Early metastasizing spindle epithelial tumor with thymus-like differentiation (SETTLE) of the thyroid

Spindle epithelial tumor with thymus-like elements is a rare thyroid lesion of children and young adults thought to be derived from branchial pouch remnants or foci of ectopic thymus. The lesion is poorly understood, and although it was originally believed to follow an indolent clinical course, its potential for late metastasis is becoming generally acknowledged. We have recently seen a unique case of this rare tumor in an 11-year-old boy, in which an unexpected and salient feature is the presence of a micrometastasis in a single lymph node at presentation. With the exception of 1 case with extensively infiltrative tumor and metastatic disease at the time of onset, in all other cases dissemination occurred years after surgical resection of the primary lesion. We review all previously reported cases and provide a detailed study of the histologic and ultrastructural appearances of this lesion.