Ki-67 Across Human Cancers: A Narrative Review of Its Prognostic Significance and Challenges in Standardization

Background/Objectives: Ki-67 is a cell proliferation marker which is tightly coupled to the cell cycle. Its importance stems from the information it provides on prognosis, response to therapy, and clinical outcomes in several tumors. However, several gaps regarding its use should be addressed before it can be used as a standard-of-care test in cancer management. Methods: A structured literature search of articles was conducted using PubMed to evaluate Ki-67 for its prognostic value across human cancers. The primary areas of interest were immunohistochemistry (IHC) methods, the impact of Ki-67 index variability on clinical judgment, and the potential for standardizing evaluation provided by AI-powered tools and automated digital image analysis (DIA). Results: In different cancers, Ki-67 overexpression is related to worse clinicopathological characteristics, shorter overall survival, and higher tumor burden. Increased Ki-67 expression in breast cancer was associated with higher risk of recurrence in the ER-positive, HER2-negative subtypes. Similarly, in prostate cancer, higher Ki-67 scores correlated well with higher Gleason scores and early biochemical relapse. In lung cancer, Ki-67 levels were found to be higher in the later stages, in metastatic disease, and in patients with poor survival. However, the cut-off values varied widely (5–50%). Conclusions: The Ki-67 index is a promising biomarker, which, with some refinements, may prove to be an invaluable tool in the management of malignant tumors. Future studies should therefore be directed towards reducing variability by increasing standardization and establishing standardized, tumor-specific cut-off values.