S3679 Appendiceal Adenocarcinoma Masquerading as Gastric and Rectal Masses: Expanding the Metastatic Landscape

Introduction: Appendiceal adenocarcinoma is a rare malignancy of the gastrointestinal tract, often incidentally discovered in patients undergoing appendectomy for acute pancreatitis. Distant metastasis outside the peritoneal cavity, especially the stomach and rectum, is an uncommon presentation. This case presents a rare occurrence of distant metastases from an appendiceal adenocarcinoma to both stomach and rectum, highlighting the challenges of diagnosing and managing such presentations. Case Description/Methods: A 48-year-old woman was diagnosed in 2013 with a low-grade mucinous appendiceal mass and disseminated peritoneal adenomucinosis, for which she underwent multiple cytoreductive surgeries and chemotherapy. She was lost to follow-up in 2016 but returned in 2019. Somatic genetic testing revealed no targetable mutations, including negative HER2, PDL1, NTRK, and PTEN. In 2024, a computed tomography scan showed gastric erosion, prompting further evaluation. EGD revealed 4 large, ulcerated gastric masses. Biopsy showed dysplastic epithelium with extracellular mucin and an intense fibroinflammatory response, consistent with appendiceal origin. Colonoscopy for anemia revealed a large, friable rectal mass with similar histology. Immunostaining of gastric and rectal masses was positive for pancytokeratin, CDX2, and CK20, and negative for CK7 and PAX8, confirming metastatic appendiceal cancer. Her course was complicated by rectovaginal fistulation. Given disease progression, she opted for palliative care focused on comfort. Discussion: Appendiceal adenocarcinoma is a rare malignancy, with mucinous and non-mucinous subtypes. The mucinous subtype often leads to pseudomyxoma peritonei, with common metastasis to the peritoneum, omentum, and ovaries. Distant metastasis to the stomach and rectum is rare. This case highlights a unique presentation with biopsy proven metastases to both the stomach and rectum, stressing the importance of long-term surveillance and necessitating vigilant monitoring for distant metastases This case highlights the complexity and variability of this malignancy. Given the rarity of distant metastases to the stomach and rectum, this case highlights the need for comprehensive diagnostic evaluations in patients with appendiceal adenocarcinomaand the importance of maintaining clinical suspicion for atypical metastatic patterns. Further research is warranted to explore the mechanisms underlying atypical metastatic patterns and to develop targeted therapies for this challenging disease.