Incidence and Predictors of Fat Necrosis After Single-Fraction Stereotactic Partial Breast Irradiation for Early-Stage Breast Cancer: Results From a Phase 2 Clinical Trial.

PURPOSE: To evaluate predictive dosimetric factors for fat necrosis (FN) after postoperative single-fraction stereotactic partial breast irradiation (S-PBI) in 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 (PTV) using GammaPod. The nominal dose was 18.4 Gy. FN was evaluated clinically at each follow-up. FN was classified into 2 categories: FN detected only by imaging (mammography or contrast-enhanced mammography) 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 evaluate predictive factors associated with FN.

RESULTS

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 1242.63 cm3 (IQR, 722.69-1394.42). Median PTV was 43.6 cm3 (IQR, 32.6-58.5). Median V19.3 (105%) was 0.55 cm3. Eighteen patients (12.1%) developed FN and 6 patients (4.1%) symptomatic FN. The incidence rates of FN at 6, 12, 18, and 24 months were 1.4%, 6.8%, 9.5%, and 11.3%, respectively. The incidence rates of symptomatic FN at 6, 12, 18, and 24 months were 0.7%, 2.7%, 3.4%, and 4.6%, respectively. Age ≥60 years and dosimetric parameter V19.3 were independently associated with an increased risk of FN. Specifically, V19.3 ≥5 cc was a strong predictor of overall FN, while V19.3% (continuous) was the only significant predictor for symptomatic FN. PTV showed only a borderline association.

CONCLUSIONS

The use of S-PBI with GammaPod is confirmed to be 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.