SU-F-T-208: An Efficient Planning Approach to Posterior Fossa Tumor Bed Boosts Using Proton Pencil Beam Scanning in Fixed-Beam Room

Purpose:A fixed-beam room could be underutilized in a multi-room proton center. We investigated the use of proton pencil beam scanning (PBS) on a fixed-beam as an alternative for posterior fossa tumor bed (PF-TB) boost treatments which were usually treating on a gantry with uniform scanning.Methods:Five patients were treated with craniospinal irradiation (CSI, 23.4 or 36.0 Gy(RBE)) followed by a PF-TB boost to 54 Gy(RBE) with proton beams. Three PF-TB boost plans were generated for each patient: (1) a uniform scanning (US) gantry plan with 4–7 posterior fields shaped with apertures and compensators (2) a PBS plan using bi-lateral and vertex fields with a 3-mm planning organ-at-risk volume (PRV) expansion around the brainstem and (3) PBS fields using same beam arrangement but replacing the PRV with robust optimization considering a 3-mm setup uncertainty.Results:A concave 54-Gy(RBE) isodose line surrounding the brainstem could be achieved using all three techniques. The mean V95% of the PTV was 99.7% (range: 97.6% to 100%) while the V100% of the PTV ranged from 56.3% to 93.1% depending on the involvement of the brainstem with the PTV. The mean doses received by 0.05 cm3 of the brainstem were effectively identical: 54.0 Gy(RBE), 53.4 Gy(RBE) and 53.3 Gy(RBE) for US, PBS optimized with PRV, and PBS optimized with robustness plans respectively. The cochlea mean dose increased by 23% of the prescribed boost dose in average from the bi-lateral fields used in the PBS plan. Planning time for the PBS plan with PRV was 5–10 times less than the US plan and the robustly optimized PBS plan.Conclusion:We have demonstrated that a fixed-beam with PBS can deliver a dose distribution comparable to a gantry plan using uniform scanning. Planning time can be reduced substantially using a PRV around the brainstem instead of robust optimization.