Determination of small field output factors and correction factors using a Monte Carlo method for a 1000 MU/min CyberKnife® system equipped with fixed collimators

Abstract

A new formalism for small field dosimetry has been proposed (Alfonso et al., 2008) with the concept of an additional correction factor (kQclin,Qmsrfclin,fmsr) which accounts for possible changes in detector response with field size. The aim of this work was to evaluate the response of eight commercially available detectors, then to provide a set of correction factors for a 1000 MU/min CyberKnife® equipped with fixed collimators and to compare them with those obtained for the 800 MU/min CyberKnife® version.Measurements were performed on a 1000 MU/min CyberKnife® with several active detectors designed for small field dosimetry (two chambers (PTW 31014 and 31018), three high resolution diodes (PTW 60016, 60017 and Sun Nuclear EDGE), a natural diamond (PTW 60003)) and two passive dosimeters (Harshaw TLD-700 7LiF:Mg,Ti thermoluminescent micro-cube and EBT3 radiochromic films). The CyberKnife® as well as the diode detectors, the PinPoint chamber, the diamond and the LiF micro-cubes were modeled with the PENELOPE Monte Carlo code in order to calculate the output factors in a point-like voxel of water (OFMC,w). A set of kQclin,Qmsrfclin,fmsr correction factors for the active detectors investigated is provided for the 1000 MU/min CyberKnife® in order to be used with the new formalism. A difference up to 2.4%, 2.0 and 1.7% in the correction factor obtained for the two different CyberKnife® models is found for the PTW 60003, the PTW 60016 and the PTW 60017 respectively. Although this difference is small, we recommend using specific kQclin,Qmsrfclin,fmsr correction factors for the 1000 MU/min CyberKnife® when they are available.