The protective effect of parity in hormone receptor-positive, Ki-67 expressing breast cancer

Background: Epidemiologic studies showed that the experience of pregnancy is associated with a reduced risk of breast cancer. We hypothesized that parity may differentially be associated with the development of invasive breast cancer by each subtype. Methods: We reviewed the clinical, radiological, and pathological records of women diagnosed with invasive ductal carcinoma of the breast at Samsung Medical Center between 2005 and 2009. Clinicopathologic results were assessed by χ² and Fisher's exact tests with a Bonferroni correction for categorical variables, and by the Kruskal-Wallis test for nonparametric continuous variables. A multinomial logistic regression model was used for multivariate analysis. Results: Among a total of 3,095 patients, 283 (9.14 %) patients were nulliparous. Older age, higher pN, and expression of HER2 were associated with parity. In the analysis between parity and molecular subtypes, parity also had a variable influence on breast cancer subtypes (p = 0.032). Intergroup analysis with multiple comparison showed that luminal B subtype was related to nulliparity compared with HER2-positive subtypes (p = 0.03). Conclusions: The effect of parity on the development of breast cancer differed by hormone receptor and HER2 expression. It seems that parity might have a protective effect against hormone receptor-positive breast cancer, especially cancers expressing HR+ and Ki-67. Further basic research to define and understand this result is ongoing.