Synchronous and metachronous bilateral breast cancer: clinicopathologic characteristics and prognostic outcomes.

The incidence of bilateral breast cancer (BBC) reportedly ranges from 1.4 to 11.8%. Women with a first primary breast cancer are at a 2- to 6-fold increased risk of developing contralateral BC. However, there have been limited studies analyzing the clinicopathologic features of BBC and conflicting data exist on the prognostic significance of BBC. In this study, we sought to analyze the incidence of BBC in the era of modern medicine and assess the clinicopathologic characteristics and prognostic outcomes compared to unilateral BC (UBC). Of the 5941 patients with stage I-III BC diagnosed between 1998 and 2013 at our institution, 110 developed BBC, including 58 synchronous BBC (SBBC, interval between the first and the contralateral BC ≤3 months) and 52 metachronous BBC (MBBC, interval >3 months). The median time to the second tumor was 67.9 months among patients with MBBC. BBC was associated with a significantly lower rate of having a ductal type, high grade, HER2-positive or node-positive disease when compared to UBC, while no difference was found for age, race, ER/PR status, or pathologic tumor stage. When compared to MBBC, SBBC was strongly associated with a lobular phenotype, non-high grade, and ER/PR-positive disease; and further demonstrated a significantly higher concordant rate for ER, PR, and HER2 status. Patients with BBC had a significantly worse distant relapse-free survival (RFS) but a similar disease-specific survival (DSS) when compared to those with UBC. Being African American, having a high histologic grade and higher pathologic tumor or node stage was significantly associated with a worse prognosis, while SBBC was associated with a favorable RFS by multivariate analysis. Nodal status was the only independent prognosticator for DSS in patients with BBC. Further investigation into the complex biologic and clinical behavior of BBC may provide novel insights into the therapeutic strategies in the pursuit of precision medicine in this unique subset of patients.