Current Status of Pathological Diagnosis of Colorectal Neuroendocrine Tumor

Abstract

In this article, the pathological diagnosis of neuroendocrine tumor (NET) and recent findings are summarized. In Japan, 90% or more of colorectal NET is found in the rectum, belonging to the hindgut system. Rectal NET has immunohistochemical characteristics different from those of midgut NET, such as a low positive rate of Chromogranin A. Regarding the classification of NET, the subclassification was changed in the 5th edition of the WHO classification of the digestive system published in 2019. The new subclassification applies both the histological findings, including cytological atypia and tumor differentiation, and the proliferation index. This makes it possible to clearly distinguish between NET, which has a high growth index, and neuroendocrine carcinoma (NEC), which has a high degree of malignancy and different genomic abnormalities and drug treatment responsiveness. This concept is closer to the Japanese way of thinking. In recent years, biomarkers other than the proliferative index have been reported and are expected to be incorporated. Appendiceal goblet cell carcinoid was renamed to appendiceal goblet cell adenocarcinoma in the 5th edition of the WHO Classification, making it clear that it was a different tumor from NET.