Primary small cell carcinoma of the maxillary sinus: a case report with immunohistochemical and molecular genetic study involving KIT and PDGFRA.

Primary small cell carcinoma of the nose and paranasal sinuses is very rare; only a few reports are present in the English literature. The author herein reports a very rare case of primary small cell carcinoma of the maxillary sinus with an emphasis on immunohistochemistry and on KIT and PDGFRA. A 64-year-old man was admitted to our hospital because of left nasal obstruction. Endoscopy revealed three nasal polyps, and imaging modalities revealed an infiltrative tumor (45 x 45 mm) in the left maxillary sinus with invasion into nasal cavity. Multiple biopsies are taken from the nasal lesions. Histologically, the tumor consists of proliferation of malignant small epithelioid cells with hyperchromatic nuclei, fine chromatin, scant cytoplasm, molded nuclei, and absent nucleoli. Immunohistochemically, the malignant cells were positive for cytokeratin (CK) 18, synaptophysin, CD56, p53, Ki-67 (labeling=95%), bcl-2, KIT, and PDGFRA. However, they were negative for pancytokeratins, high molecular weight CK, CK5/6, CK7, CK 14, CK 19, CK20, vimentin, neuron-specific enolase, chromogranin, CD15, CD45, S100 protein, CEA, CA19-9, glial fibrillary acidic protein, neurofilaments, neuroblastoma, CD99, surfactant apoprotein A, melanosome, and TTF-1. The pathologic diagnosis was small cell carcinoma. A molecular genetic analysis using PCR-direct sequencing was performed using paraffin sections, and it showed no mutations of KIT (exons 9, 11, 13, and 17) and PDGFRA (exons 12 and 18) genes. Imaging modalities including CT, MRI and PET did not reveal any tumors, including the lung, other than the maxillary sinus tumor. The present case is the first of small cell carcinoma of the maxillary sinus with a comprehensive immunohistochemical examination and a gene analysis of KIT and PDGFRA.