Answer: Sinonasal adenocarcinoma, intestinal type

Histology: The surface epithelium is ulcerated and undermined by an invasive adenocarcinoma forming villous-type projections. The invasive glands are lined by columnar epithelium with elongated and stratified nuclei. The tumor cells are immunoreactive for CK20 and CDX2; and they are not immunoreactive for CK7.

Discussion: Adenocarcinomas of the sinonasal tract fall into 2 broad categories: salivary and non-salivary types. Accordingly, the first diagnostic branch point is to determine whether the carcinoma is of salivary gland origin or surface epithelial origin. Given the large and diverse nature of salivary gland neoplasms, this is not an easy task and one that requires a very solid appreciation of salivary gland neoplasia in all of its forms. Once one is confident that the tumor is in fact of epithelial origin, the non-salivary adenocarcinomas require further subclassification as either intestinal or non-intestinal. This subclassification is generally made on the basis of whether the tumor resembles intestinal neoplasia. The W.H.O. further divides the intestinal-type sinonasal adenocarcinomas into 5 subtypes: papillary, colonic, solid, mucinous and mixed. Resolution down the subtype is important as these subtypes diverge in terms of their clinical behavior.

Resemblance to intestinal neoplasia is best developed in the colonic-subtype of intestinal-type sinonasal adenocarcinomas. This tumor is characterized by a tubule-papillary proliferation that is indistinguishable from conventional moderately differentiated colorectal adenocarcinomas. This striking likeness should raise the consideration of a metastatic nasal implant. This differential cannot be reliably made morphologically or even immunohistochemically. Like colonic adenocarcinomas, intestinal-type sinonasal adenocarcinomas are consistently immunoreactive for CK20 and the intestinal marker CDX2. Correlation with the clinical findings is the only sure way to rule out a metastasis from the colorectum.