MKI67 and breast cancer: Our goals were to use PET imaging to evaluate feasibility of the window-of-opportunity approach to assess endocrine therapy early response in breast cancer, and to measure early tumor response in order to improve treatment selection for early stage breast cancer that would provide insight into potential mechanisms of resistance to therapy using the Ki-67 assay, an established predictor of endocrine responsiveness [8, 12] as the reference standard.