Diagnostic and prognostic implications of the World Health Organization classification of neuroendocrine tumors

Background: Neuroendocrine differentiation of tumors is often difficult to establish. In the same manner, the evaluation of the prognostic role of neuroendocrine differentiation may constitute a relevant clinical problem. Although different classifications are used for neuroendocrine tumors (NET) of different origin, the last World Health Organization (WHO) classification of NET, originally proposed for gastroenteropancreatic tumors, has proved to be a practical tool to allow pathologists to uniform the diagnoses and re-classify these tumors into 3 main categories. Aim: The present study was carried out in order to evaluate diagnostic and prognostic implications of NET reclassification according to the last WHO classification of NET. Materials and methods: Thirty-one tumors with an initial diagnosis referable to a NET achieved before 1999 were independently evaluated by 3 pathologists on the basis of the 2000 WHO classification of NET. Immunohistochemistry for paneuroendocrine markers and Ki-67 was also performed in all cases. Results: Twelve, 14, and 4 tumors were respectively reclassified as well-differentiated NET, well-differentiated neuroendocrine carcinoma and poorly differentiated neuroendocrine carcinoma; 1 tumor was reclassified as mixed endocrine-exocrine tumor. Two or more neuroendocrine markers were expressed in all NET regardless of histotype, differentiation degree, and site of primary tumor. After revision, 10 of the 31 tumors under study (32%) changed histo-prognostic category when compared to the initial diagnosis. Ki-67 score was the best predictor of survival at the multivariate analysis. Conclusion: The WHO classification is suitable to accurately reclassify tumors with an initial diagnosis referable to a NET and to separate these tumors in 3 well-distinct histo-prognostic categories with relevant clinical implications. Ki-67 score seems to be a better predictor of survival than the degree of differentiation.