Early radiation effects on immediate breast reconstruction: A comparative analysis of 15- and 5-fraction postmastectomy radiotherapy regimens

Background Modern postmastectomy radiotherapy (PMRT) increasingly employs ultra-hypofractionated regimens (26 Gy/5 fractions) as an alternative to standard hypofractionation (15 fractions/40–42 Gy). However, evidence on their safety in the reconstructive setting remains limited. This study compares acute toxicity outcomes between these regimens and a non-irradiated control group following immediate breast reconstruction. Methods A single-center cohort study included 203 immediate breast reconstructions in 168 patients (implant = 59, DIEP=47, LD=33, BCS=64) performed between July 2024 and July 2025. Patients received either 3-week ( n = 63) or 5-day ( n = 50) PMRT, or no radiotherapy ( n = 90). Acute toxicity (≤3 months) was graded using the RTOG/EORTC scale. Kruskal-Wallis and ordinal logistic regression models were used to assess the influence of radiotherapy exposure, dose, and reconstruction type on toxicity severity and intervention requirements. Results Across the cohort, most reconstructions exhibited mild toxicity (47.8%), with more severe reactions (Grade 3–4) limited to 7.4% of cases. Radiotherapy exposure significantly increased toxicity severity ( H = 30.8, p < .001), whereas differences between 3-week and 5-day regimens were non-significant ( p > .05). Regression models confirmed RT exposure, particularly conventional 3-week fractionation, was the main predictor of higher RTOG/EORTC scores ( p < .001), while reconstruction type alone had no significant independent effect. The 5-day regimen had a comparable acute toxicity profile to the 3-week regimen. Erythema and oedema were the most frequent manifestations, both predominantly low-grade. Intervention rates were low across groups (