Measurement Corrections for Output Factor Measurements of Small Robotic Radiosurgery Beams Formed by a Variable Aperture Collimator

Monte Carlo (MC) detector simulations and measurements have been used to investigate the corrections needed for small field output factor measurements with the IRIS collimator on a CyberKnife System for a range of detectors. These corrections are found to be larger than 2.5% for all detectors at the smallest (5 mm) field size and to follow similar trends as those observed previously for fixed collimators on this treatment device. The correction factors for microchambers (Exradin A16 and PTW PinPoint 31014) ranged from 1.102 at 5 mm to 1.006 at 12.5 mm field sizes. The correction factors for diode detectors (PTW 60012 and Sun Nuclear Edge Detector) varied between 0.958 and 0.991 over the same field size range, and for a liquid filled ionization chamber (PTW microLion), the correction factor variation was 1.027-0.993. Before correction, measured output factors differed by up to 15% at 5 mm field size, which reduced to a maximum inter-detector variation of 3% after corrections were applied. The agreement of measured output factors to those calculated directly by MC simulation of dose to water was also improved after corrections were applied. In conclusion, these results support the use of measurement correction factors for small field output factor commissioning and QA. The correction factor dataset presented here can be used to apply this method to the IRIS Collimator on the CyberKnife System. (author)