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Abstract

Merkel cell carcinosarcoma is exceptionally rare. We report two additional cases to highlight this rare neoplasm. Case 1: A 74-year-old female presented with a thigh lesion. Histologically, the tumor was composed of sheets, nests, and cords of atypical neuroendocrine cells with enlarged nuclei with stippled chromatin.  Focal squamous differentiation was present.  There was an abrupt transition from the neuroendocrine cells to fascicles of atypical spindled cells with elongated nuclei and eosinophilic cytoplasm. Immunohistochemistry revealed the neuroendocrine cells were positive for AE1/AE3 (dot-like), INSM1, synaptophysin, and chromogranin. They were negative for CK20, TTF1, MCPyV, CK5/6, and p40.  The squamous component expressed AE1/AE3, CK5/6, and focally INSM1 and synaptophysin.  The spindle cell component was weakly positive for AE1/AE3 but negative for all other epithelial and neuroendocrine markers.  All components showed p53 positivity and loss of Rb1 expression, indicating that they were related. Case 2: An 81-year-old male presented with a XXX lesion. The tumor demonstrated similar morphology with sheets, nests, and cords of neuroendocrine cells, focal squamous differentiation, and an abrupt transition to fascicles of atypical spindled cells.  The neuroendocrine cells were positive for AE1/AE3 (dot-like), INSM1, and synaptophysin but negative for CK20, MCPyV, and TTF1.  The squamous component was positive for AE1/AE3 and p63 but negative for neuroendocrine markers. The spindle cell component was negative for all epithelial and neuroendocrine markers.  These cases highlight the hallmark biphasic morphology and immunophenotypic heterogeneity of Merkel cell carcinosarcoma, emphasizing the importance of comprehensive histopathologic and IHC evaluation in diagnosing this rare tumor.