Ectopic cervical thymus in an infant.

We report a case of accessory cervical thymus presenting as a unilateral neck mass in a 2-month-old boy. Ultrasonography (US) showed a mass isoechogenic to muscle in the left neck. Computed tomography (CT) revealed a well-defined, mildly enhanced mass located anterior to the sternocleidomastoid muscle, anterolateral to the carotid sheath, and posterior to the submandibular gland. On magnetic resonance imaging (MRI), the mass was isointense to muscle on T1-weighted images and hyperintense to muscle on T2-weighted images. Diffusion-weighted images showed relatively low apparent diffusion coefficient (ADC) values, and the mass was slightly enhanced after administration of contrast material. We suspected ectopic thymus, but we could not exclude the possibility of a malignant lesion. Therefore, the tumor was surgically resected. The histological diagnosis was ectopic cervical thymus. Ectopic thymus should be included in the differential diagnosis of a submandibular or cervical mass in infants. US and MRI can provide useful information for the diagnosis of ectopic cervical thymus.