Patterns of Ipsilateral Breast Tumor Recurrence Following Partial vs. Whole-Breast Irradiation: A Retrospective Comparative Study of Breast-Conserving Therapy

Purpose: Partial-breast irradiation (PBI) is an alternative to whole-breast irradiation (WBI) in breast-conserving therapy. This study evaluated ipsilateral breast tumor recurrence (IBTR), classified as true recurrence (TR) or new primary tumor (NP), following PBI or WBI. Methods and Materials: We retrospectively analyzed 1,395 patients treated with PBI or WBI between 2008 and 2024. PBI was delivered perioperatively using multicatheter interstitial brachytherapy (32 Gy in 8 fractions or 25.2 Gy in 4 fractions) or WBI (50 Gy in 25 fractions). The incidence, timing, and predictors of TR and NP were evaluated. Results: At a median follow-up of 6.7 years, 40 IBTRs (2.9%) occurred: 20 TRs (1.4%) and 20 NPs (1.4%). Five- and 10-year breast cancer–specific survival rates were 85.4% and 60.1% for TR, and 100% for NP (p < 0.01). TR rates were similar between the PBI and WBI cohorts (1.4% vs. 1.6%, p = 0.68), but NP occurred more frequently after PBI (1.9% vs. 0.5%, p < 0.05). Multivariate analysis revealed that younger age (<50 years) was independently associated with TR (HR, 2.46; p < 0.05), and PBI was significantly associated with NP (HR, 4.44; p < 0.05). Conclusions: PBI provides local control comparable to WBI for TR; however, it may be associated with an increased risk of NP, which represents a potential drawback of PBI.