Accelerated Partial Breast Irradiation (APBI) For HER2+ Early-Stage Breast Cancer

Historically, HER2+ breast cancer exhibited poor outcomes and, hence, has not been well-studied among trials of accelerated partial breast irradiation (APBI). We hypothesized that in contemporary practice with effective HER2-targeted agents, patients with HER2+ breast cancer now have excellent disease control and survival outcomes when treated with adjuvant APBI.Using a prospectively-maintained institutional database, we identified all HER2+ breast cancer patients treated with breast conserving surgery (BCS) and adjuvant APBI from 2000 - 2022. Salient clinicopathologic parameters were collected, as were receipt of systemic and endocrine therapies. All patients received external beam APBI to a total dose of 40Gy in 10 daily fractions over 2 weeks. We analyzed outcomes including local recurrence (LR), regional recurrence, distant recurrence, and death. Cumulative incidence functions were calculated to estimate the incidence of LR with the competing risk of death. All statistical analyses were performed in R version 4.2.2.The study cohort comprised 52 patients with HER2+ breast cancer (median age 64 years; range 44-87). Nearly all had T1 tumors (98%; median size 15 mm; range 1 - 21 mm). Approximately 10% had multifocal disease, with one exhibiting suspicion for lymphovascular invasion. Most patients had ER+ disease (88%). All patients had negative final surgical margins. Nearly all underwent sentinel node biopsy (94%), with the remainder undergoing no surgical axillary evaluation. 42 (81%) received chemotherapy, 40 (77%) endocrine therapy, and 42 (81%) HER2-directed therapy, most commonly trastuzumab. At 143.8 person-years of follow-up (range 7 - 226 months for each patient), we observed two LR events, at 14 and 26 months, yielding a 2-year LR rate of 3.8%. No regional or distant recurrences were observed, nor were any contralateral invasive breast cancer events or breast-specific mortality events. Two deaths were noted in the cohort, both without evidence of disease.Among a cohort of HER2+ early-stage breast cancer patients managed with BCS and APBI, we observed a 2-year LR rate of 3.8% with no regional or distant recurrences, and excellent overall survival. These findings merit longer term follow-up among larger cohorts, although are thus far consistent with the results of contemporary randomized trials of APBI unselected for HER2-status.