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Characterization of Surface Dose for Novel Flattening Filter Free (FFF) Straight-Through Linac with Jawless MLC Collimation

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R Scheuermann

R Scheuermann*, C Kennedy , S Anamalayil , D Mihailidis , L Dong , J Metz , University of Pennsylvania, Philadelphia, PA


SU-I-GPD-T-462 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To compare the shallow depth dose characteristics for a novel 6 MV flattening filter-free (FFF) straight-through linac with jawless MLC collimation and aluminum(Al) exit window with, MLC defined-fields in a Varian TrueBeam® 6X-FFF beam with Millennium MLC.

Methods: A PTW-23343 parallel-plate ion chamber (Markus chamber) with 0.03 mm thick polyethylene membrane was embedded in a custom milled 40x40x3 cm³ sheet of Solid Water® (Gammex RMI). The phantom was positioned at 100 cm SSD with at least 10 cm of backscatter with the chamber centered on CAX using the on-board imaging system. The percent depth dose (PDD) curve was measured using Solid Water slabs from depth of 0 to 15 mm. PDD curves were measured for square field sizes of 2, 4, 6, 8, 10, 12, 16, 20, 24, 28 cm². Measurements were performed on both the novel system and a TrueBeam® in clinical operation using 6X-FFF beams.

Results: The prototype system shows excellent agreement with the clinical TrueBeam® system. The maximum PDD difference measured in the buildup region was 1.4%. 100% of points passed a 1-d gamma analysis with 1%, 1mm criteria.

Conclusion: Despite the presence of an Al-exit window, the novel delivery system exhibits shallow depth dose characteristics which are very similar to those of a TrueBeam® 6X-FFF system. Skin doses delivered with the system are expected to be comparable to those from commercially available systems.

Funding Support, Disclosures, and Conflict of Interest: This work was performed as part of a research agreement with Varian Medical Systems.

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