Proportion of estrogen or progesterone receptor expressing cells in breast cancers and response to endocrine therapy.

Immunohistochemical determination of ER/PR status has been the gold standard in clinical practice of breast cancer for decades. A cut-off of '1%' is commonly used; however, this is not supported by strict evidence. How the proportion of ER/PR-positive cells influences the response to endocrine therapy has been scarcely reported, either. To address these issues, 486 and 663 invasive breast cancer cases treated with or without adjuvant tamoxifen respectively (median follow-up period, 12.8 years) were enrolled, and effect of tamoxifen treatment was compared among ER/PR-positive or -negative groups immunohistochemically determined using various cut-offs. Tamoxifen significantly improved 5 years disease-free survival in ER/PR-positive, but not in ER/PR-negative, cases even using immunohistochemical >0% cut-off. Cases with ≥67% ER/PR expressing cells responded to tamoxifen by far the best. Patients having tumors without any ER/PR-positive cells should be excluded from endocrine therapy, whereas this therapy should be strongly recommended for those with ≥67% ER/PR-positive cells.