Ki67: A Time-Varying Biomarker of Risk of Breast Cancer in Atypical Hyperplasia.

Background: Uncontrolled proliferation is a defining feature of the malignant phenotype. Ki67 is a marker for proliferating cells and is overexpressed in many breast cancers. Atypical hyperplasia is a premalignant lesion of the breast (relative risk ∼ 4.0). Here we asked if Ki67 expression could stratify risk in women with atypia.Methods: Ki67 expression was assessed immunohistochemically by digital image analysis in archival sections from 192 women with atypia diagnosed at the Mayo Clinic 1/1/67-12/31/91. Risk factor and follow-up data were obtained via study questionnaire and medical records. Observed breast cancer events were compared to population expected rates (Iowa SEER) using standarized incidence ratios (SIRs). We examined both short-term (within 10 years) and long-term (after 10 years) risk of breast cancer (BC) following atypia biopsy.Results: The median value for percent positive cells for Ki67 was 1.0%; the 75 th percentile value was 2.3%. Based on the empirical distribution of staining values in our cohort, we selected a cutoff of 2% cells positive to separate high from low staining. There were no differences in Ki67 levels by age at biopsy, type of atypia (ADH, ALH, or both), number of foci of atypia, or family history. 32 women developed BC over a median of 14.6 years. Among them, those with ≥2% Ki67 expression had a shorter time to breast cancer (median 5.5 years, IQR=3.2-7.2) than those with Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 909.