High-grade appendiceal mucinous neoplasm presenting as renal colic; a case report and review of literature.

High-grade appendiceal mucinous neoplasms (HAMN) are rare mucinous appendiceal tumors that are mostly incidentally discovered and histologically show marked cytoplasmic atypia. We report a 62-year-old female patient who was diagnosed with acute right flank pain mimicking renal colic. Abdominal and pelvis CT scans showed no calculi or hydronephrosis. Incidentally, a markedly distended retrocecal appendix was identified and an appendectomy was performed. Histopathology revealed an HAMN of size 5.8 cm. HAMN are one of the rare and somewhat recently described epithelial appendiceal tumors. The latest 2019 classification is based on histologic appearance and on the consensus for classification and pathologic reporting of Pseudomyxoma peritonei and associated appendiceal neoplasia (Peritoneal Surface Pathology Group International, modified Delphi process). Histologically, appendiceal mucinous lesions are classified as nonneoplastic/mucocele and neoplastic lesions with separate subcategorization. Despite its rarity, it is crucial for clinicians to be aware of HAMN to distinguish it from other clinical differentials.