OA237] The output factor measurements of cyberknife system using various detectors

Abstract

Purpose In Stereotactic Radiosurgery, the extremely high radiation dose is delivered to the target in a single or few fractions using narrow radiation fields. The radiation fields are collimated with 12 fixed circular collimators ranging from 5 to 60 mm in diameter in CyberKnife which is a robotic radiosurgery system. The accuracy of the output factors directly affects the accuracy of treatment delivery to the patients. The output factor (OF) measurement of small fields is very problematic and the small field measurements are more sensitive to physical properties of detectors. The aim of the present study is to perform the relative OF measurements of 12 fixed collimators in CyberKnife system using different detectors. Methods In this study, the OF measurements of 12 fixed collimators were made with CyberKnife system at maximum dose depth in water equivalent RW3 slab phantom using PTW 60019 microDiamond, PTW 31014 0.015 cc PinPoint, PTW 31010 0.125 cc Semiflex, EBT3 Gafchromic film and GR-200A TLD. All detectors were positioned at the center of the radiation field with the point laser from treatment head. The measured data were compared with Monte Carlo (MC) factors. Results The OF values measured at 30 mm and greater collimator sizes were found similar for all detectors. The OF values decrease rapidly as the collimator size becomes smaller. The measured data with TLD and microDiamond detector at the smallest collimator sizes such as 7.5, 10, 12.5 and 15 mm were found very similar with MC factors. Conclusions 0.125 cc Semiflex ion chamber may lead to large uncertainties in the OF measurements for the smallest collimator sizes. TLD and microDiamond detector show similar results for all collimator sizes except 5 mm. TLD is an utilizable dosimetric tool in the small field OF measurements by means of its small active volume.