Cyclin D1 expression in breast cancer patients and tamoxifen therapy

523 Background: Adjuvant tamoxifen therapy improves survival of hormone receptor-positive breast cancer patients, but there is a subset of patients who fails to respond or develops resistance to tamoxifen. Cyclin D1 plays a central role in cell cycle regulation, directly affects the estrogen receptor and may predict outcome of tamoxifen therapy. Methods: We determined expression of cyclin D1 in formalin-fixed, paraffin-embedded surgical breast cancer specimens by means of immunohistochemistry. The patients had been enrolled in the Austrian Breast and Colorectal Cancer Study Group (ABCSG) Trial 5 (test set) or ABCSG Trial 6 (validation set). Comparison of cyclin D1 expression with clinical parameters and survival of the patients was performed with cyclin D1 as a continuous variable and as a dichotomized variable. The median value of the percentage of cyclin D1-positive tumor cells was a priori chosen as cut-off point to classify cyclin D1- positive and cyclin D1-negative tumors. The cutpoint was establishe...