(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To determine what an acceptable air gap difference, defined as ? is, for the Mevion S250i Hyperscan system. During the treatment planning process for proton pencil beam scanning, an optimal air gap is defined between the snout and the patient’s surface. However due to an insufficient patient CT scan or patient anatomy changes, the air gap may be greater than what is planned. This may lead to inaccurate dose modeling in the patient.
Methods: We selected four patient plans: the IROC spine, neck, prostate and brain, created QA plans for each and then delivered one beam from each plan into our QA phantom set up. Dose distributions at isocenter were measured with the Octavius 729 XDR. In each case, we irradiated the phantom with the snout at the planned extension and then at various retracted distances, from 0.5 cm to 3.0 cm, to simulate an increased air gap. Dose distributions between the planned and retracted extensions were compared using gamma analysis.
Results: Gamma passing rates for ?=1 cm were 100% with 2%/2 mm criteria. At ?=1.5 cm, dose distribution comparisons passed at 100-88% and decreased with increasing ?. Gamma pass rates were around 70% for the brain and neck plans at ?=3.0 cm, and were still above 90% for ?=3.0 cm. Plans with large planned air gaps such as prostate cases, showed less sensitivity to ? than the brain and neck plans with smaller air gaps.
Conclusion: We determined that a snout extension difference of up to 1 cm would be acceptable for patient treatment.
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