A case-match study comparing unilateral with synchronous bilateral breast cancer outcomes

There is controversy about whether patients with synchronous bilateral breast cancer (SBBC) have similar or worse outcomes compared with patients with unilateral breast cancer. The purpose of this study was to determine whether survival outcomes for patients with SBBC can be estimated from the characteristics of their individual cancers.

Patients had invasive breast cancer, without metastases or inflammatory disease, diagnosed in British Columbia between 1989 and 2000. There were 207 cases with SBBC (diagnosed ≤ 2 months apart) and 15,497 with unilateral breast cancer. By using 10-year breast cancer-specific survival (BCSS) estimates, the higher-risk cancer of each SBBC case was determined and matched with three breast cancers from the unilateral cohort to select 621 high-risk matches. The priority sequence of matching the prognostic and predictive variables was positive lymph node number, primary tumor size, age, grade, lymphovascular invasion, estrogen receptor status, local therapy used, margin status, treating clinic, diagnosis year, and type of systemic therapy used.

With a median follow-up of 10.2 years, the overall 10-year BCSS was significantly higher for the unilateral cohort (81%; 95% CI, 81% to 82%) than for the SBBC cases (71%; 95% CI, 63% to 77%). The SBBC cases had significantly higher mean age and stage at presentation. The 10-year BCSS was 74% (95% CI, 69% to 77%) for the high-risk matches.

BCSS was not significantly different between the SBBC cases and their high-risk matches.