SU‐E‐T‐424: Improved Dosimetric Accuracy for Cyberknife Patient Plans Using a Dual‐Detector Measurement Method for Relative Output Factors

Purpose: The measurement of output factors (OFs) for small fields can lead to large dosimetric errors if detector effects are not accounted for. With its high spatial resolution and tissue equivalence, GAFCHROMIC film provides a correction free measure of OFs. We recently changed the OFs used in our Cyberknife planning system from uncorrected diode values to a dual detector method employing a diode with Monte‐Carlo corrections for the smallest collimators and a micro ion chamber for collimators >10 mm in diameter. Methods: We measured OFs for the CyberKnife G4 fixed collimators (5 to 60 mm) using an A16 microchamber and an Edge diode detector. The diode measured OFs for collimator sizes 5 mm, with differences observed for the 5 mm collimator consistent with volume averaging and a 1 mm setup uncertainty. Conclusions: OFs can be determined consistently using the dual‐detector method and verified using film. For patient specific QA, we achieve good agreement with microchambers for collimators >5 mm in diameter but film is the most appropriate detector for patient specific QA using the 5 mm collimator.