SPINDLE EPITHELIAL TUMOR WITH THYMUS-LIKE DIFFERENTIATION - A CASE-REPORT WITH CYTOLOGIC, HISTOLOGIC, IMMUNOHISTOLOGIC, AND ULTRASTRUCTURALFINDINGS

Spindle epithelial tumor with thymus-like differentiation (SETTLE) is a rare and distinctive low-grade neoplasm of thymic or related branchi al pouch differentiation. The tumor usually presents in the thyroid or lateral neck of children and adolescents and could mimic spindle-cell carcinoma, synovial sarcoma, or malignant teratoma We report the clin ical, cytologic, histologic, immunohistochemical, and ultrastructural features of a SETTLE present for 10 years in a 15-year-old boy. The fi ne-needle aspirate, initially interpreted as synovial sarcoma, contain ed numerous clusters of bland spindle cells, with a few detached sheet s of columnar mucous cells in a homogenous background of dissociated s pindle cells. Mitoses, necrosis, and atypia were not present. The exci sed tumor was a well-circumscribed, white-tan mass, with occasional mi crocysts. Microscopically, the mass consisted of a lobulated, highly c ellular, spindle-cell neoplasm arranged in intersecting, whorled, and storiform fascicles separated by fibrous bands. Entrapped within the f ibrous bands were squamous-lined cysts and benign-appearing glands lin ed by columnar epithelium with goblet cells or ciliated pseudostratifi ed epithelium. Immunohistochemically, the spindle cells showed diffuse reactivity for cytokeratins, smooth muscle actin, muscle-specific act in, and MIC-2, and they were negative for epithelial membrane antigen, calcitonin, and thyroglobulin. Ultrastructurally, numerous perinuclea r tonofilaments, some aligned with mature desmosomes, were identified in the spindle cells. Occasional cells showed thin filaments with fusi form dense bodies occupying the peripheral cytoplasm. These findings d istinguish SETTLE from ectopic thymoma, synovial sarcoma, medullary ca rcinoma, and teratoma, and they support a thymic epithelial origin for SETTLE, possibly with myoepithelial differentiation.