Comparison of Three Ki-67 Index Quantification Methods and Clinical Significance in Pancreatic Neuroendocrine Tumors

The Ki-67 index is essential in the pathological reports for pancreatic neuroendocrine tumors. There are three methods to determine the Ki-67 index including eyeball estimation, manual counting, or automated digital imaging analysis. The goal of this study was to compare the three quantification methods with the clinical outcome to determine the best method for clinical practice. Ki-67 immunostaining was performed on 97 resected pancreatic neuroendocrine tumors. The three methods of quantification were employed: (1) an average of eyeball estimation by three pathologists; (2) manual counting of at least 500 tumor cells; and (3) digital imaging analysis quantitation by selecting 8–10 hot spot regions. All tumors were graded according to the 2010 WHO grading system. The three quantification methods for the Ki-67 index had almost perfect agreement. The concordance between manual counting and digital imaging analysis and between manual counting and average eyeball estimation were 0.97 and 0.88, respectively. The concordance among the three pathologists’ eyeball estimation was 0.86. All three methods correlated with patients’ survival using the 2010 WHO grading system. Eyeball estimation scores were significantly less than those of the other two methods and tended to downgrade more tumors to grade 1, but they had higher predictive ability for survival and recurrence. The WHO system using the mitotic rate could also separate patients with different survival and even downgraded more tumors to grade 1. The results suggest the necessity of a consensus among pathologists for the method to determine the Ki-67 index and proper cutoff of the Ki-67 index for better clinical correlation.