This observational study aimed to investigate whether the reported association between family history (FH) of breast cancer (BC) or ovarian cancer (OC) and OC risks in BRCA1/2 mutation carriers can be explained by mutation position on the gene. In total, 3310 female BRCA1/2 mutation carriers participating in a nationwide prospective cohort (Hereditary Breast and Ovarian Cancer in the Netherlands) were included. FH was classified according to cancer occurrence in first-degree relatives (BC only, OC only, both, neither) and mutations were classified according to their position on the gene (OC cluster region (OCCR), BC cluster region, neither). The main outcome was OC occurrence. Cox proportional-hazard models were applied to investigate the association between FH and OC risks before and after adjusting for mutation position. Of all women included, 202 were diagnosed with OC. A BC-only FH tended to be associated with lower OC risks when compared with a FH without BC/OC (HR: 0.79, 95% CI: 0.52-1.17; HR: 0.59, 95% CI: 0.33-1.07 for BRCA1 and BRCA2, respectively) while an OC-only FH tended to be associated with higher risks (HR: 1.58, 95% CI: 0.90-2.77; HR: 1.75, 95% CI: 0.70-4.37 for BRCA1 and BRCA2, respectively). After adjusting for mutation position, association between FH and OC risks was slightly smaller in magnitude (HR: 0.85, 95% CI: 0.55-1.30; HR: 0.64, 95% CI: 0.34-1.21 for BC-only FH in BRCA1 and BRCA2, respectively; HR: 1.46, 95% CI: 0.80-2.68; HR: 1.49, 95% CI: 0.44-4.02 for OC-only FH in BRCA1 and BRCA2, respectively), indicating that mutation position explains only part of the association. Considering the magnitude of the observed trend, we do not believe FH should be used to change counseling regarding OC prevention.
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