Low-Grade Appendiceal Mucinous Neoplasm vs. Appendiceal Diverticulum: Distinction with Histomorphologic Features

Background: Low-grade appendiceal mucinous neoplasms (LAMNs) are rare lesions of the vermiform appendix and characterized by mucinous epithelial proliferation, extracellular mucin, and the absence of destructive invasion. Appendiceal diverticulum (AD) is also an uncommon condition that may be challenging to differentiate from acute appendicitis when it is superimposed by diverticulitis or perforation. Some recently published studies emphasized that complicated AD with mucosal hyperplasia can be confused with LAMNs, leading to overdiagnosis. The present study aimed to determine the histopathological features which can be used in the differential diagnosis of LAMNs and ADs, particularly complicated diverticula, in a large cohort. Methods: Cases comprising LAMNs and ADs diagnosed between 2011 and 2021 were included in the study. All cases were evaluated for the epithelial lining, the wall of the lesions, and the presence of cellular or acellular mucin, with its localization in terms of level and site of involvement within the appendix also recorded. Results: The hypermucinous epithelium characteristic of LAMNs, fibrosis, and calcification in the wall and the absence of lamina propria and muscularis mucosa proved to be the most discriminatory features in the differential diagnosis of LAMNs and ADs. Conclusions: The distinction between mucinous neoplasia and its mimics is critically important, since mucinous neoplasia requires surveillance imaging and potential surgery or chemotherapy depending on the extent of the disease, whereas non-neoplastic lesions are treated by an appendectomy and require no future intervention or surveillance.