Ki-67—Playing a key role in breast cancer but difficult to apply precisely in the real world

The Ki-67 index, which is a proliferative index, has become more important in making treatment decisions for patients with breast cancer (BC) and plays both a predictive role and a prognostic role. However, a few factors limit its use in clinical practice, particularly the assessment of the percentage of Ki-67-positive cells and the cutoff value of Ki-67. In this study, we examined the expression of Ki-67 via immunohistochemistry and systematically evaluated the value of the Ki-67 index in patients with BC. This was a retrospective study including 280 patients diagnosed with BC. There were marked differences in overall survival (OS) between patients with BC when the Ki-67 index ranged from 46 to 68% (χ2 = 5.87, P = 0.0154; χ2 = 7.64, P = 0.0057, respectively), and the same results were also found when the staining density was added to the Ki-67 index; however, the staining density alone has limited value in assessing the value of Ki-67. There were marked differences in disease-free survival (DFS) among BC patients when the Ki-67 index ranged from 50 to 58% (χ2 = 7.31, P = 0.0069; χ2 = 7.88, P = 0.005). When 14% was used as a cutoff point to classify the molecular type of BC, the luminal A-type patients were significantly different from patients with HER2-overexpressing subtype BC in terms of OS (χ2 = 5.33, P = 0.021). There was a significant difference in the OS of patients with human epidermal growth factor receptor 2 (HER-2)-overexpressing subtype BC when the Ki-67 index fell within the range of 49-60% (χ2 = 4.86, P = 0.0275; χ2 = 5.50, P = 0.019, respectively). There were significant differences between luminal A-type BC and HER2-overexpressing subtype BC in terms of OS (χ2 = 5.53, P = 0.019), according to suggestions of the 2019 CSCO consensus. There were significant differences between the two groups of luminal B HER-2(-) BC when the Ki-67 index was 52% (χ2 = 6.61, P = 0.0101). The differentiated Ki-67 index can be used to assess the OS and DFS of patients with BC, and the staining density of Ki-67 has little value in assessing prognosis in these patients. Different molecular classification methods may influence the assessment of prognosis and the results of molecular subtype in patients with BC. To predict the prognosis of BC patients, it is more scientifically feasible to use the interval values of Ki-67 than a specific value.