Merkel cell carcinosarcoma: Merkel cell carcinoma with embryonal rhabdomyosarcoma-like component

Area of Science:

• Dermatopathology
• Surgical Pathology
• Oncology

Background:

• Merkel cell carcinoma (MCC) is a rare neuroendocrine skin cancer.
• Divergent differentiation in MCC is uncommon, with rhabdomyosarcomatous change rarely documented.

Observation:

• A case of cutaneous MCC with biphasic morphology is presented.
• The tumor exhibited alternating areas of neuroendocrine small round cells and embryonal rhabdomyosarcoma-like spindle cells.
• This biphasic pattern led to the proposed designation 'Merkel cell carcinosarcoma'.

Findings:

• Immunohistochemistry confirmed distinct cellular components.
• Neuroendocrine areas stained positive for pan-cytokeratin, cytokeratin 20, synaptophysin, and chromogranin.
• Rhabdomyosarcomatous areas showed positive staining for desmin and myogenin.

Implications:

• This case expands the understanding of MCC's divergent differentiation potential.
• The biphasic morphology was observed in both the primary tumor and its recurrence/metastasis.
• Recognizing this rare carcinosarcoma subtype is crucial for accurate diagnosis and treatment planning.