Rare Rectal Involvement of Metastatic Appendiceal Adenocarcinoma Mimicking Primary Rectal Cancer

Primary appendiceal adenocarcinoma is a rare gastrointestinal malignancy that typically metastasizes to the peritoneum. Metastatic rectal involvement is exceptionally rare. We describe the case of a previously healthy 44-year-old man diagnosed with metastatic appendiceal adenocarcinoma. Sixteen months after cytoreductive surgery and hyperthermic intraperitoneal chemotherapy, he developed new-onset constipation and tenesmus. Subsequent colonoscopy revealed a large ulcerated rectal mass, consistent with possible metastatic poorly differentiated adenocarcinoma based on histology, immunoprofiling (CK20-positive, CDX2-positive, CK7-negative), and clinical history. To our knowledge, this represents the second possible documented case of rectal metastasis from appendiceal cancer in the literature, and the first to propose a potential mechanism for its spread.