ABSTRACT
Objective To compare the clinicopathological characteristics between primary and contralateral cancers in patients with metachronous bilateral breast cancer (MBBC) who carried a BRCA1/2 germline pathogenic variant. Methods A total of 496 BRCA1/2 carriers with primary unilateral breast cancer were included (196 with BRCA1 and 300 with BRCA2). Clinicopathological information of patients was collected, and the median follow-up for the entire cohort was 10.4 years (0.4-20.8 years). Results Among all patients, 31 (15.8%) of the 196 BRCA1 carriers and 49 (16.3%) of the 300 BRCA2 carriers had MBBC, respectively. Among the 31 BRCA1 carriers who developed MBBC, the proportion of triple-negative breast cancer (TNBC) in primary cancer and contralateral cancer was 61.3% and 67.7%, respectively. If the primary cancer of BRCA1-mutated MBBC was TNBC, the probability of the contralateral breast cancer with TNBC was 89.5% (17/19), which was significantly higher than that if the primary cancer was non-TNBC (33.3%, 4/12) (P=0.004). Among the 49 BRCA2 carriers who developed MBBC, the predominant molecular phenotype of the primary and contralateral cancers was HR+ & HER2- (77.6% and 67.3%, respectively; P=0.53). Conclusion Approximately 60% of BRCA1 carriers exhibit TNBC. If a BRCA1 carrier with a TNBC primary breast cancer had an MBBC, the probability of the contralateral breast cancer being TNBC phenotype is almost 89.5%.