Primary malignant tumors were found in 1.4% of appendices in the same large series. Carcinoid and argentaffin tumors comprise the majority of appendiceal cancers, and the appendix is the commonest location of carcinoid tumors of the gastrointestinal tract. Carcinoid tumors of the appendix are usually benign, but the uncommon tumor that is over 2 cm in diameter may exhibit malignant behavior. Most appendiceal carcinoids are found in the tip of the organ, while a few are at the base. About half of these tumors are discovered during an appendectomy for acute appendicitis, and the remainder are identified incidentally. Lesions less than 2 cm in diameter invade the appendiceal wall in 25% of cases, but only 3% metastasize to lymph nodes, and hepatic metastases and the carcinoid syndrome are truly rare. Appendectomy alone is adequate treatment unless the lymph nodes are visibly involved, the tumor is more than 2 cm in diameter, mucinous elements are present in the tumor (adenocarcinoid), or the mesoappendix or base of the cecum is invaded. Right hemicolectomy is recommended for these more aggressive lesions.
Mucocele of the appendix is a cystic, dilated appendix filled with mucin. Simple mucocele is not a neoplasm and results from chronic obstruction of the proximal lumen, usually by fibrous tissue. If the appendiceal contents distally are sterile, mucous cells continue to secrete until distention of the lumen thins the wall and interferes with nutrition of the lining cells; histologically, simple mucocele is lined by flattened cuboidal epithelium or no epithelium at all. Simple mucocele is cured by appendectomy.
Less commonly, mucocele is caused by a neoplasm—cystadenoma, or adenocarcinoma grade 1 in the older terminology. This lesion may arise de novo or (perhaps) in a preceding simple mucocele. In cystadenoma, the lumen is filled with mucin but the wall is lined by columnar epithelium with papillary projections. Tumor does not infiltrate the appendiceal wall and does not metastasize, although it may recur locally after appendectomy. Cystadenoma is believed to undergo malignant change in some instances. Appendectomy is adequate treatment.
Benign tumors, including carcinoids, were found in 4.6% of 71,000 human appendix specimens examined microscopically. Benign neoplasms may arise from any cellular element and are usually incidental findings. Occasionally, a neoplasm obstructs the appendiceal lumen and produces acute appendicitis. No treatment other than appendectomy is indicated.