Proton Beam Therapy After Breast-Conserving Surgery For Breast Cancer: Multi-Institutional Prospective Pcg Registry Analysis

This study investigates adverse events (AEs, CTCAE v4.0) and clinical outcomes for proton beam therapy (PBT) after breast-conserving surgery for breast cancer. A total of 82 patients were treated with PBT after breast-conserving surgery for breast cancer in the prospective multi-institutional Proton Collaborative Group (PCG) registry. Acute AEs occurred within 180 days from start of radiation. Late AEs began or persisted beyond 180 days. Fisher’s exact and Wilcoxon tests were used to test association of clinical and treatment characteristics with AEs. Statistical analysis was performed using a data and decision management software. Median follow-up was 8.1 months (range, 0-39.9 months). Median dose was 50.4 Gy Relative Biologic Effectiveness (RBE) in 28 fractions (range, 42.5-54 Gy RBE in 16-30 fractions) and the majority of patients (n=74, 90%) received a boost to the lumpectomy bed. Most patients received regional nodal irradiation (n=68, 83%), which included a combination of supraclavicular nodes (n=59, 72%), axillary nodes (n=58, 71%), and internal mammary nodes (n=54, 66%). Regional nodal irradiation and receiving a boost were not associated with severe AEs (p=1.0 for both). Overall, 6 patients (7.3%) developed grade 3 AEs consisting of 5 patients (6.1%) with grade 3 dermatitis and 5 patients (6.1%) with grade 3 breast pain. Body Mass Index (BMI) was associated with grade 3 dermatitis. Median BMI of patients with grade 3 dermatitis was 38.2 versus 27.4 without grade 3 dermatitis (p=0.015). During the follow-up period, there were 3 deaths (3.7%) including 2 breast cancer related deaths (2.4%) with 4 patients (4.9%) developing metastatic disease. Local control was 97.6% with 2 failures. To our knowledge, this is the largest prospective cohort reporting on PBT after breast-conserving surgery for breast cancer. PBT including regional nodal irradiation after breast-conserving surgery is well-tolerated with only 7.3% grade 3 toxicities. Elevated BMI was associated with grade 3 dermatitis.