Monte Carlo simulation was used to calculate correction factors for output factor (OF), percentage depth-dose (PDD), and off-axis ratio (OAR) measurements with the CyberKnife M6 system. These include the first such data for the InCise MLC. Simulated detectors include diodes, air-filled microchambers, a synthetic microdiamond detector, and point scintillator. Individual perturbation factors were also evaluated. OF corrections show similar trends to previous studies. With a 5mm fixed collimator the diode correction to convert a measured OF to the corresponding point dose ratio varies between -6.1% and -3.5% for the diode models evaluated, while in a 7.6mm x 7.7mm MLC field these are -4.5% to -1.8%. The corresponding microchamber corrections are +9.9% to +10.7% and +3.5% to +4.0%. The microdiamond corrections have a maximum of -1.4% for the 7.5mm and 10mm collimators. The scintillator corrections are 15%, reducing to dmax were <2% for all detectors except IBA Razor where a maximum 4% correction was observed at 300mm depth. OAR corrections were smaller inside the field than outside. At the beam edge microchamber OAR corrections were up to 15%, mainly caused by density perturbations, which blurs the measured penumbra. With larger beams and depths, PTW and IBA diode corrections outside the beam were up to 20% while the Edge detector needed smaller corrections although these did vary with orientation. These effects are most noticeable for large field size and depth, where they are dominated by fluence and stopping power perturbations. The microdiamond OAR corrections were <3% outside the beam. This paper provides OF corrections that can be used for commissioning new CyberKnife M6 systems and retrospectively checking estimated corrections used previously. We recommend the PDD and OAR corrections are used to guide detector selection and inform the evaluation of results rather than to explicitly correct measurements.

A systematic study of the PTW microDiamond (MD) output factors (OF) is reported, aimed at clarifying its response in small fields and investigating its suitability for small field reference dosimetry. Ten MDs were calibrated under ⁶⁰ Co irradiation. OF measurements were performed in 6 MV photon beams by a CyberKnife M6, a Varian DHX and an Elekta Synergy linacs. Two PTW silicon diodes E (Si-D) were used for comparison. The results obtained by the MDs were evaluated in terms of absorbed dose to water determination in reference conditions and OF measurements, and compared to the results reported in the recent literature. To this purpose, the Monte Carlo (MC) beam-quality correction factor, k Q M D , was calculated for the MD, and the small field output correction factors, k Q c l i n , Q m s r f c l i n , f m s r , were calculated for both the MD and the Si-D by two different research groups. An empirical function was also derived, providing output correction factors within 0.5% from the MC values calculated for all of the three linacs. A high reproducibility of the dosimetric properties was observed among the ten MDs. The experimental k Q M D values are in agreement within 1% with the MC calculated ones. Output correction factors within +0.7% and −1.4% were obtained down to field sizes as narrow as 5 mm. The resulting MD and Si-D field factors are in agreement within 0.2% in the case of CyberKnife measurements and 1.6% in the other cases. This latter higher spread of the data was demonstrated to be due to a lower reproducibility of small beam sizes defined by jaws or multi leaf collimators. The results of the present study demonstrate the reproducibility of the MD response and provide a validation of the MC modelling of this device. In principle, accurate reference dosimetry is thus feasible by using the microDiamond dosimeter for field sizes down to 5 mm.