Prognosis of squamous cell carcinoma of the breast by biological subtypes and the efficacy of radiotherapy: The Japanese Breast Cancer Registry analysis.

Abstract

561 Background: Primary squamous cell carcinoma (SCC) of the breast is a rare disease, accounting for about 0.1% of all breast cancers. We aimed to determine the clinical features and prognosis of breast SCC, and the effect of radiotherapy by biological subtypes using the Japanese Breast Cancer Registry. Methods: We conducted a matched-pair analysis of 350,977 breast cancer patients with SCC or invasive ductal cancer (IDC) in the Japanese Breast Cancer Registry between 2004 and 2014. Inclusion criteria were female breast cancer patients who underwent surgery with stage I–III diseases, SCC or IDC, and age ≥ 18. Patients who received preoperative radiotherapy were excluded. We performed 1:1 SCC-IDC exact matching analysis by predominant subtypes in SCC: ER(-) and PgR(-)/HER2(-) or ER(+) and/or PgR(+)/HER2(-). Overall survival (OS), breast cancer-specific survival (BCSS), recurrence-free survival (RFS), and first locoregional or distant metastasis recurrence incidence were compared between IDC and SCC by Kaplan-Meier methods and log-rank test. In-field area recurrence was compared between those who received radiotherapy and those who did not. Results: We included 452 SCC and 182,707 IDC patients for the analysis. SCC patients were more likely to present with advanced stage disease compared to those with IDC. Among SCC patients, 59% was ER(-)/PgR(-)/HER2(-), 19% was ER(+)/PgR(+)/HER2(-), 8% was ER(-)/PgR(-)/HER2(+), 2% was ER(+)/PgR(+)/HER2(+), and 12% was data missing. Ten-year OS, BCS, and RFS rates were 70%, 80%, and 66% in SCC, and 88%, 93%, and 81% in IDC, respectively, and SCC carried a significantly worse prognosis than IDC (all P < 0.001). For exact-matched analysis, 10-year BCS of ER(-)/PgR(-)/HER2(-) SCC patients was significantly worse than IDC (P = 0.02), whereas there were no significant differences between SCC and IDC of ER(+)/PgR(+)/HER2(-) patients in OS, BCS, and RFS. Among patients who received radiotherapy, no difference was found in in-field area recurrence between SCC and IDC in either ER(-)/PgR(-)/HER2(-) or ER(+)/PgR(+)/HER2(-) subtype. Conclusions: SCC of the breast was often diagnosed in an advanced stage and had a worse prognosis than IDC. More than half of them were triple-negative subtype. In the triple-negative subtype, SCC was an independent poor prognostic factor. No significant differences in in-field area recurrence after radiotherapy between SCC and IDC suggested that the effect of radiotherapy on local control for SCC was similar to IDC.