Primary Cutaneous Ewing Sarcoma With Unusual Features: Potential Mimic of Merkel Cell Carcinoma

Area of Science:

• Dermatopathology
• Oncology
• Molecular Pathology

Background:

• Primary Cutaneous Ewing Sarcoma (PCES) is a rare malignancy.
• PCES can mimic other small round blue cell tumors, notably Merkel cell carcinoma (MCC).
• Aberrant neuroendocrine marker expression in PCES poses a diagnostic challenge.

Purpose of the Study:

• To report a case of PCES that mimicked MCC.
• To emphasize the diagnostic difficulties in differentiating PCES from MCC.
• To highlight the importance of a multimodal diagnostic approach.

Main Methods:

• Histopathological examination of a skin nodule.
• Extensive immunohistochemical analysis (CD99, NKX2.2, S100, SOX10, CK20, Neurofilament).
• Fluorescence in situ hybridization (FISH) for EWSR1::FLI1 gene fusion detection.

Main Results:

• The tumor showed features of a small round cell tumor.
• Immunohistochemistry revealed a mixed immunophenotype, including dot-like CK20 and Neurofilament positivity, mimicking MCC.
• FISH confirmed the pathognomonic EWSR1::FLI1 gene fusion for PCES.

Conclusions:

• PCES can present with an immunophenotype that closely resembles MCC.
• Dot-like CK20 and Neurofilament positivity are not exclusive to MCC.
• Accurate diagnosis of cutaneous round cell tumors requires a comprehensive approach, including molecular techniques.