Memorial Sloan Kettering Cancer Center New York, New York, United States
L. Z. Braunstein1, L. A. Boe2, B. A. Mueller3, D. A. Roth OBrien4, I. J. Choi5, J. J. Cuaron5, A. J. Xu3, M. B. Bernstein3, B. McCormick3, S. N. Powell3, and A. J. Khan3; 1Memorial Sloan Cancer Center, NYC, NY, 2Memorial Sloan Kettering, New York, NY, 3Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, 4Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, NY, 5Memorial Sloan Kettering Cancer Center, New York, NY
Purpose/Objective(s): Invasive lobular carcinoma (ILC) represents 10-15% of invasive breast cancers with limited representation among trials of accelerated partial breast irradiation (APBI). Contemporary guidelines advise against treating ILC with APBI given a paucity of supportive evidence. Here, we evaluated oncological outcomes among patients with ILC treated with APBI. Materials/Methods: Patients treated from 2000 – 2022 with APBI following breast conserving surgery for ILC (or mixed ILC with other histologies) were ascertained from a prospectively-maintained institutional database. All patients received external beam APBI to 40Gy in 10 daily fractions. Outcomes of interest included local recurrence (LR) and overall survival (OS). Results: Of 1248 patients who underwent PBI at our center, 132 (11%) had ILC, either exclusively or mixed with another histology, comprising the study cohort (median age 63). Median tumor size was 1.1cm (IQR: 0.8, 1.5), nearly all had estrogen receptor positive disease (99%) and received endocrine therapy (91%), and most underwent sentinel node biopsy (89%) with the remainder having no axillary surgery. At 530 person-years and a median follow-up of 39 months, two LRs were observed yielding a 48-month cumulative incidence of LR of 3.0% (95% CI 0.56 – 9.5%). Both events arose in patients with mixed lobular histology (none arose in patients with pure ILC). Two unrelated deaths were also observed yielding a 48 month overall survival of 98% (95% CI: 95% - 100%) Conclusion: Among patients with ILC who received APBI followed BCS, we observed a 4-year LR rate of 3%. No regional or distant recurrences were observed, and overall survival was excellent. The safety of APBI for ILC will require confirmation among larger trials with longer follow-up, although the excellent outcomes observed here are consistent with those seen for invasive ductal carcinomas among contemporary trials of APBI.
