The association between parity, age of first full term pregnancy and invasive lobular breast cancer: a case-only analysis.
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PURPOSE: The risk for estrogen receptor positive/HER2-negative (ER+/HER2-) breast cancer (BC) is transiently augmented in the first years after giving birth especially if the first birth is occurring at a later age. Later in life, high parity and early first full-term pregnancy (1st FTP) appear to be protective for ER+/HER2- BC. The impact of reproductive factors on invasive lobular carcinoma (ILC) is poorly studied. We aimed at retrospectively investigating the association between reproductive factors (parity and age at 1st FTP) and the prevalence and clinicopathological features of ER+/HER2- ILC. METHODS: We considered patients primarily diagnosed with non-metastatic ER+/HER2- BC at University Hospitals Leuven, Belgium, between January 2000 and November 2020. Pure ILC cases were compared in a case-only design to a reference group including all other histological subtypes per parity and age at 1st FTP using linear regression models. RESULTS: 7360 patients were included, of which 1121 (15.2%) were diagnosed with pure ER+/HER2- ILC. A parity greater than two children was associated with a higher prevalence of pure ILC compared to uniparous patients (OR 1.28, 95%CI 1.06-1.55, p-value 0.011). No significant association was seen for parity equal to two compared to one or for age at 1st FTP <21 vs. 21-25, 26-30 and ≥30. In patients with pure ER+/HER2- ILC, no significant associations were seen between parity and the clinicopathological features. CONCLUSIONS: Within an ER+/HER2- BC cohort, higher parity seems to be associated with a higher prevalence of pure ILC. No association was found between parity and clinicopathological features of ER+/HER2- pure ILC.