Fat necrosis after accelerated partial breast irradiation or hypofractionated whole breast irradiation: A case-control study.

Purpose: This study aimed to compare the incidence of fat necrosis after accelerated partial breast irradiation (APBI) vs hypofractionated whole breast irradiation (WBI) in patients with early-stage breast cancer.

Materials and methods: Data from early-stage breast cancer patients who underwent breast-conserving surgery and adjuvant radiotherapy between 2009 and 2022 were retrospectively collected. Radiation therapy consisted of APBI of 30 Gy in 5 daily fractions (Fx) (delivered in one week, consecutively) to the tumour bed or WBI (42.4 Gy in 16 Fx). Reports on fat necrosis were extracted from yearly mammograms and breast ultrasound imaging. The primary endpoint was the incidence of radiologically detected fat necrosis.

Results: A total of 536 patients were included among the APBI and WBI cohorts, with 268 and 268 patients respectively. The three-year Kaplan-Meier actuarial rate of fat necrosis was 32.8% (95% CI: 30.0% - 35.6%) for APBI and 22.3% (95% CI: 19.7% - 24.9%) for WBI patients. Univariate Kaplan-Meier survival analysis revealed a Hazard Ratio of 1.6 [95% CI: 1.1 - 2.2; p = 0.0055] for the fat necrosis rate within the APBI group compared to WBI. Multivariate Cox proportional hazard regression confirmed significant associations between fat necrosis and APBI (HR = 2.2 95% CI: 1.2 - 4.0; p = 0.01).

Conclusions: The occurrence of radiologically diagnosed fat necrosis was higher in the APBI group compared to the WBI. Further investigations aiming to identify a lower-dose schedule with comparable efficacy to 30 Gy in 5 Fx but fewer toxicities, particularly for high-risk patients, are warranted.