Spindle epithelial tumor with thymus-like differentiation: a case report with cytologic, histologic, immunohistologic, and ultrastructural findings.

Spindle epithelial tumor with thymus-like differentiation (SETTLE) is a rare and distinctive low-grade neoplasm of thymic or related branchial 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 clinical, cytologic, histologic, immunohistochemical, and ultrastructural features of a SETTLE present for 10 years in a 15-year-old boy. The fine-needle aspirate, initially interpreted as synovial sarcoma, contained numerous clusters of bland spindle cells, with a few detached sheets of columnar mucous cells in a homogeneous background of dissociated spindle cells. Mitoses, necrosis, and atypia were not present. The excised tumor was a well-circumscribed, white-tan mass, with occasional microcysts. Microscopically, the mass consisted of a lobulated, highly cellular, spindle-cell neoplasm arranged in intersecting, whorled, and storiform fascicles separated by fibrous bands. Entrapped within the fibrous bands were squamous-lined cysts and benign-appearing glands lined by columnar epithelium with goblet cells or ciliated pseudostratified epithelium. Immunohistochemically, the spindle cells showed diffuse reactivity for cytokeratins, smooth muscle actin, muscle-specific actin, and MIC-2, and they were negative for epithelial membrane antigen, calcitonin, and thyroglobulin. Ultrastructurally, numerous perinuclear tonofilaments, some aligned with mature desmosomes, were identified in the spindle cells. Occasional cells showed thin filaments with fusiform dense bodies occupying the peripheral cytoplasm. These findings distinguish SETTLE from ectopic thymoma, synovial sarcoma, medullary carcinoma, and teratoma, and they support a thymic epithelial origin for SETTLE, possibly with myoepithelial differentiation.