Spindle epithelial tumor with thymus-like elements (SETTLE): a surgical case diagnosed preoperatively using fine-needle aspiration cytology.

Summary: Spindle epithelial tumor with thymic-like elements (SETTLE) is an extremely rare tumor that occurs primarily in the thyroid gland. Histologically, SETTLE is characterized by the presence of spindle-shaped epithelial cells and glandular structures. However, it is known that diagnosis via fine-needle aspiration cytology can be challenging. SETTLE predominantly occurs in younger individuals and has a less favorable prognosis compared to differentiated thyroid carcinoma. Therefore, ensuring accurate diagnosis and appropriate treatment is crucial. We encountered a case of spindle epithelial tumor with thymus-like differentiation in a 10-year-old patient for whom the preoperative diagnosis was successfully established through fine-needle aspiration cytology, which facilitated appropriate surgical resection. Comprehensive histopathological examination and immunohistochemical analysis are essential to ensure appropriate management and surveillance of SETTLE. Learning points: A rare thyroid tumor, spindle epithelial tumor with thymic-like elements (SETTLE), was diagnosed preoperatively and treated surgically. SETTLE presents with characteristic histological features that must be recognized for accurate diagnosis. In addition, diagnosis through cytology is often challenging. The primary treatment for SETTLE is surgical intervention as radiotherapy and pharmacological treatments are generally not expected to be highly effective. Radical resection is the only effective treatment, making the selection of the surgical procedure according to the stage of the disease essential.