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

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.

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.

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).

This study supported BC etiologic heterogeneity across subtypes, particularly for triple negative.