High-Grade Appendiceal Mucinous Neoplasm Mimicking Appendiceal Tubulovillous Adenoma: A Case Report and Literature Review.

High-grade appendiceal mucinous neoplasm (HAMN) has been separated from appendiceal adenocarcinoma recently as an independent entity and categorized into appendiceal mucinous neoplasms. These neoplasms demonstrate distinct histological characteristics, including architectures and appendiceal mural changes similar to low-grade appendiceal mucinous neoplasm but with high-grade cytology, and no infiltrative invasion. Overt mucinous feature are not evident in some cases as the high-grade neoplastic epithelium may show intracytoplasmic mucin reduction. Occasionally, the neoplastic epithelial cells show florid proliferation and tubulovillous configuration and may be misdiagnosed as appendiceal tubulovillous adenoma. We report the case of a 67-year-old woman with appendicular dilatation and luminal mucin. She underwent an ileocecoectomy. The appendiceal lesion was found histologically to be a HAMN, which closely resembled appendiceal tubulovillous adenoma. The tumor cells demonstrated wild-type p53 expression and mismatch repair proficiency by immunochemistry. Molecular testing showed 1 KRAS mutation, 2 PIK3CA mutations, and 1 BRCA2, EP300, TGFBR2, CHD4, CREBBP, FANCC, PKHD1 mutation each in the tumor. The patient was followed up for 1 year with no evidence of disease.