Incidence and Predictors of Fat Necrosis After Single-Fraction Stereotactic Partial Breast Irradiation for Early-Stage Breast Cancer: Results From a Phase II Clinical Trial.

Abstract

to evaluate predictive dosimetric factors for fat necrosis (FN) after post-operative single-fraction stereotactic partial breast irradiation (S-PBI) in early-stage breast cancer. 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)using GammaPod™. The nominal dose was 18.4 Gy. FN was evaluated clinically at each follow-up. FN was classified into two categories: FN detected only by imaging (mammography or contrast-enhanced mammography, CEM) and FN detected by imaging and associated with clinical symptoms. To describe the cumulative incidence of FN over time after S-PBI we used the Kaplan Meier method. Univariate and stepwise logistic regression analyses were conducted to predictive factorsassociated with FN. Between January 2022 and May 2024, 148 patients    were enrolled. Median age at diagnosis was 68 years (IQR 58-74). Median follow-up was 29.9 months (IQR 23.5-38.8). Median whole breast volume was 1242.63cc (IQR722.69-1394.42). Median PTV volume was 43.6cc (IQR 32.6-58.5). MedianV19.3(105%) was 0.55cc. Eighteen patients (12.1 %) developed FN and 6 patients (4.1%) symptomatic FN. The incidence rate FN at 6-12-18-24 months were 1.4%, 6.8%, 9.5% and 11.3% respectively. The incidence rate symptomatic FN at 6-12-18-24 months were0.7%, 2.7%, 3.4% and 4.6% respectively. Age ≥60 years and dosimetric parameter V19.3 were independently associated with an increased risk of fat necrosis. Specifically, V19.3 ≥5 cc was a strong predictor for overall FN, while V19.3% (continuous) was the only significant predictor for symptomatic FN. PTV showed only a borderline association, CONCLUSION: The use of S-PBI with GammaPod™ is confirmed as a well-tolerated strategy, with a relatively low incidence of FN. Advanced age and volume of mammary gland exposed to higher doses represent the main risk predictors.