Accelerated Partial Breast Irradiation for Early-Stage Invasive Lobular Carcinoma.

Purpose: Invasive lobular carcinoma (ILC) represents 10% to 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 oncologic outcomes among patients with ILC treated with APBI.

Methods and materials: Patients treated from 2010 to 2022 with APBI after 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 40 Gy in 10 daily fractions. Outcomes of interest included local recurrence (LR) and overall survival (OS).

Results: Of 1248 patients who underwent APBI at our center, the study cohort comprised 132 (11%) who had ILC, either exclusively or mixed with another histology (median age 63). Median tumor size was 1.1 cm (interquartile range: 0.8-1.5), nearly all had estrogen receptor positive disease (99%) and received hormone 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, 2 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 OS of 98% (95% CI: 95%-100%).

Conclusion: Among patients with ILC who received APBI after breast conserving surgery, we observed a 4-year LR rate of 3%. No regional or distant recurrences were observed, and OS 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.