Postoperative Single-Fraction Stereotactic Partial Breast Irradiation for Early-Stage Breast Cancer With GammaPod Technology: A Phase 2 Clinical Trial.

Abstract

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. 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. 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. 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.