Background: The spindle epithelial tumor with thymuslike differentiation (SETTLE) is a rare thyroid tumor believed to be derived from ectopic thymus tissue or the embryonic remnants of branchial pouches, which displays primitive thymic differentiation. Histologic observation of spindle cells may lead to confusion with other tumors the development and prognosis of which are very different. Differential diagnosis using histochemical markers is essential since although there is a tendency to develop blood-borne metastases, tumor growth is slow and the survival rate in patients followed up is as high as 70%. It mainly affects children and young adults (mean age 15), although cases have been reported in patients ranging from 2 to 59.
Case: A 2-year-old boy presented with a tumor on the anteroinferior aspect of the neck, which had been growing since birth, suggesting a congenital origin. Histologic examination showed spindle cell nodules separated by fibrous bands containing mucous glands and cysts filled with mucoid material. Immunohistochemical markers revealed primitive differentiation; tumor cells stained positive for cytokeratin and vimentin but negative for markers indicative of greater differentiation, such as calcitonin, chromogranin, calretinin, synaptophysin and S-100 protein.
Conclusion: This is the first reported case of SETTLE in the youngest patient which had been growing since birth.