The impact of selected risk factors among breast cancer molecular subtypes: a case-only study

Purpose: Breast cancer (BC) risk factors have been differentially associated with BC subtypes, but quantification is still undefined. Therefore, we compared selected risk factors with BC subtypes, using a case-case approach. Methods: We retrieved 1321 invasive female BCs from the Piedmont Cancer Registry. Through record linkage of clinical records, we obtained data on estrogen (Er) and progesterone (Pr) receptors, Ki67 and HER2+ status, BC family history, breast imaging reporting and data system (BI-RADS) density, reproductive risk factors and education. We defined BC subtypes as follows : luminal A (Er+ and/or Pr+ , HER2− , low Ki67), luminal BH- (Er+ and/or Pr + , HER2− , Ki67 high), luminal BH+ (Er+ and/or Pr + , HER2+), HER2+ (Er − , Pr − , HER2+),) and triple negative (Er − , Pr − , HER2−). Using a multinomial regression model, we estimated the odds ratios (ORs) for selected BC risk factors considering luminal A as reference. Results: For triple negative, the OR for BC family history was 1.83 (95% confidence interval (CI) 1.13–2.97). Compared to BI-RADS 1, for triple negative, the OR for BI-RADS 2 was 0.56 (95% CI 0.27–1.14) and for BI-RADS 3–4 was 0.37 (95% CI 0.15–0.88); for luminal BH +, the OR for BI-RADS 2 was 2.36 (95% CI 1.08–5.11). For triple negative, the OR for high education was 1.78 (95% CI 1.03–3.07), and for late menarche, the OR was 1.69 (95% CI 1.02–2.81). For luminal BH + , the OR for parous women was 0.56 (95% CI 0.34–0.92). Conclusions: This study supported BC etiologic heterogeneity across subtypes, particularly for triple negative.