MKI67 and neuroendocrine carcinoma: Based on their morphology, NENs are classified into two main subgroups: the generally more indolent, well‐differentiated neuroendocrine tumours (NETs) that account for ~ 80% of NENs and are subclassified by their proliferative index in grade 1 (Ki‐67 < 3%), 2 (Ki‐67 3–20%) or 3 (Ki‐67 > 20%), and the more aggressive, poorly differentiated neuroendocrine carcinomas (NECs) that always present high proliferative rates (grade 3 or Ki‐67 > 20%) and are associated with a very poor prognosis [3, 4, 5].