Diagnosis

Discussion

Spindle Epithelial Tumor with Thymus-Like Differentiation (SETTLE) is a rare, primary, low-grade malignant neoplasm of the thyroid gland presumably derived from branchial pouch remnants or ectopic thymus. The tumor has been described as presenting in a wide age range (4 to 59 years) but is more common in children to young adults with an average age of 15 years. The male to female ratio is 1.8:1. The tumor generally presents as a mass, slowly increasing in size although it may present with rapid enlargement of an otherwise longstanding thyroid mass.

Histologically, the tumor tends to be biphasic, with spindle cell and epithelioid components. The spindled component tends to predominate, and may compose from 40% to almost 100% of the tumor. The spindled cells form short intersecting or streaming fascicles with a sharp interface with the fibrovascular stroma. They display elongated, pale nuclei with delicate chromatin and inconspicuous nucleoli. Although usually highly cellular, the mitotic rate tends to be low (1/10 HPF or less). The epithelial component may be composed of narrow tubules, small papillae, trabeculae, glandular lumens, small nests, or solid sheets that blend imperceptibly with the spindled component. The epithelial cells are cuboidal-to-columnar with moderate cytoplasm, and round-to-oval, pale bland nuclei. Glandular spaces lined by mucinous or respiratory-type epithelium, and foci of squamous differentiation, reminiscent of Hassall's corpuscles, have also been described. Irregular sclerotic fibrous bands characteristically separate the neoplastic cells resulting in the formation of incompletely demarcated nodules. Immunohistochemically, the neoplastic cells (both the spindle and epithelial) are reactive for epithelial markers such as 34bE12 indicating expression of high molecular weight cytokeratins. Reactivity for smooth muscle actin has been reported and was seen in this case. Thyroid follicular or C-cell related markers are consistently negative.

SETTLE is a low-grade malignant neoplasm which may metastasize to the lung after a period of slow, long-term growth (mean, 11.4 years; median 5 years). Despite metastatic spread, long survival had been reported attesting to the indolent nature of this neoplasm. The differential diagnosis includes anaplastic carcinoma, medullary thyroid carcinoma, and synovial sarcoma.