Spindle epithelial tumor with thymus-like differentiation (SETTLE): a distinctive malignant thyroid neoplasm with significant metastatic potential.

Spindle epithelial tumor with thymus-like differentiation (SETTLE) is a very rare tumor of the thyroid believed to be derived from branchial pouch or thymic remnants and showing primitive thymic differentiation. Although this tumor is prone to develop delayed blood-borne metastases, the metastatic risk is unclear because the case reports in the recent literature had very short follow-up periods. We report one case, the oldest patient reported so far, who had been followed up until death. The 59-year-old man had an enlarged thyroid for all of his adult life, and presented with recent rapid enlargement of the thyroid. Neck exploration revealed a hard tumor in the right lobe, with adhesion to sternothyroid muscle. Histologic examination showed an invasive biphasic neoplasm traversed by sclerotic septa. Tight to loose fascicles of bland-looking spindly cells were intimately intermingled with tubulopapillary structures, diagnostic of the SETTLE. This patient developed left pulmonary metastases at 2 years and subsequently developed bilateral pulmonary and widespread metastases. He died 8 years after initial presentation. This case illustrates the protracted clinical course of the tumor, and survival for many years despite the occurrence of metastases. Review of the literature shows that SETTLE occurs predominantly in young patients with a median age of 15 years and male predominance. There is a significant metastatic rate of 71% for patients with more than 5 years of follow-up in spite of the otherwise indolent nature of the tumor.