Dermal <scp>CIC</scp>‐Rearranged Sarcoma With Neuroendocrine Differentiation Mimicking Merkel Cell Carcinoma

Capicua transcriptional repressor (CIC)-rearranged sarcoma (CRS) is a rare and recently described tumor that most commonly affects patients between 15 and 30 years of age. It is an undifferentiated round cell malignancy, with a disease defining CIC fusion, with double homeobox 4 (DUX4) being the most common partner. Here, we report a 77-year-old woman who presented with a cutaneous thigh mass with a clinical morphology suggesting Merkel cell carcinoma. Immunohistochemically, there was positivity for INSM1 (extensive) and synaptophysin (patchy) and granular expression of neurofilament (extensive), CAM5.2, and CK20 (focal, nonspecific). The majority of the tumor showed histopathologic features within the range of what can be seen in Merkel cell carcinoma, but there were divergent features, including a myxoid zone with corded and stranded tumor cells and a Ewing-sarcoma-like zone with confluent concentric membranous CD99 expression. WT-1 was strongly expressed, prompting RNA-based next generation sequencing for gene fusions, which identified the CIC:DUX4 [t(19;4)(19q13.2;4q35.2)]. A novel IRAK3:HMGA2 fusion was also identified. This example of CRS simulated MCC clinically, histopathologically, and immunohistochemically and represents a likely underrecognized diagnostic pitfall.