Dosimetric and On-treatment Clinical Results of a Volumetric-based Skin-sparing Planning Technique for Patients Treated to the Breast and Chest Wall With Pencil-Beam Scanning Proton Therapy.

Purpose: This study evaluates the hypothesis that a volumetric skin-sparing planning technique (SSPT) will reduce acute dermatitis in patients treated to the breast or chest wall (CW) with proton pencil-beam scanning (PBS). Methods and Materials: In January 2022, our center incorporated volumetric-based skin-sparing objectives in addition to skin hot spot evaluation as an SSPT. The SSPT incorporated an objective to limit the volume of a skin evaluation structure (skin-eval) receiving 95% of the prescription dose or more (V95%Rx) to ideally < 50%. We compared target coverage, robustness, skin-eval dosimetry, and acute on-treatment skin toxicity in patients treated with and without incorporation of this SSPT. Patients with skin/dermal lymphatic invasion or inflammatory breast cancer were excluded. Results: A total of 84 patients who received breast/CW PBS were included (43 planned without and 41 with the SSPT). There was no difference in percentages of patients treated with intact breast/CW/immediate CW reconstruction between groups. Mean skin-evalV95%Rx was 72% vs 30%, Conclusions: The use of an SSPT may reduce acute skin toxicity in patients with breast cancer treated with PBS.