Abstract

Oncogenic NTRK gene fusions are now being increasingly identified in various cutaneous and soft tissue tumors. Recognizing these gene fusions has significant therapeutic relevance and opens pathways towards new treatment options. A subset of cutaneous mesenchymal tumors harboring the LMNA-NTRK1 fusion has been described recently, and the diagnosis of this rare tumor is often challenging due to the highly variable morphology and nonspecific immunophenotype. A 6-year-old boy presented with a congenital, slowly growing scalp lesion. The excisional biopsy specimen demonstrated an infiltrative dermal and subcutaneous spindle cell proliferation with highly vascular myxoid stroma and prominent lymphocytic infiltrates. The tumor cells had oval to elongated nuclei with inconspicuous cytoplasm. An unusual feature of this lesion was the presence of scattered epithelioid and dendritic cells with prominent melanin pigment. No significant cytologic atypia or mitotic activity was seen. By immunohistochemistry, the tumor cells were diffusely positive for CD34 and pan-TRK. S-100 protein was also partially positive. Additional melanocytic markers, including Melan A, SOX-10, and HMB-45, selectively highlighted the pigmented epithelioid and dendritic cells. Finally, next-generation sequencing revealed a fusion transcript joining exon 13 of the LMNA gene and exon 12 of the NTRK1 gene. The lesion was excised entirely, and there has been no evidence of recurrent disease or metastasis to date. With the awareness that cutaneous LMNA-NTRK1 fusion mesenchymal tumors may show melanocytic differentiation, careful morphological evaluation and ancillary molecular studies allow for correct diagnosis.

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