Impact of Adjuvant Systemic Therapy and Differences in Synchronous and Metachronous Bilateral Early Breast Cancer in Long-Term Outcome.

[AIMS] The aim of this study was twofold: to analyse the 20-year outcomes and differences of invasive synchronous bilateral breast cancer (SBBC) and invasive metachronous bilateral breast cancer (MBBC) and to determine the impact on outcomes of the increased use of adjuvant systemic therapy (AST).

[MATERIAL AND METHODS] Data were obtained from our prospective population-based cohort study that included women, diagnosed with early breast cancer (BC) between 1984 and 2015. Bilateral breast cancer (BBC) was defined as SBBC (≤3 months of the first primary) or MBBC (>3 months after the first primary).

[RESULTS] The incidence of SBBC was 1.2% and that of MBBC 9.5%. MBBC status was an independent significant predictor of local failure (hazard ratio [HR]: 1.7; 95% confidence interval [CI]: 1.4-2.2). SBBC status was an independent predictor of distant metastases (HR: 2.3; 95% CI: 1.4-3.7) and showed a worse disease-specific survival (DSS) (HR: 2.4; 95% CI: 1.5-4.0) than unilateral breast cancer (UBC). The use of AST nearly doubled between 1984 and 1998 and 1999 and 2015. The 20-year MBBC-free survival was better for those with AST with an HR of 0.5 (95% CI: 0.4-0.6) than for those without AST. This increased use of AST had a positive effect on DSS for SBBC, MBBC, and UBC.

[CONCLUSION] The increased use of AST over time had a positive effect on incidence of MBBC and on survival for SBBC, MBBC, and UBC. Women suffering from SBBC have almost a three times higher BC mortality rate than women suffering from UBC. Patients with MBBC demonstrated a worse local relapse-free survival than those with unilateral breast cancer (UBC) for both periods.