of polarization and ion recombination correction factors and leakage of small megavoltage photon beams

Abstract

Background: Small field dosimetric challenges lead to a deviation from the reference dosimetry. The aim of this study is to investigate the changes of polarization (kpol) and ion recombination (ks) correction factors and determination of leakage dose in small fields. Materials and Methods: All values were measured on a RW3 slab phantom, at 100 cm Source-to-Surface Distance, 10 cm depth and 6, 10 and 18 MV photon beams for square fields (0.5 to 10 cm). Three ionization chambers (PTW Pinpoint 31014 and 31015, Semiflex 31010) were hired. After the electrometer readout, the correction factors were computed according to the protocol No. 398 of International Atomic Energy Agency's Technical Report (IAEA TRS-398). Results: The kpol (min) and the kpol (max) value occurred in 0.5×0.5 cm 2 and 10×10 cm field size, respectively. Dosimeters with a larger sensitive volume showed greater kpol values. In all three dosimeters, an increasing trend detected in normalized dosimeter reading after working voltage. The level of leakage in all of the values and radiation conditions was at the level of a few Nano colons. Conclusion: The values of kpol and ks in the small fields were different from the reference field. The saturation voltage of the small field dosimeters was greater than the dosimeter working voltage. The leakage values of the dosimeter-electrometer combination in the present study were negligible for all radiation conditions. The correction factors should be considered due to the differences between small fields and reference dosimetric conditions.