Breast cancer laterality and molecular subtype likely share a common risk factor

BACKGROUND: To investigate the epidemiological features of breast cancer laterality and molecular subtypes in southern China. MATERIALS AND METHODS: A total of 2,049 cases who were diagnosed with unilateral breast cancer in the past 5 years were classified based on laterality and molecular subtypes. Molecular subtypes were defined in accordance with the 2013 St. Gallen recommendations. RESULTS: Breast cancer was more likely to be diagnosed in the left breast than in the right at a rate of around 5%. In the case of invasive carcinomas, the right breast was more commonly affected than the left in young (<40 years old) patients (left-to-right [L:R] ratio 0.80, 95% CI 0.65, 0.98), whereas the opposite trend was found in old (≥40 years old) patients (L:R ratio 1.06, 95% CI 1.02, 1.73). Except for invasive mucinous and invasive medullary breast cancers, the other histological types occurred more frequently on the left side than on the right. In situ cancer with a defined subtype was likely to be diagnosed as luminal B(HER-2+). Except for invasive medullary and invasive nonspecific cancers, other invasive carcinomas with a defined subtype were most likely to be diagnosed as luminal B(HER-2−). The age of ≥40 years was a risk factor for luminal B(HER-2+), and a significant correlation was present between the right breast and luminal B(HER-2+). CONCLUSION: We explored the risk factors of breast cancer laterality and various molecular subtypes and found that age may be a predictor of breast cancer laterality. We found that age and laterality are the probable risk factors of the luminal B(HER-2+) type of breast cancer. These results provide a basis for the epidemiological characterization of breast cancer.