Room: Exhibit Hall
Purpose: To determine the accuracy of the proton pencil-beam algorithm in the Eclipse treatment planning system (TPS) in predicting small fields produced by the Mevion S250 passively scattered proton therapy system for brain targets.
Methods: Eclipse treatment plans were created for five spherical pseudo-targets of 0.5, 1, 2, 3 and 4 cm diameter using circular apertures of 1, 2, 3, 4 and 5 cm, respectively, in a RANDO phantom head. Three plans for each target were generated using one, two, and three beams to deliver 180 cGy to the target. The plans were delivered to Gafchromic EBT3 film inserted into the phantom in the target plane. The 3%/3mm gamma passing rates were computed with both 10% and 50% dose thresholds using an in-house MATLAB program. Film quenching was also simulated and considered.
Results: As beam number was increased, the gamma passing rates with 50% threshold increased for all aperture sizes. The gamma passing rates of one-beam plans were not clinically acceptable (<70%), whereas all three-beam plans had gamma passing rates of at least 93%. The gamma passing rates with 10% threshold were less than 90% for all plans due to higher measured dose in entrance regions compared to Eclipse calculation. Quenching simulations showed that film measurements parallel to beam produced lower target dose towards distal falloff and surface dose escalation of up to ~10%. It was found that higher surface dose of up to 20% in film measurements resulted from film quenching and fluence depletion in small fields.
Conclusion: Eclipse TPS underestimates entrance dose for small proton beams using the Mevion S250 proton therapy system. Plans with multiple fields produce better target coverage and mitigate dose increases at the proximal end of spread-out Bragg peaks. Quenching should be considered when Gafchromic EBT3 film is used for depth dose measurement.