Dermal CIC-Rearranged Sarcoma With Neuroendocrine Differentiation Mimicking Merkel Cell Carcinoma.

Area of Science:

• Oncology
• Genetics
• Pathology

Background:

• Capicua transcriptional repressor (CIC)-rearranged sarcoma (CRS) is a rare, undifferentiated round cell malignancy typically affecting young adults.
• CIC-rearranged sarcoma is characterized by a defining CIC gene fusion, most commonly with double homeobox 4 (DUX4).

Observation:

• A 77-year-old woman presented with a cutaneous thigh mass clinically resembling Merkel cell carcinoma.
• Histopathology and immunohistochemistry showed features overlapping with Merkel cell carcinoma, but with divergent myxoid and Ewing-sarcoma-like areas.
• Immunohistochemical markers included INSM1, synaptophysin, neurofilament, CAM5.2, CK20, and WT-1.

Findings:

• Next-generation sequencing identified a CIC:DUX4 fusion, confirming the diagnosis of CIC-rearranged sarcoma.
• A novel IRAK3:HMGA2 fusion was also detected in the tumor.
• This case demonstrates that CRS can present in an older individual and mimic Merkel cell carcinoma.

Implications:

• CIC-rearranged sarcoma represents a diagnostic challenge due to its varied presentation and potential to mimic other malignancies.
• Accurate diagnosis requires integrated analysis of clinical, histopathological, immunohistochemical, and molecular findings.
• Recognition of CRS as a mimic of Merkel cell carcinoma is crucial for appropriate patient management and treatment.