Appendiceal Mucinous Adenocarcinoma Presenting as Flank and Thigh Abscess: An Unusual Extra-abdominal Manifestation in a Rural Setting.

Appendiceal malignancies are rare clinical entities, most commonly diagnosed incidentally following appendicectomy. When symptomatic, these tumors frequently mimic acute appendicitis, which may obscure the underlying diagnosis. Presentation with extra-abdominal symptoms, including skin and soft tissue infection, is exceptionally rare. We report a case of a 63-year-old male presenting to a rural hospital with sepsis and a right flank abscess. CT demonstrated a large retroperitoneal collection extending from the right paracolic gutter into the flank and groin, with an associated appendicolith. In the absence of interventional radiology, surgical incision and drainage were undertaken. Despite multiple surgical drainages, recurrence of collections necessitated laparoscopic right hemicolectomy. Initial histopathology revealed perforated appendicitis with a tubulovillous adenoma demonstrating high-grade dysplasia, without definitive invasive carcinoma. Despite apparent source control, the patient re-presented two months later with a larger collection, now extending into the right thigh. Repeat debridement and biopsy of the flank and right thigh abscess tissue confirmed a diagnosis of mucinous adenocarcinoma of appendiceal origin. Immunohistochemistry demonstrated intact mismatch repair protein expression, and molecular testing detected a BRAF mutation. The patient elected for palliative management. This case highlights a rare presentation of appendiceal mucinous adenocarcinoma mimicking perforated appendicitis with flank and thigh abscess formation. Retroperitoneal tracking along fascial planes may allow for extra-abdominal disease manifestations. Appendiceal mucinous adenocarcinoma has been associated with high rates of perforation, and underlying malignancy should be considered in adult patients presenting with perforated appendicitis, especially in the context of persistent or recurrent abscess formation.