2657 - Comparison between Proton LET-Dependent RBE Dose and Photon Physical Dose for Acute Radiation Dermatitis in Caucasian and African-American Breast Cancer Patients Undergoing Radiation Therapy

N. C. Biswal^1,2, S. W. Lee¹, B. Zhang¹, J. Jatczak², S. A. McAvoy¹, S. Cheston¹, B. Y. Yi^1,2, E. M. Nichols¹, and W. Yao^1,2; ¹Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, ²Maryland Proton Treatment Center, Baltimore, MD

Purpose/Objective(s): Radiation-induced skin toxicity (RIST), in the forms of Radiation dermatitis (RD) and/or skin hyperpigmentation (SH) is an important endpoint affecting quality of life. In this study, proton radiobiological effectiveness (1.1RBE) and linear energy transfer (LET)–dependent RBE (LET-RBE) doses were calculated and corelated as predictors for RD. The dose thresholds were compared between proton and photon treatments. Materials/Methods: Patients treated with intensity-modulated proton therapy (IMPT) and photon therapy (3DCRT and VMAT) at an academic proton center in 2022 and 2023 were retrospectively analyzed in this IRB approved study. For the IMPT group, we investigated 11 lumpectomy and 19 mastectomy Caucasian patients and 16 African-American lumpectomy patients without chemotherapy. All the patients were prescribed to a dose of 50.4 Gy (RBE = 1.1) in 28 fractions, to the targets (cropped by 3 mm or 5 mm from the skin surface), followed by a 9 -16.2 Gy boost to surgical scars. For the photon therapy group, 11 Caucasian and 8 African-American lumpectomy patients prescribed either 266 ×16 cGy or 200 × 25 cGy and boost by 200 × 5 cGy were studied. Toxicity scores were recorded weekly during the course of treatment. For each patient, the skin was contoured with 3 mm inside from the patient surface, within 50% isodose line, for 1.1RBE and LET-RBE proton dose and photon physical dose analysis. For IMPT plans, the LETd was calculated using a Monte-Carlo dose engine and then 3 RBE models were employed to convert the LET to RBE dose. The averaged RBE dose from the 3 models were used for our analysis.

Results: Skin D_10%(Gy) was calculated as dosimetric parameter for RD prediction. Surprisingly, no correlation between RD and the full course skin D_10% were found, however, there was strong correlation found between RD and accumulated skin D_10% on the day when the RD was reported. For the proton group, RD scores were positively correlated with both the accumulated 1.1RBE D_10% (the correlation coefficient r = 0.727, 0.617and 0.885 for Caucasian lumpectomy, Caucasian mastectomy and African-American lumpectomy patients respectively) and LET-RBE D_10% (r =0.762, 0.575, 0.889 for Caucasian lumpectomy, Caucasian mastectomy and African-American lumpectomy patients respectively). For the photon group, RD scores were positively correlated with physical D_10% dose with r=0.96 and 0.86 for African-American and Caucasian patients respectively. Furthermore, the mean of the accumulated proton 1.1RBE D_10% was 41 Gy, much less than the threshold 50.25 Gy from photon therapy, but was 48 Gy when the LET effect was considered (LET-RBE D_10%).

Conclusion: Breast skin dose toxicity shows strong evidence of LET-RBE effect for breast cancer patients undergoing proton beam therapy, compared to photon beam therapy.