Comparison of SRS QA Tools On IROC Head Phantom Plan
C Teng1*,2, N Vassell2, E Sudentas1, T Nguyen3, Y Lo4, (1)Mount Sinai West, New York, NY, (2)Mount Sinai Chelsea, New York, NY, (3) MD Anderson Cancer Center, Houston, ,(4) Mount Sinai Health System, New York, NY
Presentations
PO-GePV-T-352 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: Intracranial stereotactic radiosurgery (SRS) requires high positional and dosimetric accuracy. This study evaluated three different SRS QA tools for IROC SRS phantom test.
Methods: Portal dosimetry (PD), SunNuclear SRS MapCHECK, and IROC film/TLD were used to evaluate the agreement between delivered and planned dose for IROC SRS phantom test plan QA. Gamma Index for each of the three SRS QA methods was computed for a combination of criteria from 1-3mm distance-to-agreement (DTA), and 1%-5% dose difference (DD) for all detector points measured > 10% threshold. A VMAT plan consisting of 6-field non-coplanar beams of 6MV FFF was delivered to a 1.9 cm diameter target using TrueBeam V2.7 and 120 Millennium MLC. Plan was calculated by Eclipse AAA (Version 15.6.03) in 1mm calculation grid.
Results: PD, MapCheck and film/TLD all are good tools for SRS QA. For tightest tolerance of 1%/1 mm, the Gamma passing rate were 95.4%/92.5% for PD/MapCheck, respectively. For clinical use 2%/2mm tolerance, the passing rates were 99.6%/98.2%/94.7% for PD/Mapcheck/film, respectively. We found that PD had slightly higher passing rate than MapCHECK QA in all combination of DTA and DD criteria, with the largest difference at the tightest bound of 1mm/1%. Gamma Index difference was reduced to <1.5% at 2mm/2%, and <1% at 2mm/3%.
Conclusion: Portal dosimetry does not require the use of a specialized phantom, provides sub-millimeter detector resolution, and is convenient to use. However, due to the fixed geometry of the portal imager with the gantry, the PD QA does not simulate couch walk, and hence does not represent the true delivered dose distribution. Before an institution commissions the protocol for patent-specific QA using PD, it is recommended to use multiple SRS QA tools and determine a machine specific gamma passing criteria adequate for an accurate SRS treatment delivery.
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