Appendiceal adenocarcinoma (mucinous and non-mucinous, including signet ring cell)

Appendiceal adenocarcinoma invades the wall of the appendix as a carcinoma and is subdivided into mucinous adenocarcinoma (more than half the tumour is extracellular mucin), non-mucinous or colonic-type adenocarcinoma, and signet ring cell carcinoma when half or more of the cells are signet ring cells. Most are found after appendicectomy for suspected appendicitis or at operation for peritoneal disease, and staging follows the colorectal TNM system. The genetics differ from colorectal cancer: KRAS and GNAS mutations are common, APC mutations rare, microsatellite instability uncommon, and TP53 and SMAD4 mutations mark high-grade tumours; a 2025 analysis of appendiceal cancers also found that many were diagnosed in people under 50, mirroring early-onset colorectal cancer.

Because the disease is rare, treatment is extrapolated from colon cancer. Right hemicolectomy with lymphadenectomy is recommended for all adenocarcinomas, and adjuvant FOLFOX or CAPOX for node-positive or high-risk disease, without trial evidence specific to the appendix. Peritoneal metastases, the dominant pattern of spread, are treated with cytoreductive surgery and HIPEC in patients with a limited peritoneal cancer index and non-signet-ring histology, where series report median survival of several years, and with perioperative systemic chemotherapy; signet ring cell carcinoma with a high peritoneal cancer index does poorly even after cytoreduction. Unresectable or distant metastatic disease receives FOLFOX or CAPOX with or without bevacizumab, then FOLFIRI, and the rare mismatch-repair-deficient tumour is a candidate for pembrolizumab. Retrospective data suggest that non-mucinous tumours respond to chemotherapy like colorectal cancer while mucinous tumours respond less, and prospective trials in appendiceal cancer specifically are only now beginning.