(Saturday, 4/7/2018)
Room: Foyer
Purpose: Due to limited industry guidelines for small field dosimetry, this study will demonstrate the clinical implementation and end-to-end testing of Varian SRS cones.
Methods: 6MV and 6FFF plans were created for each of the Varian SRS cones (17.5mm, 15mm, 12.5mm, 10mm, 7.5mm, 5mm, and 4mm) in Eclipse Cone Planning. Each plan consisted of a 50° and 80° arc. A water phantom was used for calculation at 95cm SSD. Point doses were calculated on the central axis at 5cm depth. Point doses were measured and verified using a PTW 60012 in a Sun Nuclear 1D SCANNER water tank and a Sun Nuclear EDGE detector in both a solid water phantom and 1D SCANNER water tank. In addition, independent MU and dose verification was performed using RadCalc® software for each plan.
Results: For the EDGE detector, the maximum difference between the planned point dose and the measured point dose was 3.19% and 1.26% for 6MV and 6FFF energies, respectively, using a solid water phantom and 6.77% and 3.72% for 6MV and 6FFF energies, respectively, using a water tank. For the PTW 60012, the maximum difference between the planned point dose and the measured point dose was 2.38% and 6.92% for 6MV and 6FFF energies, respectively. The RadCalc® software showed agreement within 1.29% for MU and 1.28% for point dose calculations for 6MV energy plans and 0.67% for MU and 0.67% for point dose calculations for 6FFF energy plans.
Conclusion: Recently published MPPG 9a is currently the only guideline specific to SRS/SBRT treatment but gives no detailed instruction to small field considerations. IAEA TRS 498 is referenced but is currently unpublished. This experience provides a model for successfully implementing Varian SRS cones for clinical use and developing a second independent MU and dose verification method utilizing the widely used RadCalc® software.
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