Spindle epithelial tumor with thymus-like differentiation (SETTLE): Case report with longest follow up & latency to metastasis

Spindle epithelial tumor with thymus-like differentiation (SETTLE) is an extremely rare, low-grade malignant spindle cell tumor of the thyroid gland with delayed metastatic potential. It occurs mainly in children, adolescents and young adults and is thought to be derived from thymic or branchial pouch remnants. Nearly 42 cases have been reported in the English literature. Metastatic disease has been described up to 22 and 25 years after the initial operative intervention. SETTLE should be considered in the differential diagnosis of any spindle cell tumor in thyroid region. Pathologic diagnosis requires spindle and epithelial components and the use of immunohistochemistry. The differential diagnosis is broad and includes synovial sarcoma, spindle cell anaplastic thyroid carcinoma, spindle cell variant of medullary thyroid carcinoma, thymoma, fibrosarcoma, leiomyoma and schwannoma. Owing to delayed metastasis, close long term follow-up is mandatory. We present a case where metastatic disease appeared in the lungs 35 years after the initial diagnosis, representing the longest reported latency period to metastasis, which unfortunately led to the patient's demise.