ALK-rearranged epithelioid mesenchymal neoplasm. Expanding the spectrum of tyrosine kinase-altered mesenchymal tumors

The anaplastic lymphoma kinase (ALK) gene encodes a receptor tyrosine kinase, and fusions involving this gene have been reported in a variety of mesenchymal neoplasms. ALK-altered tumors with epithelioid morphology have been described in epithelioid inflammatory myofibroblastic sarcoma and epithelioid fibrous histiocytoma. Herein, we describe the clinicopathologic features of 7 ALK-rearranged mesenchymal tumors with epithelioid morphology occurring predominately in the pediatric population. Tumors occurred in 4 females and 3 males with an age ranging from 1 month to 28 years. Five tumors were superficial and solitary, while 1 presented with multiple peritoneal/ omental nodules, and 1 presented as a large mediastinal mass. Morphologically, all tumors comprised epithelioid cells arranged in sheets, anastomosing cords, or small clusters embedded in a myxohyaline stroma. The cells had slightly variably sized ovoid nuclei with moderately prominent nucleoli and abundant eosinophilic cytoplasm. Four cases had sparse mitotic figures without ne- crosis. The remaining 3 tumors (2 deep and 1 superficial) had more than 10 mitoses per 10 high- power fields as well as foci of necrosis. ALK fusions were identified in all cases. The fusion part- ners included HMBOX1 (n 1⁄4 1), VCL (n 1⁄4 1), PRRC2B (n 1⁄4 1), MYH10 (n 1⁄4 1), STRN (n 1⁄4 1), and EML4 (n 1⁄4 2). One tumor recurred locally 2 years after initial resection; 1 patient had widely metastatic disease (mediastinal tumor). At the time of last follow-up (n 1⁄4 6), 4 patients were alive without evidence of disease, 1 died due to complications of therapy (peritoneal tumor), and 1 was alive with disease. Our findings expand the spectrum of ALK-rearranged mesenchymal tumors. Our cases predominately occurred in older children and mainly exhibited epithelioid to round cell morphology, as opposed to spindle cell morphology. We also show that tumors in a deep location with higher-grade features follow a more aggressive clinical course.