Cutaneous melanocytic tumor with CRTC1::TRIM11 fusion

Summary: Cutaneous melanocytic tumor with CRTC1::TRIM11 fusion (CMTCT) is a newly identified dermal-based tumor that shows epithelioid and spindle cell morphology with melanocytic differentiation and harbors an in-frame translocation, CRTC1::TRIM11. Given the few reported cases describing its biological behavior, it is crucial to distinguish this entity from histologic mimics, including clear cell sarcoma (CCS), cellular/amelanotic blue nevus, and metastatic or primary dermal melanoma. Herein, we report a 39-year-old woman with CMTCT on the left leg histopathologically mimicking dermal melanoma. The patient developed a tender nodule on the left lateral malleolus one year prior to presentation, which enlarged gradually. A punch biopsy from the lesion demonstrated a dense spindle cell tumor in the dermis. There were fascicles of achromic spindle cells, some of which showed mildly enlarged nuclei. A mitotic rate of 4/mm2 was noted. The lesional cells are diffusely positive with SOX10 and MITF, with rare S100 and HMB45 staining. Melan-A, Pan Cytokeratin, p63, and SMA were negative. With detection of CRTC1::TRIM11 by next generation sequencing and lack of CCS-associated cytogenetic translocations, a diagnosis of CMTCT was established. She was treated with Mohs micrographic surgery. No metastasis or local recurrence has been found in the 16 months since treatment.