SU‐E‐T‐704: Skin Dose in Proton Pencil Beam Scanning for Pediatric Patients

Purpose: Optimize intensity modulated proton therapy (IMPT) treatment planning guidelines with regard to skin dose reduction in pediatric patients. Methods: Phantom and patient CTs were used to evaluate skin dose using a pre‐clinical proton beam modeled using Monte Carlo (TOPAS) in a commercial radiotherapy planning system (RTP). Single‐spot and single‐beam doses were analyzed in phantom environment. Fractional surface dose was evaluated vs. spot spacing and maximum range for beams optimized for increasingly deep targets. Both RTP automatic variable spacing (0.5xσprojected) and constant spacing (0.2–1.2cm) were evaluated. 3 spine IMPT plans were obtained with/without objectives for skin dose, with automatic and constant spot spacing. Dosimetric parameters and spot weight/energy statistics were evaluated. Results: Single‐spot primary fluence depletion raises the spot axial relative skin dose above Bragg peak dose for E0>139MeV. Dose heterogeneity and maximum skin dose increases with maximum beam energy and spot spacing non‐linearly. Exponential fitting of maximum skin‐to‐target dose ratio against spot spacing suggested an optimum constant spacing 50% MU weight was carried by spots with energy >135MeV, and had spots >155MeV. Introducing skin optimization objectives/constraints, decreased high energy spots weights, while that for >135MeV spots increased. Hence, skin D50 increased with skin‐optimized plans, while coverage remained constant. Average dose, D1 and D0.1cc decreased compared to plans without skin objectives. Constant spacing did not produce superior skin dose distributions without skin optimization objectives. Conclusion: IMPT inverse planning can produce high skin doses with heterogeneous distributions, effects that can be mitigated via optimization. Careful coordination between RTP techniques and accurate skin dosimetric parameters will allow minimization of clinical skin effects and establishment of threshold doses for acute/late effects in pediatric population treated with IMPT.