Metastatic spindle epithelial tumor with thymus-like element (SETTLE) to the lung: Case report of a rare tumor and a brief review of the literature

Spindle Epithelial Tumor with Thymus-Like Differentiation (SETTLE) is an extremely rare thymus-like thyroid tumor that mostly occurs in children, teenagers, and young adults. We report a case of a 26-year-old male with a slowly growing, painless neck mass that developed over 4-month duration. Thyroid function tests, serum calcitonin, and carcinoembryonic antigens values were all within normal values. MRI showed a well-demarcated, lobulated, 4 × 3 cm heterogeneous, mixed solid-cystic mass in the thyroid with marked internal vascularity. FNA cytology showed spindle cells, but there was no definitive diagnosis. Total thyroidectomy was performed, including the entire mass. There was no local invasion, and the parathyroid glands were preserved. The histomorphologic features and immunohistochemistry (IHC) profile were supportive of the diagnosis of SETTLE.
Two pulmonary nodules of metastatic SETTLE were discovered 3 years later and were completely surgically removed, and no adjuvant therapy was given. At 16 months following a metastasectomy, the patient was free of disease before being lost to follow-up. SETTLE is a rare tumor, which represents a diagnostic challenge because it is histomorphologically similar to other tumors of the thyroid and the thymus. Awareness of this tumor is important so it can be included in the differential diagnosis of a neck mass. Metastasis can occur years after initial treatment, and long-term follow-up is mandatory.