SU-E-T-267: Proton Pencil Beam Scanning for Mediastinal Lymphoma: 4-Dimensional Feasibility Study

Purpose: To assess the feasibility of proton pencil beam scanning (PBS) for the treatment of mediastinal lymphoma. Methods: A group of 6 patients were planned using an anterior field with PBS. Spots with ∼5 mm σ were used for all patients, while large spots (∼10 mm σ) were employed for patients with motion perpendicular to the beam (≥5 mm). We considered volumetric repainting such that, in each fraction, the same field would be delivered twice. Four-dimensional dose was calculated on initial and verification 4-dimensional computed tomography (4D-CT) scans (2—3) based on respiratory trace and beam delivery sequence. This was implemented by binning the spots into separate plans on each 4D-CT phase respectively. Four starting phases were sampled for each painting and 4 energy switching times (0.5 s, 1 s, 3 s, and 5 s) were tested, resulting in 2560 dose distributions for the cohort. Plan robustness was measured for target and critical structures in terms of the percentage difference between delivered dose and planned dose. Results: For 5 of the 6 patients, the ITV (internal target volume) D98% was degraded by