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

Small field dosimetric challenges lead to a deviation from thereference dosimetry. the aim of this study is to investigate the changes ofpolarization (kpol) and ion recombination (ks) correction factors anddetermination of leakage dose in small fields. materials and methods: allvalues were measured on a rw3 slab phantom, at 100 cm source-to-surfacedistance, 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 correctionfactors were computed according to the protocol no. 398 of internationalatomic energy agency's technical report (iaea trs-398). results: the kpol (min)and the kpol (max) value occurred in 0.5×0.5 cm2 and 10×10 cm2 field size,respectively. dosimeters with a larger sensitive volume showed greater kpolvalues. in all three dosimeters, an increasing trend detected in normalizeddosimeter reading after working voltage. the level of leakage in all of thevalues 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 thereference field. the saturation voltage of the small field dosimeters wasgreater than the dosimeter working voltage. the leakage values of thedosimeter-electrometer combination in the present study were negligible forall radiation conditions. the correction factors should be considered due tothe differences between small fields and reference dosimetric conditions.