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Improved Dosimetric Accuracy for Cyberknife Patient Plans Using a Dual-Detector Measurement Method for Relative Output Factors

E Vandervoort

E Vandervoort1*, D La Russa1, N Ploquin2, J Szanto1, E Henderson1, P Francescon3, (1) The Ottawa Hospital Cancer Centre, Ottawa, ON, (2) Tom Baker Cancer Centre, Calgary, AB, (3) Ospedale Di Vicenza, Vicenza,

SU-E-T-424 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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 <10 mm were corrected using Monte-Carlo correction factors. OFs were also measured using GAFCHROMIC film. We evaluated how this change in OFs influenced the dosimetric accuracy of patient specific QA measurements for 13 patient plans (9 before and 4 after the OF change) using film and the A16 chamber.

Results: The OFs measured using the dual-detector method agree with film to within two standard deviations for the full range of collimator sizes. When the dual detector method OFs are used, we achieve better dosimetric agreement (<2 sigma for pixels within the 80% isodose) than with uncorrected diode OFs for all patient specific QA plans measured using film. For patient specific QA using the microchamber, we get good agreement (<3%) 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.

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