The Evolution of Ki-67 and Breast Carcinoma: Past Observations, Present Directions, and Future Considerations

Abstract

The 1983 discovery of a mouse monoclonal antibody-the Ki-67 antibody-that recognized a nuclear antigen present only in proliferating cells represented a seminal discovery for the pathologic assessment of cellular proliferation in breast cancer and other solid tumors. Cellular proliferation is a central determinant of prognosis and response to cytotoxic chemotherapy in patients with breast cancer, and since the discovery of the Ki-67 antibody, Ki-67 has evolved as an important biomarker with both prognostic and predictive potential in breast cancer. Although there is universal recognition among the international guideline recommendations of the value of Ki-67 in breast cancer, recommendations for the actual use of Ki-67 assays in the prognostic and predictive evaluation of breast cancer remain mixed, primarily due to the lack of assay standardization and inconsistent inter-observer and inter-laboratory reproducibility. The treatment of high-risk ER-positive/human epidermal growth factor receptor-2 (HER2) negative breast cancer with the recently FDA-approved drug abemaciclib relies on a quantitative assessment of Ki-67 expression in the treatment decision algorithm. This further reinforces the urgent need for standardization of Ki-67 antibody selection and staining interpretation, which will hopefully lead to multidisciplinary consensus on the use of Ki-67 as a prognostic and predictive marker in breast cancer. The goals of this review are to highlight the historical evolution of Ki-67 in breast cancer, summarize the present literature on Ki-67 in breast cancer, and discuss the evolving literature on the use of Ki-67 as a companion diagnostic biomarker in breast cancer, with consideration for the necessary changes required across pathology practices to help increase the reliability and widespread adoption of Ki-67 as a prognostic and predictive marker for breast cancer in clinical practice.

Keywords: Ki-67; abemaciclib; analytic validity; breast cancer; clinical validity; monarchE; predictive; prognostic; proliferation; standardization.

Figure 1

(A–H): Ki-67 staining (10×) category fields based on the International Ki-67 Working Group recommendations, with corresponding hematoxylin and eosin (H&E), (4×) staining. (A,B): Negative/Very Low (<1%) Ki-67 staining, with corresponding (H&E); (C,D): Low (≤5%) Ki-67 staining, with corresponding (H&E); (E,F): Medium (6–29%) Ki-67 staining, with corresponding H&E; (G,H): High (≥30%) Ki-67 staining, with corresponding (H&E).

Figure 2

(A): Ki67-QC International Working Group whole section scoring protocol (global method): Specification of tissue fields and Ki-67 staining percentage in invasive tumor fields. * Fields are either non-invasive tissue (Field 0) or invasive carcinoma tissue (Fields 1–4). ** The percentage of Ki-67 staining in each invasive carcinoma field (fields 1–4) represents relative determinations of Ki-67 staining as a percentage of all the tissue on the glass slide (F0, F1, F2, F3, F4). Percentages should be considered based on the IKWG considerations of a low (≤5%) and high (≥30%) Ki-67 expression. Very low might be considered as <1%. *** Non-invasive tissue includes non-tumor tissue and in situ carcinoma. (B): Ki67-QC International Working Group whole section scoring protocol (global method): Calculation of overall Ki-67 staining percentage. * The percentage of each field (F0–F4) represents the area of that field relative to the area of the entire slide. For example, the percentage of invasive carcinoma field 1 (%F1) = %F1 = (^{area of F1}/_{area F0 + area F1 + area F2 + area F3 + area F4 + area F5}) × 100; ** For example, the relative percentage of invasive tumor nuclei in invasive carcinoma field 1 (%F1) = (^{% F1}/_{%F1 + %F2 + %F3 + %F4}) × 100; *** All invasive carcinoma fields may not be present (i.e., only fields of low and high Ki-67 staining may be present); however, in order to count a total of up to 400 cells, a total of four (4) invasive carcinoma fields must be considered. If a particular invasive carcinoma field is not present, then the count for that invasive carcinoma field would be allocated to another invasive carcinoma field that is present, per the IKWG Step 2 protocol: https://www.ki67inbreastcancerwg.org/wp-content/uploads/2018/12/Ki67-Phase-3b-WS-protocol-v1.pdf (accessed on 23 January 2023); **** In each field, count until either 100 invasive tumor nuclei have been counted, or all invasive tumor nuclei in the entire scoring field have been counted, whichever comes first. ***** The IKWG weighted Ki-67 score considers the effects of Ki-67 heterogeneity. IKWG unweighted Ki-67 = (^{total # of positive Ki-67 cells from all fields}/_{total # of invasive tumor cells from all fields}) × 100; IKWG weighted Ki-67 = ∑ _{(%F1, %F2, %F3, %F4)} %F (^{# of positive Ki-67 cells in %F}/_{# of invasive tumor cells in %F}) × 100.

Figure 2

(A): Ki67-QC International Working Group whole section scoring protocol (global method): Specification of tissue fields and Ki-67 staining percentage in invasive tumor fields. * Fields are either non-invasive tissue (Field 0) or invasive carcinoma tissue (Fields 1–4). ** The percentage of Ki-67 staining in each invasive carcinoma field (fields 1–4) represents relative determinations of Ki-67 staining as a percentage of all the tissue on the glass slide (F0, F1, F2, F3, F4). Percentages should be considered based on the IKWG considerations of a low (≤5%) and high (≥30%) Ki-67 expression. Very low might be considered as <1%. *** Non-invasive tissue includes non-tumor tissue and in situ carcinoma. (B): Ki67-QC International Working Group whole section scoring protocol (global method): Calculation of overall Ki-67 staining percentage. * The percentage of each field (F0–F4) represents the area of that field relative to the area of the entire slide. For example, the percentage of invasive carcinoma field 1 (%F1) = %F1 = (^{area of F1}/_{area F0 + area F1 + area F2 + area F3 + area F4 + area F5}) × 100; ** For example, the relative percentage of invasive tumor nuclei in invasive carcinoma field 1 (%F1) = (^{% F1}/_{%F1 + %F2 + %F3 + %F4}) × 100; *** All invasive carcinoma fields may not be present (i.e., only fields of low and high Ki-67 staining may be present); however, in order to count a total of up to 400 cells, a total of four (4) invasive carcinoma fields must be considered. If a particular invasive carcinoma field is not present, then the count for that invasive carcinoma field would be allocated to another invasive carcinoma field that is present, per the IKWG Step 2 protocol: https://www.ki67inbreastcancerwg.org/wp-content/uploads/2018/12/Ki67-Phase-3b-WS-protocol-v1.pdf (accessed on 23 January 2023); **** In each field, count until either 100 invasive tumor nuclei have been counted, or all invasive tumor nuclei in the entire scoring field have been counted, whichever comes first. ***** The IKWG weighted Ki-67 score considers the effects of Ki-67 heterogeneity. IKWG unweighted Ki-67 = (^{total # of positive Ki-67 cells from all fields}/_{total # of invasive tumor cells from all fields}) × 100; IKWG weighted Ki-67 = ∑ _{(%F1, %F2, %F3, %F4)} %F (^{# of positive Ki-67 cells in %F}/_{# of invasive tumor cells in %F}) × 100.

Figure 3

(A,B): Ki-67 immunohistochemical stain of breast cancer showing heterogeneous expression of Ki-67(10×). (A): Biopsy material; (B): Whole excision material.