A new study has reported that women with higher breast density have an increased risk of hormone receptor positive (ER+/PR+) and mixed hormone receptor (ER+/PR- or ER-/PR+) tumors, but not hormone receptor negative (ER-/PR-) disease. Mammographic breast density is a strong risk factor for breast cancer, but it is not clear whether breast density predicts subtypes of breast cancer defined by estrogen receptor (ER) and progesterone receptor (PR) expression. The study included 607 breast cancer cases and 667 cancer-free controls. The women were Caucasian, Japanese, and Native Hawaiian participants in the Hawaii component of the Multiethnic Cohort Study. The Hawaii Tumor Registry was used to obtain hormone receptor status as follows: 341 ER+/PR+; 50 ER-/PR-; 64 mixed hormone receptor (ER+/PR- or ER-/PR+); and 152 unstaged or unknown. Average percent breast density was calculated for women who had more than one mammogram.

Average breast density was found to be significantly greater for ER+/PR+ tumors (37.3%) than cancer-free controls (29.4%) after adjusting for age. On the other hand, breast density for ER-/PR- tumors (28.9%) was not significantly different from that of controls (29.4%). Risk of breast cancer increased for each 10% increase in percent density for ER+/PR+ and mixed hormone receptor tumors. However, breast density was not found to be a linked to risk of ER-/PR- tumors. The results were not found to vary according to ethnicity, menopausal status, number of children, or hormone replacement therapy (HRT) use.

Comments regarding the study

The study results differ from previous reports that high breast density is also associated with risk of ER-/PR- tumors. Hormone replacement therapy, particularly combined estrogen plus progestin HRT, heightens the risk of breast cancer associated with dense breasts among postmenopausal women. Since HRT use has been found to be associated with increased risk of estrogen receptor positive (ER+/PR+ and ER+/PR-) tumors in particular, it is surprising that the study did not find that HRT use increased the likelihood of ER+ disease among women with dense breasts.

Please see our article on the latest research concerning breast density and breast cancer for more information.