Clinical Introduction of a Volumetric-Based Skin-Sparing Planning Technique for Breast and Chest Wall Patients Treated with PBS Proton Therapy: A Review on Feasibility and Impact on Target Coverage and Robustness

Abstract

<h3>Purpose/Objective(s)</h3> Acute and late skin toxicities are common and bothersome for patients undergoing breast or chest wall (CW) irradiation. There is clinical concern that acute skin toxicity may be higher with proton radiation compared with photon radiation because of differences in their energy deposition properties. Limited literature exists on dosimetric strategies to mitigate skin toxicity for patients treated with intensity modulated proton therapy (IMPT). We characterize the outcomes of a volumetric-based skin-sparing planning technique for IMPT treated breast and CW patients. <h3>Materials/Methods</h3> Prior to a consensus strategy, skin dose evaluation in IMPT Breast and CW plans was heterogeneous across physicians and dosimetrists. Uniformly, clinical targets were delineated by cropping off the skin surface by 3 mm for CW and 5 mm for intact breast; however, further skin-sparing strategies were previously restricted to limiting hot spots. In January 2022 our center established a consensus strategy to mitigate skin toxicity that added volumetric-based skin sparing objectives in addition to hot-spot evaluation. To start, a consistent skin evaluation structure (skin-eval) was defined within 5 mm of the CTV and included a rind of 3 mm for CW and 5 mm for intact breast, bound by the patient's surface. IMPT planning incorporated an objective to limit the volume of skin-eval receiving 95% of the prescription dose or more (V95%) to ideally less than 40% (goal of less than 50%) of the volume of the skin-eval structure, Skin-eval V95%Rx<40% (goal 50%), while still prioritizing CTV coverage and robustness. We report the early dosimetric experience with this volumetric-based objective. Boost plans were not included in this study. <h3>Results</h3> Ten patients underwent IMPT planning incorporating the described skin-eval objective, including 6 with intact breasts and 4 CW. Median skin-eval V95%Rx=36.4% (range, 20.3-49.9%). Median skin-eval max dose to 0.03cc, 0.3cc, and 1cc was 102.8% (range 101.4%-103.9%), 101.8% (range 100.5%-102.9%), and 101.1% (range 99.5%-102.3%), respectively. Median CTV V97.5%Rx was 98.2% (range, 94.9%-99.5%). 9 of the 10 plans passed all robust evaluation scenarios at V95%Rx receiving at least 95% (median 97.7%, range 96.1%-98.5%). Only 1 plan did not meet CTV goal coverage in all robust scenarios, failing only 1 scenario. This plan uniquely incorporated a couch kick to address atypical setup challenges. <h3>Conclusion</h3> A volumetric-based objective of skin-eval V95%Rx<50% appears consistently achievable while still maintaining excellent nominal and robustly optimized target coverage in patients receiving IMPT to the breast and CW. Future studies with both retrospective and prospective designs can validate dose constraints that may be more relevant for toxicity mitigation with IMPT.