Answer: Sinonasal ameloblastoma

Histology: The sinus mucosa is undermined by anastomosing sheets and ribbons of cells in a collagenized stroma. These anastomosing ribbons have a well defined structural pattern: the periphery is comprised of a zone of palisading columnar cells, and the central zone is characterized by loosely cohesive polygonal to fusiform cells. The palisading columnar cells have well developed subnuclear vacuoles such that the nuclei are displaced away from the basement membrane (reverse polarity). The cells lack significant pleomorphism, and the mitotic rate is very low.

Discussion: Ameloblastoma is an epithelial tumor of odontogenic origin that shows enamel organ differentiation. They are locally aggressive but rarely malignant such that complete surgical removal is generally curative. The nature of the surgical approach depends on the size and extent of the tumor.

Ameloblastoma is readily identified microscopically and separated from other basaloid tumors on the basis of its characteristic histologic appearance; namely the presence of peripheral palisading, subnuclear vacuoles with reversely polarized nuclei, and the central stellate reticulum. Diagnostic problems can arise when an ameloblastoma arises in some non-expected site outside of the craniofascial bones such as the sinonasal tract. While true sinonasal ameloblastomas are well recognized (believed to arise from totipotential cells in the sinonasal mucosa), most ameloblastomas encountered in the maxillary sinus represent central tumors that have eroded through cortical bone. The distinction is not easily made histologically, but requires correlation with the radiographic findings.