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Cellular Neurothekeoma with Possible Neuroendocrine Differentiation

We report a case of cellular neurothekeoma showing unusual immunohistochemical findings and occurring on the left upper arm of a healthy 48‐year‐old woman. She presented with a 1 cm, red, asymptomatic, dermal nodule of 1 year duration. A biopsy showed dermal proliferation of plexiform fascicles of spindle‐shaped or polygonal cells with plentiful eosinophilic cytoplasms. The immunohistochemical profile included negative stains for S‐100, CD34, factor XIIa, CD68, HMB45, cytokeratins, and EMA, strongly positive stains for neuron specific enolase (NSE), synaptophysin, and chromogranin A, and focally positive ones for NKI/C3 and α‐smooth muscle actin. Ultrastructural analysis showed undifferentiated mesenchymal cells with cytoplasmic projections and abundant RER. Although we couldn't find any confirmative cell type in this cellular tumor, we believe that cellular neurothekeoma is predominantly composed of undifferentiated cells that can exhibit features of neuroendocrine cells in addition to fibroblastic or myofibroblastic ones, suggesting a divergent cell origin.