Family History and Risk of Second Primary Breast Cancer after In Situ Breast Carcinoma

Background: Incidence rates of in situ breast carcinomas have increased due to widespread adoption of mammography. Very little is known about why some women with in situ breast cancer later develop second primary breast cancers. Methods: In this population-based nested case-control study among in situ breast cancer survivors, including 539 cases with a second primary breast cancer and 994 matched controls, we evaluated the association between first degree family history of breast cancer and risk of developing a second primary breast cancer. Results: First degree family history of breast cancer was associated with an increased risk of developing a second primary breast cancer among women with a previous in situ breast cancer (odds ratio (OR)=1.33, 95% confidence interval (CI):1.05, 1.69) and those with two or more affected first degree relatives had an even higher risk (OR=1.94, 95% CI:1.15, 3.28). Those whose relative was diagnosed at less than 50 years old were more likely to develop a second primary breast cancer (OR=1.78, 95% CI:1.24, 2.57). No difference in risks associated with number or age of affected relatives were observed by menopausal status. Conclusions: Results from this study suggest that first degree family history of breast cancer may be an important risk factor for development of a second primary breast cancer among women with a previous in situ breast cancer. Impact: Given the growing population of in situ breast cancer survivors, a better understanding of risk factors associated with development of a second primary breast cancer is needed to further understand risk.