Answer: Villous adenoma

Histology: The dilated appendix is lined by adenomatous epithelium arranged both in a flat and undulating lining as well as a more overt villiform appearance. In areas, the epithelium is attenuated with abundant mucinous secretions. Mucin extravasation is seen under the adjacent colonic mucosa of the cecum. The mucin contains lymphocytes and histiocytes and elicits a surrounding giant cell reaction. No epithelial cells are noted within the mucin.

Discussion: A spectrum of lesions may be seen within the colon and appendix associated with mucin extravasation. At the most benign, one may have a dilated appendix lined by normal epithelium dilated with mucin as the result of an obstruction. Associated with the mucocele, there may be locally extravasation of mucin. Removal of the mucocele is curative. In the current case, there is a villous adenoma of the appendix with abundant mucin production. This mucin has also locally extravasated in the region of the appendix. In contrast to adenocarcinoma, the mucin shows no epithelial cells. In order to arrive at such a diagnosis, the lesion should be extensively sampled as areas of carcinoma may be present focally within the lesion. Mucinous cystadenomas, if no epithelium is present within the extravasated mucin, are cured following excision. Along the continuum of malignancy, one then can have situations with glandular epithelium floating within extravasated mucin. In some cases there is overt cytologic malignancy within the epithelium contained within the mucin at which point a diagnosis of mucinous adenocarcinoma with abundant extracellular mucin is rendered. These patients typically have a very poor prognosis and clinically patients typically have an abdomen full of mucin known as pseudomyxoma peritonei. However, some individuals may also have a mucinous cystadenoma of the appendix which ruptures where the extravasated mucin contains histologically bland to low grade adenomatous mucinous epithelium, which clinically could also result in an abdomen full of mucin (ie. pseudomyxoma peritonei). These lesions are designated as disseminated peritoneal adenomucinosis (DPAM). Whereas patients with DPAM have 10-year survival rates of close to 70%, patients with disseminated mucinous carcinoma have less than 5% long-term survival. Consequently, extensive sampling of both the primary mucinous lesion as well as extravasated mucin is necessary to both accurately characterize the primary lesion as well as determine the overall prognosis.

Reference(s):
– Cancer 92:85-91, 2001.