Answer: Carcinoid tumor and mucinous cystadenoma of the appendix

Histology: The tumor is composed of two components. The first is a nested proliferation of bland neuroendocrine type cells with the typical features of carcinoid tumor. These tumor cells feature uniform nuclei with open “salt and pepper” chromatin, and are present in nests associated with retraction artifact. These nests of tumor are centered on the appendiceal submucosa and extend through the muscularis propria and into the subserosa. The second component of the tumor is histologically distinct. This component consists of papillary mucinous epithelium, which colonizes the appendiceal lumen. Its cigar-shaped nuclei demonstrate stratification and hence this lesion can be considered adenomatous. Because of its cystic and diffuse nature, the lesion is considered a mucinous cystadenoma.

Discussion: Carcinoid tumor would lack the neoplastic mucinous component present in this case. An adenocarcinoma of the appendix should not be associated with a classic carcinoid tumor, and would be characterized by infiltrative, cytologically malignant glands involving the appendiceal wall. An adenocarcinoid combines features of a carcinoid tumor (neuroendocrine cells) and an adenocarcinoma (goblet cells); however, these tumors tend to have a uniform morphology throughout and do not have two distinctive morphologies, as the present case does. Occasional cells within adenocarcinomas demonstrate both mucinous and endocrine features simultaneously.
Adenocarcinoids are typically centered on the submucosa.

Rupture of appendiceal mucinous cystadenoma is one of the causes of pseudomyxoma peritonei (PMP). As opposed to PMP resulting from rupture of a mucinous adenocarcinoma, rupture of an appendiceal adenoma produces PMP characterized by bland cytology, low cellularity, minimal parenchymal tissue invasion and more indolent clinical course. This syndrome has been labeled by Ronnett et al. (AJSP 1995; 19:1390-1408) as disseminated peritoneal adenomucinosis (DPAM).