Risk factors for second primary breast cancer by laterality, age, and race and ethnicity.

Background: Epidemiologic studies of risk factors for second primary breast cancer (SBC) have been conducted primarily in non-Hispanic White (NHW) women.

Methods: A racially- and ethnically-diverse population-based pooled cohort of 9,639 women with first primary stage I-III invasive breast cancer (FBC) was linked with the California Cancer Registry; 618 contralateral SBC (CSBC) and 278 ipsilateral SBC (ISBC), diagnosed >6 months after FBC, were identified. Using Fine and Gray models accounting for competing risks, we assessed associations of CSBC and ISBC risk with FBC clinical characteristics and epidemiologic factors.

Results: In younger women (FBC at age <50 years), higher CSBC risk was associated with ER/PR-negative FBC [hazard ratio (HR)=1.68], breast cancer family history (HR = 2.20), and nulliparity (HR = 1.37). In older women (FBC at age ≥50 years), higher risk was associated with breast cancer family history (HR = 1.32), premenopausal status (HR = 1.49), overweight (HR = 1.39), and higher alcohol consumption (HR = 1.34). For ISBC, higher risk was associated with married status (HR = 1.94) in younger women, and overweight (HR = 1.60) among older women. For CSBC, HR estimates were generally similar across racial and ethnic groups. Even after adjustment for these risk factors, compared with NHW women, risk remained elevated for CSBC in younger African American, Asian American, and Hispanic women, and for ISBC in older African American and Hispanic women with ER/PR-positive FBC.

Conclusions: Our findings support genetic risk evaluation, enhanced screening, and lifestyle changes in women at higher risk of SBC. Additional risk factors must contribute to the unequal burden of SBC across racial and ethnic groups.