Answer: Low grade appendiceal mucinous neoplasm

Histology: The appendiceal mucosa is hypermucinous, but demonstrates adenomatous features in the form of nuclear enlargement and pseudostratification. The epithelium produces extensive mucin which has ruptured through the appendiceal wall and into the peritoneal cavity. Within the extra-appendiceal mucin in the peritoneal cavity, there are strips of adenomatous epithelium similar to that found within the appendix. This is the morphology of a mucinous cystadenoma of the appendix which has ruptured and seeded the abdominal cavity, producing the clinical appearance of pseudomyxoma peritonei. The lesion has a variety of terms in literature including mucinous cystadenoma with rupture, low grade mucinous carcinoma, mucinous tumor of uncertain malignant potential, and low grade appendiceal mucinous neoplasm.

Discussion: A simple mucocele is uncommon in the appendix, and occurs in the setting of obstruction. These lesions are typically smaller than 2 cm, and the epithelium is atrophic and lacks dysplasia. High grade mucinous carcinomas demonstrate loss of nuclear polarity, nuclear contour irregularities, and frequent mitoses. There is also typically destructive invasion of the appendiceal wall and architectural complexity (complex papillae, cribiform spaces). Prostatic adenocarcinoma would lack the intracellular mucin seen in the current case and would not replace the lining of the appendix.

Reference(s):
– Molavi D, Argani P. Distinguishing benign dissecting mucin (stromal mucin pools) from invasive mucinous carcinoma. Adv Anat Pathol. 2008; 15:1-17.