Single-fraction stereotactic partial breast irradiation shows promising early control in early-stage breast cancer

PURPOSE: To evaluate the feasibility, safety, and preliminary oncologic outcomes of postoperative single-fraction stereotactic partial breast irradiation (S-PBI) delivered with the GammaPod system in patients with early-stage breast cancer. METHODS AND MATERIALS: This prospective, single-institution phase 2 trial enrolled 148 patients who received breast-conserving surgery followed by S-PBI. Treatment consisted of a single 17.5-Gy fraction prescribed to 95% of the planning target volume, delivered in prone position using GammaPod, a dedicated stereotactic radiation therapy device. The primary objectives of the study were the cumulative incidence of acute toxicity (within 3 months from S-PBI) and moderate-to-severe late toxicity at 2 years following S-PBI. Secondary objectives include assessing local recurrence, regional recurrence, and locoregional recurrence. RESULTS: Between January 2022 and May 2024, 148 patients were enrolled. The median age at diagnosis was 68 years (IQR, 58-74). Median follow-up was 24.9 months (IQR, 18.3-33.7). All patients had a minimum follow-up of 12 months. The median time from surgery to radiation was 2.5 months (IQR, 2.1-2.9). Most tumors were pT1, pN0, hormone receptor-positive. The median planning target volume was 43.6 cc. Grade 2 fibrosis occurred in 5 patients (3.3%), with no grade ≥3 toxicities. The incidence rate of grade 2 fibrosis at 3, 6, and 12 months was 0.7%, 2%, and 3.4%, respectively. Grade 1 fibrosis was reported in 36.5% of patients. Local recurrence, regional recurrence, and locoregional recurrence rates at 24 months were 1.8%, 0.8%, and 1.8%, respectively. CONCLUSIONS: Single-fraction S-PBI using GammaPod was associated with excellent tolerability and promising early oncologic control in early breast cancer. With minimal late toxicity, this approach offers an efficient, patient-friendly alternative to multifraction regimens. Ongoing randomized comparison with a 5-fraction PBI schedule will further define its role in clinical practice.