Merkel Cell Carcinoma of the Maxillary Sinus: An Unusual Presentation of a Common Tumor.

Merkel cell carcinoma is most commonly seen in the skin of sun exposed areas, particularly the head and neck and is associated with Merkel cell polyomavirus. Merkel cell carcinoma at an extracutaneous mucosal site of the head and neck is rare. We report a case of a 74-year-old women who presented with an enlarging thyroid mass found to be neuroendocrine carcinoma consistent with Merkel cell carcinoma (positive for synaptophysin, chromogranin, CK20). Subsequent work up revealed a maxillary sinus mass with extension into the nasal cavity. Biopsy was diagnostic for Merkel cell carcinoma (positive for synaptophysin, chromogranin, CK20 and Merkel cell polyomavirus). There are only case reports and small case series of Merkel cell carcinoma arising in the mucosal sites of the head and neck most commonly in the oral cavity, rarely the sinonasal mucosa. Merkel cell carcinoma metastasizing to the thyroid has only been reported in three other case reports, all from skin primaries. In addition to our case, we review the literature of extracutaneous sinonasal Merkel cell carcinoma and metastases to the thyroid.