Study of commercial detector responses in non-equilibrium small photon fields of a 1000 MU/min CyberKnife system

Abstract Purpose The purpose of this study was to analyze the detector responses in non-equilibrium small photon fields. Methods Five detectors (PTW 31014 ionization chamber, PTW 60016, PTW 60017 and Sun Nuclear EDGE diodes and PTW 60003 diamond detector) and one passive dosimeter (Harshaw micro-LiF) as well as a 1000 MU/min CyberKnife were modeled with the PENELOPE Monte Carlo code. Field factors, Ω Q clin , Q msr f clin , f msr were calculated and perturbations due to volume averaging effect, active material effect and coating effect were quantified for the five detectors and passive dosimeter. Results The PTW 31014 ionization chamber under-response is mainly due to the fluence perturbation caused by the presence of air as detecting material. Regarding diodes, the high density materials used in their active volume and in their coating is responsible for their over-response. Regarding the PTW 60003 diamond, its under-response for the 5 mm field size is due to a large volume averaging effect whereas for largest field sizes a nearly perfect compensation between the volume averaging effect and the material effect due to the diamond density occurs. Despite its small size, a volume averaging effect was observed for the micro-LiF for the 5 mm field size. Conclusion Perturbations due to volume averaging effect, active material effect and coating effect were investigated and quantified for five active detectors. Since these perturbations can cause opposite effects, wrong conclusions may be drawn regarding the radiological water-equivalence of detectors. Thus, we recommend performing such a study for each novel detector available on the market.