Carcinoma with Triphasic Differentiation Arising from Inverted Papilloma in Sinonasal Sinus: A Rare Case with Molecular Characterization.

Small cell neuroendocrine carcinoma (SNEC) is a rare subset of tumors in the sinonasal sinus. Combined tumors are exceedingly rare. Here, we describe a 65-year-old male with a mixed tumor of SNEC and sarcomatoid carcinoma arising in an inverted papilloma, containing squamous cell carcinoma in situ (SqCCis) in the sinonasal sinus. We evaluated the molecular characteristics of the two separate carcinoma components using next-generation sequencing. The patient presented with a nasal obstruction. Computed tomography showed a mass infiltrating the right ethmoid and maxillary sinuses. An excisional biopsy was performed. The tumor was found to have three morphologically distinct components. The first was SqCCis arising in an inverted papilloma, which was positive for cytokeratin and P40. The second consisted of nests of densely packed small round cells representing SNEC-positive neuroendocrine markers. The third was a solid sheet of anaplastic spindle cell proliferation, which was negative for the above markers. Oncogenic mutations such as FBXW7, TP53, and EGFR were detected in both SNEC and sarcomatoid carcinoma, and MYCL amplification was observed only in the SNEC component. This case highlights an extremely rare presentation of combined SNEC and sarcomatoid carcinoma arising from an inverted papilloma in the sinonasal sinus.