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A Comparison of the Histopathologic Growth Patterns Between Non–Merkel Cell Small Round Blue Cell Tumors and Merkel Cell Carcinoma

Background: Merkel cell carcinoma (MCC) is a rare neuroendocrine cutaneous malignancy that shares cytologic, histopathologic, and immunohistochemical features with other small round blue cell (SRBC) tumors. Although the trabecular pattern is anecdotally associated with MCC, objective data are lacking. Methods: This was a retrospective institutional review board–approved observational study conducted on microscopic images of 79 MCCs and 74 other SRBC tumors (desmoplastic small round cell tumor, primitive neuroectodermal tumor, neuroblastoma, embryonal rhabdomyosarcoma, alveolar rhabdomyosarcoma, synovial sarcoma, carcinoid, metastatic small cell lung cancer, non-Hodgkin small cell lymphoma, retinoblastoma, medulloblastoma, nephroblastoma, small cell osteosarcoma, and round cell liposarcoma). An expert dermatopathologist evaluated blinded and randomized microscopic specimens and recorded histologic patterns (diffuse, infiltrative, large anastomosing nests, small islands, any trabecular, focal trabecular, mixed trabecular, and predominately trabecular). Results: Trabecular features were identified in over 72% of MCCs but only rarely in non-MCC SRBC tumors. The presence of any amount of a trabecular pattern favored a diagnosis of MCC over SRBC tumors with a sensitivity of 72.2% and a specificity of 87.8%. If “any” and “focal” trabecular patterns were discounted, specificity rose to 93.2%. Conclusion: The presence of a trabecular pattern helps to differentiate MCC from other SRBC tumors, and specificity approaches that achieved with immunostaining.