To report early outcome analysis of a prospective institutional phase 2 trial of weekly hypofractionated breast irradiation (WHBI) for patients undergoing breast-conserving surgery (BCS).

Patients who underwent BCS for American Joint Committee on Cancer stage 0, I, or II breast cancer with negative surgical margins received whole-breast radiation therapy to 30 or 28.5 Gy in 5 weekly fractions with or without an additional boost. The eligibility criteria were the same as for NSABP (National Surgical Adjuvant Breast and Bowel Project) B39/RTOG (Radiation Therapy Oncology Group) 0413, and there were no restrictions on age, breast size, tumor grade, receptor status, or the use of cytotoxic chemotherapy for otherwise eligible patients. The primary endpoint was ipsilateral breast tumor recurrence. Patients were also evaluated for acute toxicity (Common Terminology Criteria for Adverse Events version 3.0), cosmesis (Harvard Scale), development of distant metastatic disease, and overall survival.

Between January 2011 and October 2015, 158 eligible patients underwent WHBI immediately following BCS. The median age was 60 years (range, 30-84 years), and the median follow-up period was 3 years. Ipsilateral breast tumor recurrence developed in a total of 2 patients (1.3%), 1 in conjunction with widespread metastatic disease. Distant metastatic disease developed in 4 patients (2.5%), and the 3-year disease-free survival and overall survival rates were 97.5% and 96.2%, respectively. The most common grade 1 or 2 acute toxicities were breast pain, radiation dermatitis, and fatigue. There were 2 grade 3 events (1.3%): pain requiring narcotic analgesics (1) and posttreatment infection requiring hospitalization (1). The rate of excellent or good cosmesis versus fair or poor cosmesis was 82.3% versus 17.7%. The rate of significant cosmetic change from baseline to last follow-up (dropping from excellent or good to fair or poor) was 11.6%.

Early outcomes after WHBI are favorable and parallel those seen with daily hypofractionated whole-breast irradiation. With broader entry criteria than all previous reports of WHBI, this study will facilitate comparison to the results of NSABP B39/RTOG 0413. With continued follow-up, future reports will assess cosmetic stability and disease-specific outcomes.