The risk of contralateral breast cancer in daughters of women with and without breast cancer

We aimed to estimate the 15‐year and lifetime risks of contralateral breast cancer in breast cancer patients according to the age of diagnosis of the first cancer and the history of breast cancer in the mother. The risks of contralateral breast cancer were estimated for all 78,775 breast cancer patients in the Swedish Family‐Cancer Database (age at diagnosis of first breast cancer <70 years). The risk of experiencing a contralateral breast cancer within 15 years of diagnosis was 8.4% [95% confidence interval (CI): 8.1–8.7%] for women with an unaffected mother, was 12% (95%CI: 11–13%) for a woman with a mother with unilateral breast cancer and was 13% (95%CI: 9.5–17%) for women with a mother with bilateral breast cancer. In early‐onset diagnosed women (<50 years) with an unaffected mother, the risk of contralateral breast cancer until age 80 was 23% (95%CI: 20–26%) and for late‐onset (50–69 years) diagnosed women it was 17% (95%CI: 14–21%). In a woman with a mother with an early‐onset unilateral breast cancer, risk of contralateral breast cancer by age 80 was 35% (95%CI: 25–46%). Women with a mother with early‐onset bilateral breast cancer had 31% (95%CI: 12–67%) lifetime risk of contralateral breast cancer. The risk of contralateral breast cancer is higher for daughters of breast cancer patients than for daughters of women without breast cancer. Maternal cancer history and age at onset of first breast cancer in women should be taken into account when counseling breast cancer patients about their risk of contralateral breast cancer.