Expanding the Spectrum of Cutaneous ALK Fusion Mesenchymal Tumors: A Case with Unique Histological Features.

Genomic rearrangements resulting in ligand independent constitutive activation of anaplastic lymphoma kinase (ALK) gene have been shown to be a genomic driver in neoplasms of variable lineage, including mesenchymal tumors. Inflammatory myofibroblastic tumor and epithelioid fibrous histiocytoma are typical well-described ALK-driven neoplasms in the skin and subcutis. However, recent literature has indicated additional ALK-rearranged mesenchymal tumors distinct from inflammatory myofibroblastic tumor and epithelioid fibrous histiocytoma, showing variable morphology, clinical presentation, and malignant potential. These tumors typically coexpress CD34 and S100, lack SOX10 expression, and are challenging to diagnose histologically owing to their morphological features being on a spectrum. This report describes a mesenchymal tumor with EML4-ALK fusion, displaying a unique histological pattern of compacted nested growth of uniform polygonal epithelioid cells with minimal stroma, nondescript vessels, low mitotic activity, and without necrosis. The features of this case do not exactly fit within the spectrum of cases described in the literature and expands the immunomorphological range of ALK-rearranged mesenchymal neoplasms.