A Case of Sporadic Desmoid Fibromatosis of the Appendix: A Rare Site of Presentation

Background: Desmoid fibromatosis is a locally aggressive benign fibroblastic soft tissue tumor representing nearly 0.03% of all neoplasms. They can be sporadic or may be associated with Familial Adenomatous Polyposis (FAP). Around 50% of fibromatosis are intrabdominal or arise in the abdominal wall. Rarely, these tumors can arise from the intestinal wall. Appendix and mesoappendix are extremely rare sites of presentation with only 3 reported cases. Case Presentation: A healthy 43-year-old male presented with abdomen distension. CECT scan showed a well-defined enhancing mass in the lower abdomen attached to the appendix with no surrounding infiltration, fat stranding, or evidence of distant metastasis. A wide resection was performed, and gross examination showed a well-circumscribed mass measuring 8 cm attached to the appendix. Histological examination revealed a well-circumscribed cellular spindle cell neoplasm with focal infiltrative borders. The tumor cells showed nuclear immunoreactivity for Beta-catenin and focally for Desmin, while they were negative for DOG1, CD117, CD34, STAT6, S100 & Pan Cytokeratin. The morphology and immunohistochemistry were compatible with Desmoid fibromatosis. CT scans were negative for recurrence or distant metastases after 8 months of follow-up. Conclusion: Appendix and mesoappendix desmoid fibromatosis are extremely rare and can present as an abdominal mass or features of acute appendicitis. Gastrointestinal stromal tumor (GIST) is an important differential diagnosis at this site and is crucial to differentiate from fibromatosis for patient management and follow-up. Identifying beta-Catenin (CTNNB1) mutation is a diagnostic criterion to differentiate from other spindle cell tumors, especially on small biopsies. Multi-disciplinary treatment approach is crucial for management.