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|Classification and external resources|
The most common tumor of the appendix is carcinoid, but not all appendiceal carcinoids are malignant. Malignant features include size over 1 to 2 cm, presence in lymph nodes, or invasion of tissues adjacent to the appendix.
Adenocarcinomas are more common primary malignancies of the appendix than carcinoids. Even so, adenocarcinoma originating in the appendix is rare, and usually resembles colonic adenocarcinoma microscopically. The signet ring cell carcinoma form carries a worse prognosis. Most mucinous tumors of the appendix are benign mucinous cystadenomas. The rare malignant form, mucinous cystadenocarcinoma is identified by invasion through the wall of the appendix or epithelial cells in mucus found in the peritoneum. Appendiceal tumors can present with peritoneal seeding, resulting in peritoneal carcinomatosis or pseudomyxoma peritonei (PMP).
Gastrointestinal stromal tumors are rare tumors with malignant potential. Primary lymphomas can occur in the appendix. Breast cancer, colon cancer, and tumors of the female genital tract may metastasize to the appendix.
A study of primary malignacies in the United States found a rate of 0.12 cases per 1,000,000 population per years. Carcinoids that were not identified as malignant were not included in this data. Carcinoid is found in roughly 1 in 300 appendectomies for acute appendicitis. A Hong Kong case series of 1492 appendectomies identified 17 neoplasms. Eight were carcinoids without specification of malignant features. Three cases of adenocarcinoma, and one each of cystadenocarcinoma, psedomyxoma peritoneii, and metastasic carcinoma were identified. The remaining tumors were benign.
The treatment for tumors varies. Small carcinoids (<2 cm) without features of malignancy may be treated by appendectomy if complete removal is possible. Other carcinoids and adenocarcimoas may require right hemicolectomy.
Pseudomyxoma peritonei treatment includes cytoreductive surgery which includes the removal of visible tumor and affected essential organs within the abdomen and pelvis. The peritoneal cavity is infused with heated chemotherapy known as HIPEC in an attempt to eradicate residual disease. The surgery may or may not be preceded or followed with intravenous chemotherapy or HIPEC.
Famous cases 
- Jaffe, Bernard M.; Berger, David H. (2010) . "30. The appendix". In Brunicardi, F. Charles; Andersen, Dana K.; Billiar, Timothy R.; Dunn, David L.; Hunter, John G.; Matthews, Jeffrey B.; Pollock, Raphael E. Schwartz's principles of surgery (9th ed.). New York: McGraw Hill Medical. pp. 1088–1089. ISBN 0-07-154769-X [Amazon-US | Amazon-UK].
- McCusker, M. E.; Cot�, T. R.; Clegg, L. X.; Sobin, L. H. (2002). "Primary malignant neoplasms of the appendix". Cancer 94 (12): 3307. doi:10.1002/cncr.10589. PMID 12115365. Text "12115365" ignored (help)
- Rosai, Juan (2004) . "11. Gastrointestinal tract". Rosai and Ackerman's surgical pathology (9th ed.). Mosby. pp. 761–769.
- Ma, KW; Chia, NH; Yeung, HW; Cheung, MT (2010). "If not appendicitis, then what else can it be? A retrospective review of 1492 appendectomies". Hong Kong medical 16 (1): 12–7. PMID 20124568.
- Selim, Jocelyn (Fall 2009), "The Fairest of All", CR (Philadelphia: American Association for Cancer Research) 4 (4), retrieved January 22, 2011