Nonneoplastic Mimics of Appendiceal Mucinous Neoplasms

With the widespread acceptance of the term low-grade appendiceal mucinous neoplasm (LAMN) for relatively bland mucinous tumors of the appendix with pushing invasion and a propensity to disseminate to the peritoneal cavity, there has been increasing concern among pathologists of overlooking one of these tumors when an appendix shows extrusion of mucin into the wall or onto the serosa. The increased awareness of LAMN and its histologic features has caused some benign entities to be misinterpreted as LAMN based largely on the presence of mucin extrusion and hyperplastic and reactive appendiceal mucosa. The misinterpretation of some of these mimics as LAMN can have profound clinical consequences for patients, in terms of both overtreatment and anxiety. In this review, the most common nonneoplastic mimics of LAMN are reviewed. The most common of these mimics is ruptured diverticular disease of the appendix, which is frequently mistaken for ruptured LAMN. Retention mucoceles of the appendix can also cause confusion with LAMNs. Finally, a rare but curious form of endometriosis with intestinal metaplasia that occurs largely in the cecum and appendix can histologically mimic a mucinous neoplasm of the appendix.