Post-Operative Single Fraction Stereotactic Partial Breast Irradiation (S-PBI) for Early-Stage Breast Cancer with Gammapod™ Technology: A Phase II Clinical Trial

Abstract

Purpose: To evaluate the feasibility, safety, and preliminary oncologic outcomes of post-operative 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 II 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 (PTV), delivered in prone position using GammaPod™, a dedicated stereotactic radiotherapy device. The primary objectives of the study were the cumulative incidence of acute toxicity (within 3 months from S-PBI) and moderate/severe late toxicity at two years following S-PBI. Secondary objectives include assessing local recurrence (LR), regional recurrence (RR) and loco-regional recurrence (LRR).

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 PTV 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 3-6-12 months were 0.7%, 2% and 3.4% respectively. Grade 1 fibrosis was reported in 36.5% of patients. LR, RR, and LRR 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 multi-fraction regimens. Ongoing randomized comparison with a 5-fraction PBI schedule will further define its role in clinical practice.