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

A new formalism for small field dosimetry has been proposed (Alfonso et al., 2008) with the concept of an additional correction factor (k_Qclin,Qmsr^f_clin,f_msr) 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 (OF_MC,w). A set of k_Qclin,Qmsr^f_clin,f_msr 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 k_Qclin,Qmsr^f_clin,f_msr correction factors for the 1000 MU/min CyberKnife^® when they are available. - Highlights: • A Monte Carlo model of a 1000 MU/min CyberKnife^® was developed using PENELOPE. • The response of active detectors, TLD micro-LiF and radiochromic film was studied. • A set of correction factors for several active detectors is provided. • Correction factors are compared to the previous 800 MU/min CyberKnife^® version