Dosimetric and 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

Abstract

Given clinical concerns for more brisk acute skin toxicity with proton compared with photon RT due to differences in the energy deposition properties, our institution implemented a novel volumetric skin-sparing planning technique (SSPT) for intensity modulated proton therapy (IMPT) for patients treated to the breast or chest wall (CW). This study evaluates SSPT dosimetry and the hypothesis that a SSPT will reduce acute dermatitis during IMPT to the breast and CW. Prior to the development of a consensus technique, skin dose evaluation in IMPT plans was limited to mitigation of skin hot spots and cropping off the skin surface by 3mm for CW and 5mm for intact breast targets (except indications for deliberate skin dosing). In January 2022 our center added volumetric-based skin sparing objectives in addition to hot-spot evaluation as a SSPT. A skin evaluation structure (skin-eval) was defined as a skin rind of 3 mm for CW and 5 mm for intact breast within 5mm of the CTV, bound by the patient's surface. The SSPT incorporated an objective to limit the volume of skin-eval receiving 95% of the prescription dose or more (V95%Rx) to ideally <50% (goal<60%) while still prioritizing CTV coverage and robustness. We compared target coverage, robustness, and skin-eval dosimetry as well as 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. 84 patients who were planned to receive breast or CW IMPT were included (43 planned without and 41 with the SSPT). There was no difference in percentages of patients treated to the intact breast/CW/immediate CW reconstruction between the groups (37%/23%/40% without and 34%/27%/39% with SSPT, p>0.05). Mean skin-evalV95%Rx was 72% vs. 30%, p<0.0001, for those treated without vs. with a SSPT. Maximum %Rx to the skin volume of 0.03 cc, 0.3cc, and 1cc, was higher in patients treated without compared to those with a SSPT (103.1% vs. 101.5%; 101.3% vs. 100.4%; and 101.8% vs. 99.7% (all p=<0.0001)), respectively. There was a small statistical difference in the mean CTV V97.5%Rx in patients treated without vs. with the SSPT (97.8% vs. 96.5%, p=0.0003). Patients planned utilizing the SSPT demonstrated reduced rates of Grade 1 breast pain at Week 2 (12% vs. 33%, p=0.0424) and Grade 2 and 3 dermatitis at Week 5 (grade 2 38% vs. 42%; grade 3, 0% vs. 11%, p=0.0016). There were numerically more patients requiring a treatment break or not completing full intended prescription (4 vs. 1) in the pre-SSPT cohort. A volumetric-based SSPT appears to reduce the frequency of brisk onset dermatitis and near-end of RT significant dermatitis while still maintaining acceptable target coverage and robustness in patients receiving IMPT to the breast and CW.