Answer: Mucinous cystadenoma

Histology: The lumen of the appendix is dilated and filled with mucinous material. The appendiceal mucosa has a somewhat villiform appearance and in some areas nuclei are small and basally located, similar to normal goblet cells. However, in other areas, there are clearly stratified, columnar shaped, enlarged nuclei typical of adenomatous epithelium.

Discussion: Mucosal hyperplasia morphologically resembles a hyperplastic polyp of the colon with a serrated epithelial lining and minimal to no nuclear atypia. A mucinous cystadenocarcinoma shows greater cytologic atypia and would demonstrate desmoplastic stromal invasion by malignant mucinous epithelial cells. An adenocarcinoid typically shows the growth pattern of a carcinoid tumor (submucosal, encircling the appendiceal lumen), but demonstrates cytologic features of both carcinoid tumor and goblet cells. Endocrine differentiation and submucosal spread were not present in the current case.

Rupture of mucinous cystic lesions of the appendix into the peritoneal cavity may lead to the syndrome of pseudomyxoma peritonei. Rupture of appendiceal mucinous cystadenomas leads to an indolent form which has been termed Disseminated Peritoneal Adenomucinosis (DPAM). This features bland cytology, low cellularity, abundant mucin and minimal parenchymal tissue invasion. Rupture of an appendiceal mucinous cystadenocarcinoma leads to the more aggressive Peritoneal Mucinous Carcinomatosis (PCMA). PCMA features cytologic atypia, higher cellularity, minimal mucin, and greater tissue invasion. This case did not show rupture, and therefore the patient should be cured of this lesion.