Clinicopathological characteristics of synchronous bilateral breast cancer compared to unilateral breast cancer: a propensity score-matched analysis

Although synchronous bilateral breast cancer (SBBC) is increasingly detected following advances in imaging and routine screening, its clinicopathological characteristics and prognosis remain unclear. Prior studies have been limited by small cohorts and insufficient adjustment for baseline differences. This study aimed to clarify these issues by comparing SBBC with unilateral breast cancer (UBC) using propensity score matching. We conducted a retrospective cohort study of patients diagnosed with breast cancer at the Seoul National University Bundang Hospital between 2004 and 2023. In total, 383 patients with SBBC and 9,573 patients with UBC were identified, and a 4:1 propensity score–matched cohort was constructed for comparison. Recurrence-free survival (RFS) and overall survival (OS) were assessed using the Kaplan–Meier method, and prognostic factors were evaluated using Cox regression analysis. SBBCs were more often hormone receptor-positive, human epidermal growth factor receptor (HER2)-negative, and of low histological grade than UBCs. In total, 1,532 patients with UBC were compared with 383 patients with SBBC after matching for baseline clinicopathological characteristics. No significant differences in RFS or OS were observed between patients with SBBC and those with UBC before and after matching. Subgroup analysis revealed no significant survival differences among the unilateral, subtype-concordant SBBC, and subtype-discordant SBBC groups, although the subtype-concordant SBBC group showed a non-significant trend toward reduced OS. Despite distinct clinicopathological characteristics, SBBC does not confer a worse prognosis than UBC. These results support current treatment approaches guided primarily by tumor biology rather than disease laterality. The trend toward reduced OS in the subtype-concordant SBBC subgroup was based on a small sample ( n = 12) and should be interpreted as hypothesis-generating. However, larger cohorts are needed to clarify the prognostic impact of subtype discordance within SBBC.