Was Reexcision Less Frequent for Patients with Lobular Breast Cancer After Publication of the SSO-ASTRO Margin Guidelines?

Background. The Society of Surgical Oncology and American Society for Radiation Oncology consensus guidelines defined a negative margin for breast-conserving surgery (BCS) as no ink on tumor, and implementation has reduced rates of additional surgery for patients with invasive ductal cancer (IDC). The outcomes for invasive lobular cancer (ILC) patients are uncertain. Methods. This study identified patients who had stage 1 or 2 ILC treated with BCS from January 2010 to February 2018. The guidelines were adopted 1 January 2014. Clinicopathologic characteristics, margin status, and reexcisions were compared before and after adoption of the guidelines and with those of IDC patients treated from May 2013 to February 2015. Results. Among 745 early-stage ILC patients undergoing BCT, 312 (42%) were treated before the guidelines and 433 (58%) after the guidelines. Most clinicopathologic characteristics were similar between the two groups, with differences in lobular carcinoma in situ, lymphovascular invasion, and node-positivity rates. The overall rates of additional surgery declined significantly after the guidelines (31.4 to 23.1%; p = 0.01), but the difference did not reach significance for reexcisions (19.9 to 15.2%; p = 0.12) or conversions to mastectomy (11.5 to 7.9%; p = 0.099) individually. Between eras, no difference in incidence or number of tumor on ink or <= 2 mm margins was observed (all p = 0.2). Larger tumors, younger age, and pre-guideline era were independently associated with additional surgery. Only younger age was predictive of mastectomy. Among 431 pre-guideline and 601 post-guideline IDC patients, reexcisions declined from 21.3 to 14.8% (p = 0.008), and conversion to mastectomy was rare (0.6%). The magnitude of reduction in any additional surgery (interaction, p = 0.92) and reexcisions (interaction, p = 0.56) was similar between ILC and IDC. Conclusions. Despite differences in growth pattern and conspicuity, guideline adoption significantly reduced additional surgery among ILC patients, with a magnitude of benefit similar to that among IDC patients.