Cervical ectopic thymus presenting as a painless neck mass in a child.

Ectopic cervical thymus is an uncommon entity to be considered in the differential diagnosis of neck masses in infants and children. The sonographic appearance of a hyporeflective mass with characteristic internal linear hyperreflections situated along the path of the thymopharyngeal duct should suggest diagnosis. On MRI, signal intensities differ significantly from other more common neck mass lesions in children such as a branchiogenic cyst, hemangioma and lymphangioma. Diagnosis can be confirmed by performing a sonographically guided thru-cut biopsy.