2707 - Practical Approach to Reducing Skin Toxicity in Breast IMPT

Purpose/Objective(s): Proton therapy offers dosimetric advantages over photon-based techniques in breast cancer treatment; however, due to its physical characteristics, proton therapy may increase skin toxicity unless intentionally mitigated through treatment planning. We aimed to compare radiation dermatitis severity before and after implementing specific skin-sparing objectives in intensity modulated proton therapy (IMPT).

Materials/Methods: Female patients treated at our institution for breast cancer with IMPT from 2018-2025 were reviewed. Patients with prior breast cancer, prior radiation, bilateral cancer, or cT4b-d disease were excluded. Skin toxicity was assessed using CTCAE V5 criteria based on treatment visit and follow-up notes. The primary endpoint was grade 2+ radiation dermatitis rates before and after the implementation of proton specific skin goals: average skin (first 1 mm from surface, overlapping chest wall target) <= 90% prescription on nominal plan and max dose to skin (first 3 mm from surface, overlapping all targets) < 102% prescription robustly. Clinical target volume coverage criteria was D95% = 95%. Additional univariate analyses were performed via Fisher’s exact test with binary factors of breast/chest wall boost or not, T1-2 vs T3-4, age >50 or not, and lumpectomy vs. mastectomy. Further analysis with chi-square testing on non-binary variables were performed with race and reconstruction type.

Results: A total of 119 patients met the inclusion criteria: 76 patients were treated prior to skin (1 mm and 3 mm) goals and 43 patients were treated after implementation. There were no statistically significant differences between groups regarding T-stage, race, age, boost or not, or surgery types. Before optimization, rates of G0-G1, G2, and G3 dermatitis were 67% (51/76), 29% (22/76), and 4% (3/76), respectively; after optimization (2024 onwards), these rates were 77% (33/43), 23% (10/43), and 0%, respectively. There were no statistically significant differences in rates of G0-G1 versus G2+ dermatitis pre- and post-optimization (OR = 0.62, 95% CI 0.26–1.5, p = 0.30). Additional univariate analysis on variables listed above showed no statistically significant associations with rate of G2+ dermatitis: Race, p = 0.86; age, p = 0.99; T-stage (T1-2 vs T3-4), p =0.81; surgery (lumpectomy vs mastectomy), p = 0.17, reconstruction type, p = 0.50; boost, p = 0.72.

Conclusion: Implementation of practical skin constraints in IMPT reduced the observed rate of grade 3 dermatitis in this patient cohort; however, larger patient cohorts are needed to confirm this finding. Dermatitis rates with these constraints are closely aligned with historical photon-based post-mastectomy data, suggesting further dose reduction strategies should be cautiously balanced against tumor stage, target proximity to skin, and individual patient risk factors. Conservative supportive interventions, such as topical creams or barrier films, remain important to help patients manage acute skin reactions.