KRT20-/SATB2+/MCPyV+ Sinonasal Merkel Cell Carcinoma: A Detailed Immunohistochemical and In Situ Hybridization Study.

Merkel cell carcinoma (MCC) is an uncommon aggressive neoplasm, usually arising in sun-exposed skin of the head and neck. By immunohistochemistry, KRT20 and MCPyV positivity are found in about 90% and 80% of MCCs, respectively. Noteworthy, viral status in lip/oral cavity MCCs is poorly known. A 78-year-old male was referred presenting a tumor mass on the right maxilla four months ago. Computed tomography revealed an extensive osteolytic lesion in the right maxillary sinus extending into the orbit and nasal cavity and crossing the midline. Microscopy revealed large necrotic areas admixed by sheets of small, round cells with hyperchromatic nuclei exhibiting a '"salt and pepper" chromatin pattern, which showed immunopositivity for pan-KRT AE1/AE3, EMA, CD56, CD99 (weak), CD138, S100 (focal), vimentin (focal), synaptophysin, FLI1, INSM1, MCPyV, SATB2, and Ki-67 (MKI67 [40%]). Moreover, intact RB1 and wild-type p53 expression were observed. Relevantly, KRT20, KRT5/6, chromogranin A, p40, p63, HMB45, NF, TTF1, TDT (DNTT), PAX5, p16, KIT, as well as high-risk HPV and EBER1/2 (both by ISH), were negative. This report illustrates that a detailed immunohistochemical study, including STAB2 and MCPyV markers, as well as a careful clinical workup, are essential to adequately classify high-grade neuroendocrine carcinomas, especially KRT20-negative MCC.