YAP1::TFE3-rearranged inflammatory spindle cell PEComa of the anterior mediastinum in a 5-year-old child: first pediatric case.

Perivascular epithelioid cell tumors (PEComas) are rare mesenchymal neoplasms characterized by myomelanocytic differentiation and dichotomic molecular pathogenesis (mTOR-activating mutations versus TFE3 gene rearrangements). A recently recognized subset harboring YAP1::TFE3 gene fusion, termed inflammatory spindle cell PEComa, has been described predominantly in the lungs of adults showing significant morphologic and molecular overlap with YAP1::TFE3-fused clear cell stromal tumor of the lung (CCST-L). We report the first pediatric case of YAP1::TFE3-rearranged inflammatory spindle cell PEComa arising in the anterior mediastinum of a 5-year-old child. The tumor, largely enclosed by a thick fibrous capsule, was composed of plump ovoid to spindle cells with a prominent chronic mononuclear inflammatory infiltrate and strikingly extensive multifocal stromal dystrophic calcifications that were radiologically conspicuous. Worrisome features included large size (8 cm) and multifocal necrosis. Tumor cells showed diffuse expression of melanocytic markers (HMB45 and melanoma cocktail), cathepsin K and TFE3 in the absence of smooth muscle markers. The C-terminal YAP1 staining revealed complete loss limited to the neoplastic cells. This case expands the clinicopathologic and age spectrum of YAP1::TFE3-rearranged PEComa which needs to be distinguished from other overlapping neoplasms carrying the same gene fusion.