SU‐E‐T‐235: A Comparison of Small Field Output Factors for Three SRS/SRBT Systems

Abstract

Purpose: Dosimetry measurement for small fields used in SRS/SBRT is challenging and the results can vary between institutions especially when different detectors are used. There are several reports in recent years that serious errors occurred in SRS/SRBT treatments due to incorrect output measurements in the initial commissioning processes. The present study compares small field output factors measured using a same detector for 6 MV photon beams in three SRS/SRBT systems: Cyberknife (cone), Brainlab (cone), and Varian TrueBEAM (Jaw), and determines if they are close enough to be used for reference purposes in commissioning similar SRS/SRBT systems. Methods: Relative output factors of small fields were measured by the author using a SRS diode (PTW 60012) in water for 6 MV photons with Cyberknife cones (5 to 60 mm), Brainlab cones on Varian TrueBEAM (5 to 30 mm), and TrueBEAM (Jaw: 10 × 10 mm2 to 40 × 40 mm2). The results are compared after being corrected for differences in reference field sizes, measurement distances and depths as defined in the three computer planning systems. Results: The corrected relative output factors of small fields measured for three systems are within +−2% from the average for collimator sizes larger than or equal to 20 mm, and within +−3% for smaller fields. Conclusion: The Results suggests that it may be feasible to establish a reference dataset of small field output factors for various SRS/SRBT systems and beam shaping devices (cone and MLC), similar to the RPC standard dataset for IMRT fields. Such a reference dataset is not to replace data collections during the institutional commissioning processes, but rather, is used as a secandary check to alert possible errors when large deviations from the reference data are observed.