Background: The aim of this study was to compare mitotic count (MC) and Ki-67 proliferation index as prognostic markers in pancreatic and midgut neuroendocrine neoplasms (NENs).
Methods: Two hundred eighty-five patients with metastatic NENs were recruited. Concordance between histological grade according to either Ki-67 or MC as defined by the European Neuroendocrine Tumour Society guidelines was assessed and the prognostic significance of Ki-67 or MC were evaluated.
Results: There was a discrepancy of 44 and 38% in grade assignment when using Ki-67 or MC in pancreatic and midgut NENs, respectively. In multivariate analysis, grade using Ki-67, but not MC, was a significant prognostic factor in determining overall survival (hazard ratios: midgut G2 2.34, G3 15.1, pancreas G2 2.08, G3 11.3). The prognostic value of Ki-67 was improved using a modified classification (hazard ratios: midgut G2 3.02, for G3 22.1, pancreas G2 5.97, G3 33.8).
Conclusion: There is a lack of concordance between Ki-67 and MC in assigning tumour grade. Grade according to Ki-67 was a better prognostic marker than MC for metastatic pancreatic and midgut NENs. We suggest that Ki-67 alone should be used for grading pancreatic and midgut NENs and that the current threshold for classifying G1/G2 tumours should be revised from 2 to 5%.