14.

Spindle Epithelial Tumor with Thymus-Like Differentiation, Mod Pathol 2000;13:1150; AJSP 1999; 23:712

Rare indolent tumor of neck in young patients (median age 15), with thymic or branchial pouch differentiation and delayed metastases (mean 5 years)

Metastases commonly to lung

Micro: invasive, highly cellular, sclerotic bands (thick and thin); biphasic - bland spindle cells in fascicles and tubulopapillary epithelial structures; rare mitotic activity or focal necrosis, image © AFIP

Stains: keratin (strongest for high molecular weight keratin)

Negative: CD20, CD3, TdT

DD: sarcomatoid or anaplastic carcinomas (more atypia), medullary carcinoma (more prominent septa, positive for calcitonin and chromogranin), synovial sarcoma (weak cytokeratin in spindle cells), malignant teratoma (13)

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