Assessment of Ki67 in breast cancer: recommendations from the International Ki67 in Breast Cancer working group

Abstract

Uncontrolled proliferation is a hallmark of cancer. In breast cancer, immunohistochemical assessment of the proportion of cells staining for the nuclear antigen Ki67 has become the most widely used method for comparing proliferation between tumor samples. Potential uses include prognosis, prediction of relative responsiveness or resistance to chemotherapy or endocrine therapy, estimation of residual risk in patients on standard therapy and as a dynamic biomarker of treatment efficacy in samples taken before, during, and after neoadjuvant therapy, particularly neoadjuvant endocrine therapy. Increasingly, Ki67 is measured in these scenarios for clinical research, including as a primary efficacy endpoint for clinical trials, and sometimes for clinical management. At present, the enormous variation in analytical practice markedly limits the value of Ki67 in each of these contexts. On March 12, 2010, an international panel of investigators with substantial expertise in the assessment of Ki67 and in the development of biomarker guidelines was convened in London by the co-chairs of the Breast International Group and North American Breast Cancer Group Biomarker Working Party to consider evidence for potential applications. Comprehensive recommendations on preanalytical and analytical assessment, and interpretation and scoring of Ki67 were formulated based on current evidence. These recommendations are geared toward achieving a harmonized methodology, create greater between-laboratory and between-study comparability, and allow earlier valid applications of this marker in clinical practice.

Figure 1

Schematic representation of the applications of Ki67 as a pharmacodynamic marker in endocrine neoadjuvant therapy. The circles represent the tumor at different time points during neoadjuvant treatment. The boxes contain the application of Ki67 or of change in Ki67 (ΔKi67) between the respective time points. PEPI = Preoperative Endocrine Prognostic Index.

Figure 2

Examples of Ki67 staining in breast cancer. Tumor biopsies were fixed in neutral buffered formalin and sections stained for Ki67 with the MIB1 antibody (brown stain) and counterstained with Mayer’s hematoxylin (blue stain). A) Well-fixed specimen. B) Poorly fixed specimen. The micrograph was taken using a Leica Microsystems Ariol image analyzer (Leica Microsystems, Gateshead, UK). Scale bar = 50 μm.

Figure 3

Variable levels of Ki67 staining in breast cancer. Tumor biopsies were fixed in neutral buffered formalin and sections stained for Ki67 with MIB1 antibody (brown stain) and counterstained with Mayer’s hematoxylin (blue stain). The two areas circled in red are shown at higher magnification to illustrate the differences in scores that can occur in different high-power fields. The average score across the whole section should be taken. The micrograph was taken using Aperio ScanScope image analyzer (Aperio, Vista, CA). Scale bar = 100 μm and scale bar of inset = 50 μm.