Breast fibroadenomas are not associated with increased breast cancer risk in an African American contemporary cohort of women with benign breast disease

BACKGROUND: Fibroadenomas are common benign breast lesions, and studies of European American women indicate a persistent, increased risk of breast cancer after diagnosing a fibroadenoma on biopsy. This association has not been independently assessed in African American women, despite reports that these women are more likely to present with fibroadenomas. METHODS: The study cohort included 3853 African American women with a breast biopsy completed between 1997 and 2010 in metropolitan Detroit. Biopsies were microscopically reviewed for benign breast lesions, including fibroadenoma, proliferative disease, and atypia. Risk of breast cancer within the cohort was estimated using relative risk ratios and 95% CIs calculated using multivariable log-binomial regression. Relative risk of breast cancer in this cohort compared with African American women in the broader metropolitan Detroit population was estimated using standardized incidence ratios (SIRs). RESULTS: Fibroadenomas occurred more frequently in biopsies of younger women, and other types of benign breast lesions were less likely to occur when a fibroadenoma was present (p = 0.008 for lobular hyperplasia; all other p values < 0.01). Unlike women with other benign lesions (SIR, 1.41; 95% CI, 1.20, 1.66), women with fibroadenomas did not have an increased risk of developing breast cancer compared with the general population (SIR, 0.94; 95% CI, 0.75, 1.18). Biopsies that indicated a fibroadenoma were associated with a reduced risk of breast cancer after adjusting for age at biopsy, proliferation, and atypia (relative risk, 0.67; 95% CI, 0.48, 0.93) compared with biopsies without a fibroadenoma. CONCLUSIONS: These findings have important implications for breast cancer risk models and clinical assessment, particularly among African American women, in whom fibroadenomas are common. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s13058-018-1027-6) contains supplementary material, which is available to authorized users.