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Abstract

Merkel cell carcinoma (MCC) is a rare, aggressive primary cutaneous neuroendocrine tumor with morphologic variants such as squamoid, adnexal, and sarcomatous differentiation described infrequently. Even more unusual are cases demonstrating heterologous rhabdomyoblastic differentiation. Here, we present three such rare cases, highlighting their unique histopathologic features.

The first case involved a 74-year-old male with a right calf lesion. Histologic sections revealed a dermal-based small round blue cell tumor with numerous mitoses and apoptotic bodies. The tumor exhibited dot-like paranuclear CK20 positivity and expressed CD56, synaptophysin, chromogranin, and myoD1, but was negative for desmin, CD99, TTF-1, CD3, CD20, and S100, with intact INI1.

The second case was a 79-year-old male with chronic lymphocytic leukemia and a right groin wound. Histologic findings demonstrated a high-grade neuroendocrine carcinoma with dot-like perinuclear CK20 positivity, diffuse chromogranin, MCPyV, myogenin expression, and rare desmin staining. The tumor cells were negative for S100, BRAF, NRAS, CK7, and TTF-1. Molecular testing excluded rhabdomyosarcoma due to the absence of FOXO1 fusion.

The third case was an 88-year-old female with a 10-month history of a left anterior leg lesion, initially misdiagnosed as stasis dermatitis. Histology showed a dermal polypoid tumor of biphasic basaloid cells with an immunohistochemical profile consistent with the prior cases.

The differential diagnosis for MCC with rhabdomyoblastic differentiation included transdifferentiated melanoma with rhabdomyoblastic differentiation and alveolar rhabdomyosarcoma. Accurate diagnosis requires detailed morphologic, immunohistochemical, and molecular evaluation. Further studies are warranted to determine whether this rare variant exhibits distinct clinical behavior or prognosis compared to conventional MCC.