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.