Immunophenotypic heterogeneity of primary sinonasal melanoma with aberrant expression of neuroendocrine markers and calponin.

Primary sinonasal melanoma is an aggressive tumor that often presents a diagnostic challenge owing to its rarity and variable morphology. Unusual immunophenotypic expression of sinonasal melanoma compounds the problem particularly in a limited tissue sample. We studied immunohistochemical patterns of 5 primary sinonasal melanoma using antibodies against pan-cytokeratin, S-100, HMB-45, Melan-A, chromogranin, synaptophysin, neurofilament protein, and calponin and correlated these patterns with histologic appearance. Sinus/nasal mucosa from non-neoplastic cases were stained for Melan-A to evaluate prevalence of melanocytes in the benign mucosa in comparison to the staining pattern of the mucosa adjacent to the tumor. Similar to previous report, small cell pattern appeared to be over represented in the sinonasal melanoma. Small cell population in 4 cases was almost devoid of pigment, and also was either completely negative or only focally positive for S-100. Three cases stained positive for neuroendocrine markers and neurofilament protein bringing olfactory neuroblastoma and small cell neuroendocrine carcinoma in as differential diagnosis. Three cases were focally positive for calponin without spindle cell morphology or desmoplastic reaction. Immunophenotypic heterogeneity along with the unconventional histomorphology of sinonasal melanoma compounds diagnostic problem and warrants a precautionary approach to avoid misinterpretation. It is necessary to apply a broad panel of immunohistochemistry for the purpose of screening when sinonasal melanoma is considered as a differential diagnosis.