Determination of the k_(Qclin, Qmsr)~Ifclin,f_(msr) correction factors for detectors used with an 800 MU/msn CyberKnife~R system equipped with fixed collimators and a study of detector response to small photon beams using a Monte Carlo method

摘要

Purpose: In a previous work, output ratio (ORdet) measurements were performed for the 800 MU/min CyberKnife~R at the Oscar Lambret Center (COL, France) using several commercially available detectors as well as using two passive dosimeters (EBT2 radiochromic film and micro-LiF TLD-700). The primary aim of the present work was to determine by Monte Carlo calculations the output factor in water (OFmc.w) and the k_(Qclin, Qmsr)~Ifclin,f_(msr) correction factors. The secondary aim was to study the detector response in small beams using Monte Carlo simulation. Methods: The LINAC head of the CyberKnife~R was modeled using the PENELOPE Monte Carlo code system. The primary electron beam was modeled using a monoenergetic source with a radial gaussian distribution. The model was adjusted by comparisons between calculated and measured lateral profiles and tissue-phantom ratios obtained with the largest field. In addition, the PTW 60016 and 60017 diodes, PTW 60003 diamond, and micro-LiF were modeled. Output ratios with modeled detectors (OR_(MC,det)) and OF_(Mc,w) were calculated and compared to measurements, in order to validate the model for smallest fields and to calculate k_(Qclin, Qmsr)~Ifclin,f_(msr) correction factors, respectively. For the study of the influence of detector characteristics on their response in small beams; first, the impact of the atomic composition and the mass density of silicon, LiF, and diamond materials were investigated; second, the material, the volume averaging, and the coating effects of detecting material on the detector responses were estimated. Finally, the influence of the size of silicon chip on diode response was investigated.