Ki-67 evaluation at the hottest spot predicts clinical outcome of patients with hormone receptor-positive/HER2-negative breast cancer treated with adjuvant tamoxifen monotherapy.

Background: The clinicopathological importance of Ki-67 in breast cancers has been intensely studied; however, there have been few systematic large studies of patients treated with predefined adjuvant therapy. Further, Ki-67 evaluation methodology differed among studies, which prevents Ki-67 from being used for clinical practice. We performed a large systematic study using routinely processed tissues and compared various scoring methods.
Methods: Representative slides of archival tissue blocks of 442 consecutive invasive breast cancers from women treated with adjuvant tamoxifen monotherapy and having a long follow-up period were subjected to immunohistochemistry using anti-Ki-67 monoclonal antibody, Mib-1. Both the average score across the section and the score at the hottest spot were assessed.
Results: Ki-67 evaluated at the hottest spot, not the average score across the section, independently predicted poor clinical outcomes of patients with hormone receptor-positive/HER2-negative cancer. Ki-67 was not a predictor of clinical outcome in patients with triple-negative breast cancer. Overall, high Ki-67 level significantly correlated with classic unfavorable clinicopathological factors, correlating negatively with the status of estrogen receptor (ER)-α and progesterone receptor (PR), and positively with HER2 status and grade. ER-β status positively correlated with the Ki-67 level.
Conclusions: Ki-67 evaluation at the hottest spot was superior to that determined by average score across the section as a predictor of outcome in patients with hormone receptor-positive/HER2-negative breast cancers treated with endocrine monotherapy. The different result obtained in patients with triple-negative carcinomas needs to be further investigated.