Cytokeratin and Neuroendocrine Positivity in Cutaneous Small Blue Round Cell Tumor—Is It Always Merkel Cell Carcinoma?

We present a case of a 39-year-old woman initially diagnosed with Merkel cell carcinoma (MCC), a highly aggressive neuroendocrine carcinoma, due to the presence of cytokeratin and neuroendocrine marker expression. The tumor was dermal based, showing small round blue cells with fine chromatin, scant cytoplasm, and scattered mitotic figures arranged in sheets, small cohesive nests, and cords within sclerotic to edematous stroma. Provided immunohistochemical stains showed strong pancytokeratin expression coupled with perinuclear dot-like staining for cytokeratin 20 in a distinct regional distribution, predominantly in areas where the tumor cells formed cohesive nests and cords within sclerotic stroma. Stains for neuroendocrine markers, including synaptophysin, INSM1, and CD56, were positive, albeit focal or regional in the more cohesive areas. Given the patient's age and unusual regional staining patterns, additional testing was performed, which revealed diffuse membranous CD99 staining and EWSR1::ERG fusion. These findings led us to revise the diagnosis to cutaneous Ewing sarcoma (ES). The distinction between MCC and cutaneous ES is crucial due to their different survival rates and treatment approaches. This case underscored the importance of considering alternative diagnoses when encountering cutaneous small round blue cell tumors in the presence of unusual histologic and immunohistochemical findings, particularly in younger patients.