Imaging description

A five-month-old boy presented with a palpable left neck mass. MRI shows a left parapharyngeal mass that is isointense to muscle on T1-weighted imaging (Fig. 7.1a) and hyperintense to muscle and homogeneous in signal on T2-weighted imaging (Fig. 7.1b). A coronal contrast-enhanced T1-weighted image (Fig. 7.1c) shows mild enhancement of this mass and again shows homogeneous signal. The MR imaging characteristics are similar to intrathoracic thymus but there is not definite contiguity. The diagnosis of normal thymic tissue was confirmed following CT-guided fine-needle aspiration.

Importance

The primordial thymus arises from the third and fourth pharyngeal pouches during the sixth week of gestational life. During the seventh week, thymic primordia migrate medially and caudally, forming the thymopharyngeal duct, to their final anatomic location in the anterior mediastinum. Ectopic and accessory thymic tissue may occur anywhere along the descending pathway of the thymopharyngeal duct (from the angle of the mandible to the superior mediastinum) due to arrest in migration, sequestration, or failure of involution. Ectopic thymic tissue and remnants are congenital lesions that may be solid and/or cystic in nature. Most ectopic cervical thymic tissue is unilateral and usually on the left for reasons that are unclear. Manifestation of ectopic thymic tissue as a neck mass may be mistaken for a pathologic process.