Differences in breast cancer risk factors according to the estrogen receptor level of the tumor.

One hundred and forty-eight postmenopausal breast cancer cases and 585 postmenopausal controls were included in an investigation of whether various risk factors for breast cancer are associated with the level of estrogen (E) receptor (ER) protein in the tumor. In an intracase analysis, the tumor ER level was positively associated with nulliparity, late age at first live birth, a history of benign breast disease, and having breast-fed at least 1 child and was negatively associated with previous use of E replacement therapy. A case-control analysis suggested that the first three variables, established risk factors for breast cancer, are associated with an increased risk for malignant breast tumors that are ER-positive but not for those that are ER-negative. This analysis did not provide a clear interpretation of the findings in the intracase comparisons with regard to prior breast-feeding and the use of E replacement therapy.