Accelerated Partial Breast Irradiation for Early-Stage Invasive Lobular Carcinoma

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.

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).

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%).

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.