The assessment of new therapies in the adjuvant setting in early breast cancer requires large numbers of patients and many years of follow-up for results to be presented. Therefore, the neoadjuvant study setting, which allows for early prediction of treatment response in smaller patient sets, has become increasingly popular. Ki67 is the most commonly used and extensively studied intermediate biomarker of treatment activity and residual risk in neoadjuvant trials on endocrine therapy, new biological therapies, and chemotherapy. It is increasingly being used as a primary endpoint for new therapies particularly those added to endocrine therapy. The PeriOperative Endocrine Therapy for Individualizing Care (POETIC) trial, including more than 4000 postmenopausal, estrogen receptor (ER)-positive patients randomly assigned to receive 2 weeks of presurgical treatment with an aromatase inhibitor or no further treatment, is the largest window-of-opportunity trial conducted and is assessing the clinical utility of on-treatment Ki67 as a predictor of long-term outcome. For generalizability, Ki67 measurements in the POETIC and other trials need to use standard methodology. The International Working Group on Ki67 in Breast Cancer is conducting a series of studies to bring this to reality.
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