An unexpected case of goblet cell adenocarcinoma of the appendix

Goblet cell adenocarcinoma (GCA) of the appendix is a unique entity newly introduced in the 2019 WHO Classification of Digestive Tumours. It is a rare and aggressive type of primary appendiceal adenocarcinoma showing amphicrine differentiation with goblet-like mucinous cells and variable numbers of endocrine cells and Paneth-like cells. A 49-year-old man was found to have an incidental mass after presenting with acute appendicitis and undergoing a routine appendicectomy. Histology showed small tubules and clusters of atypical cells circumferentially infiltrating into the wall of the appendix without a stromal reaction. There was a characteristic mix of mucinous, endocrine and Paneth-like cells with mild nuclear atypia and infrequent mitoses. Immunohistochemistry showed positivity for cytokeratin AE1/AE3, and neuroendocrine markers, including synaptophysin and CD56. Recommended management is generally a right hemicolectomy, although due to its rarity, there are no well-established consensus guidelines for management of GCA. In summary, GCA is a rare and aggressive primary appendiceal neoplasm exhibiting both mucinous and neuroendocrine differentiation with a characteristic histological appearance for diagnosis, albeit often a surprise finding. This case aims to highlight this unique entity and enrich the current knowledge base regarding GCAs.