Spindle epithelial tumor with thymus-like differentiation (SETTLE): clinical-pathological features, differential pathological diagnosis and therapy.

Spindle epithelial tumor with thymus-like differentiation (SETTLE) is a very rare tumor of the thyroid gland. An algorithm for the diagnosis and treatment of SETTLE has yet to be established. The aim of this study was to identify all case reports of SETTLE and to compare the clinical-pathological features and therapy of the cases identified. We performed a PubMed search for case reports of SETTLE in English published up to November 2014 in which "SETTLE" and "Spindle epithelial tumor with thymus-like differentiation" were keywords. We identified 35 articles for a total of 42 cases. We found that SETTLE usually occurs in children and adolescents as an asymptomatic neck mass. Thyroid function tests and tumor markers are invariably within normal range in all patients, and fine needle aspiration biopsy is rarely diagnostic for SETTLE. All 42 patients had undergone thyroidectomy. After surgical resection, chemotherapy (adjuvant or first/second-line treatment) and/or radiotherapy were administered to control tumor growth in cases with metastatic involvement. Although SETTLE presents a low-grade malignancy, it can metastasize to lymph nodes, the mediastinum, lung, vertebrae, and kidney even many years after the initial diagnosis. SETTLE may have a good prognosis if appropriately treated at initial presentation and if patients undergo long-term monitoring with regular clinical and morphological evaluations.