Room: ePoster Forums
Purpose: To directly compare gamma pass rates of VMAT plans by four commercially-available QA methods.
Methods: VMAT treatment plans were generated in Eclipse for three challenging treatment sites: 1. Spine SBRT, 2. Cavernous sinus SRS, and 3. Glioblastoma. All plans were created for delivery on a Varian Truebeam with a standard Millenium 120 MLC. In-house software was used to change these baseline plans by uniformly shifting the MLC leaves by a known amount (i.e. 0.5 mm, 1 mm, 3 mm, and 5 mm), representing errors that could occur from an MLC miscalibration. Subsequently, each plan was delivered and measured using the ArcCheck (Sun Nuclear), Delta4 Phantom+ (ScandiDos), and Delta4 Discover (ScandiDos) IMRT QA phantoms as well as the EPID for use in portal dosimetry. The gamma (2%,2mm, 10% dose threshold) pass rate from each QA method was compared.
Results: All four IMRT QA modalities examined showed relatively little response to MLC shifts of up to 1 mm. There is a noticeable dip (more than 10%) in gamma pass rates in all modalities for shifts of greater than 1 mm, with portal dosimetry showing the steepest decline in gamma pass rates for these plans. The Delta4 Discover demonstrated the next steepest drop in gamma pass-rate with increasing error magnitude, while the Delta4 Phantom+ and the ArcCheck generally agreed with one another.
Conclusion: In general all devices explored showed the expected behavior, with the gamma pass rates dropping significantly once the shifts introduced were more than the gamma distance criterion. Portal dosimetry and Delta4 Discover exhibited steeper declines in gamma pass rates for large errors, presumably due to their high spatial resolution and planar geometry always being perpendicular to the beam.