Le thymus cervical ectopique (TCE) est une anomalie embryologique rare et bénigne de l’enfant. Il peut être révélé par une masse cervicale compressive prise à tort pour une tumeur maligne. Avant la chirurgie, la présence d’un thymus médiastinal doit être confirmée pour éviter une thymectomie totale. L’IRM peut aider à une meilleure définition des limites de l’ectopie thymique par rapport au thymus médiastinal. À travers un cas de TCE chez un nourrisson de 19 mois, nous nous proposons de mettre l’accent sur cette entité et ses différentes caractéristiques. Le TCE est une anomalie bénigne rare.

Ectopic cervical thymus (ECT) is a rare embryological abnormality in children. It can be revealed by a compressive neck mass mistaken for a malignant tumor. Through a new case of ECT, we review the embryopathogenesis, diagnostic difficulties and therapeutic features.

A 19-month-old girl presented a right cervical mass that quickly increased in size, causing intermittent dyspnea. The physical examination objectified a 6-cm, soft and compressible, painless right cervical tumefaction, extending from the mastoid area to the ipsilateral supraclavicular fossa. The diagnosis suggested based on CT was a cervicomediastinal cystic lymphangioma. The diagnoses discussed based on MRI were a collection of necrotic lymphadenopathy, rhabdomyosarcoma or neurofibroma debris. The mass was surgically excised through a laterocervical incision. A whitish multilobular tissular mass was found, adherent to the neurovascular axis of the neck. Pathological examination concluded in normal ectopic thymus tissue. The postoperative course was uneventful.

Although ECT is a rare benign anomaly, it should be considered as a possible cause of a neck mass in children. Surgery is the curative treatment. Before surgery, the presence of a mediastinal thymus must be confirmed to avoid the risk of a total thymectomy in children. MRI is helpful in delineating thymic ectopia compared to the mediastinal thymus.