5-Year Results of a Prospective Phase 2 Trial Evaluating 3-Week Hypofractionated Whole Breast Radiation Therapy Inclusive of a Sequential Boost.

PURPOSE:To report 5-year outcomes of a phase 2 trial of hypofractionated whole breast irradiation (HF-WBI) completed in 3 weeks, inclusive of a sequential boost. METHODS AND MATERIALS:Women with stage 0-IIIA breast cancer (ductal carcinoma in situ through T2N2a) were enrolled on a prospective, phase 2 trial of accelerated HF-WBI. We delivered a whole breast dose of 36.63 Gy in 11 fractions of 3.33 Gy, with an equivalent dose to the regional nodes when indicated, followed by a tumor bed boost of 13.32 Gy in 4 fractions of 3.33 Gy over a total of 15 treatment days. The primary endpoint was locoregional control; secondary endpoints included acute/late toxicity and physician-assessed and patient-reported breast cosmesis. RESULTS:Between 2009 and 2017, we enrolled 150 patients, of whom 146 received the protocol treatment. Median age was 54 years (range, 33-82) and median follow-up was 62 months. Patients with higher-risk disease comprised 59% of the cohort, including features such as young age (33% ≤50 years), positive nodes (13%), triple-negative disease (11%), and treatment with regional nodal irradiation (11%) and/or neoadjuvant/adjuvant chemotherapy (36%). Five-year estimated locoregional and distant control were 97.7% (95% confidence interval [CI], 93.0%-99.3%) and 97.9% (95% CI, 93.6%-99.3%), respectively. Five-year breast cancer-specific and overall survival were 99.2% (95% CI, 94.6%-99.9%) and 97.3% (95% CI, 91.9%-99.1%), respectively. Acute/late grade 2 and 3 toxicities were observed in 30%/10% and 1%/3% of patients, respectively. There were no grade 4 or 5 toxicities. Physicians assessed breast cosmesis as good or excellent in 95% of patients; 85% of patients self-reported slight to no difference between the treated and untreated breast. CONCLUSIONS:Our phase 2 trial offers one of the shortest courses of HF-WBI; at 5 years of follow-up there continues to be excellent locoregional control and low toxicity with favorable cosmetic outcomes in a heterogeneous cohort of patients.