Established breast cancer risk factors and risk of intrinsic tumor subtypes.

Breast cancer is a heterogeneous disease with multiple intrinsic tumor subtypes. These subtypes vary in tumor gene expression and phenotype, and are most commonly grouped into four major subtypes: luminal A, luminal B, HER2-overexpressing and triple negative (or basal-like). A growing number of studies have evaluated the relationship between established breast cancer risk factors and risk of one or more intrinsic tumor subtypes. We conducted a systematic review of 38 studies to synthesize their results and identify areas requiring more research. Taken together, published studies suggest that most established breast cancer risk factors reflect risk factors for the luminal A subtype of breast cancer, and some breast cancer risk factors may be differentially associated with other intrinsic tumor subtypes. Future breast cancer research will need to consider etiologic differences across subtypes and design studies focused on understanding the etiology and prevention of less common tumor subtypes.