Author: Milad BaradaranGhahfarokhi, Ashley Elizabeth Bonner, James R. Burns, Annie Cooney, George X. Ding, Kenneth L. Homann, Guozhen Luo 👨🔬
Affiliation: Corewell Health William Beaumont University Hospital, Vanderbilt University Medical Center 🌍
Purpose: The goal of this study was to evaluate the performance of a new plastic scintillating detector (PSD) system with 0.5×0.5 mm2 and 1.0×1.0 mm2 probes (referred to as Medscint0.5 and Medscint1.0 in this study). The evaluation was conducted for small field dosimetry using 6 and 10 MV FFF beams.
Methods: The output factors measured with this PSD system for Varian TrueBeamTM Stereotactic cone sizes of 4, 5, 7.5, 10, 12.5, 15, and 17.5 mm, were evaluated to determine if correction factors were needed in extremely small fields. The results of output factor measurements were compared with Monte Carlo (MC) simulations, using EGSnrc BEAMnrc (Version 4-r2.3.1, National Research Council, Canada) and Gafchromic EBT3 film (Ashland LLC, NJ, USA) dosimetry.
Results: Medscint0.5 and Medscint1.0 showed agreement within 2.12% and 1.24%, respectively, with MC for all SRS cone sizes for both 6 and 10 MV FFF beams. As expected, the maximum discrepancy was found for cone sizes <5.0 mm, though differences were less than 1.10% for cone sizes 10 to 17.5 mm. The average discrepancies of output factors measured by Medscint0.5 and Medscint1.0 for all cone sizes were 1.02% and 0.27%, respectively, for both 6 and 10 MV FFF beams. Gafchromic EBT3 film dosimetry also demonstrated good agreement with MC simulations (<2.64%) for all SRS cone sizes. For all cone sizes, average discrepancies of the measured output factors using film dosimetry were 1.75% and 1.53% for 6 and 10 MV FFF beams, respectively. The maximum discrepancies of output factors measured with EBT3 film compared to Medscint0.5 and Medscint1.0 were 1.44% and 2.33%, respectively.
Conclusion: The latest small size PSD system is well-suited for small field dosimetry offering the unique advantage of measuring field output factors without need of any correction factor even in measuring small circular fields down to 4-mm diameter.