Diagnosis of Ectopic Cervical Thymus in an Infant Using Noninvasive Imaging Methods.

Ectopic cervical thymus (ECT) is a rare congenital anomaly caused by the incomplete descent of thymic tissue during fetal development. Imaging modalities such as ultrasonography and magnetic resonance imaging (MRI) are useful tools for diagnosing this condition. However, ECT is often difficult to diagnose based solely on imaging findings because of its rarity. Surgical excision is frequently performed for differentiation from malignant neck tumors. We encountered a four-week-old male infant with left-sided neck swelling noted at three weeks of age. Ultrasonography revealed a well-defined hypoechoic lesion showing a fine mosaic-like internal pattern identical to that of the normal mediastinal thymus. MRI demonstrated a mass with signal intensity similar to the thymus, without airway or vascular compression. Based on these findings, ECT was diagnosed noninvasively, and the patient was managed conservatively, showing gradual regression over time. This case emphasizes that direct comparison of imaging characteristics between the cervical lesion and normal thymic tissue can make the diagnosis of ECT more definitive, thus allowing avoidance of invasive procedures and enabling safe conservative management.