Do you rely on Ki67 as an estimate of proliferation to guide the decision for chemotherapy in premenopausal women with hormonally-driven breast tumors?

Far from it. Especially in premenopausal women, Ki-67 is affected by circulating hormones and time of the month, and it is not a reliable indicator of endocrine resistance. In postmenopausal women, the most valuable Ki-67 reading is on endocrine therapy if any, not baseline either. Ki-67 should not be used as a stand alone decider for chemotherapy. It should be taken within the context of other variables if it were to be done at all.

Remember that among the 16 genes in the Recurrence Score assay, there are 5 proliferation genes; Ki-67 is one of them. It is understandable that there is interest in a poor man's predictor that can be readily available, but it would be misleading to use baseline Ki-67, not to mention technical issues in reading...