Ectopic cervical thymus in an infant

Clinical History

A 2-month-old male infant with a neck mass which appeared shortly after birth and gradually increased in size.

Imaging Findings

A 2-month-old male infant was referred to Radiology Department for the evaluation of a neck mass. The mass was noticed by the parents. It appeared shortly after birth and gradually increased in size. He was otherwise healthy. On examination, there was a non-tender 4cm mass on the right side of the neck just below the angulus mandibula.The mass was not very firm on palpation and there was no accompanying eryhthema or fluctuation. No limitations in the neck movements was noted. On ultrasound examination, the lesion was approximately 4x3cm in size, well-defined and smooth-contoured and homogeneously hypoechoic. A lymphadenomegaly was suspected, and antibiotics were prescribed. The patient came back for follow-up with no response to treatment. A CT scan to better delineate the extent of the lesion and to see whether there are other smaller accompanying lesions was done prior to excisional biopsy. Upper mediastinum was included in the examination. On CT (Fig 1), the mass was homogeneous and isodense with muscle, and did not enhance with intravenous contrast. It was located inferior to mandibular angle, anteromedial to sternocleidomastoid muscle and adjacent to the carotid space. There was no additional lesion. The mass was completely and easily excised. The histopathology came out as thymic tissue. The CT films were evaluated again, a normal thymus was noted in the anterior mediastinum excluding the suspicion that the only thymic tissue in the infant was removed (Fig 2).

Discussion

Ectopic thymus is a rare cause of cervical mass in an infant. It can be located in the line of the descent of thymus, from the mandibular angulus to the superior mediastinum. There are only eleven reported cases in the literature, mostly being cystic masses (1-5). Due to its rarity, a preoperative diagnosis is seldom made and it is often mistaken for an enlarged lymph node or a branchial cleft cyst. Since most pediatric non-vascular masses are biopsied and/or excised when there is no response to antibiotic therapy, the possibility must be kept in mind for this mass can be the only thymic tissue in the infant. Ectopic cervical thymus is a rare entity with only 93 cases reported out of which only 11 occurred in infants. Ectopic cervical thymus is usually diagnosed between the age of 2 and 13 years, have a male preponderance and occurs more commonly on the left side (5). To completely understand the pathogenesis of cervical thymic masses, the embryologic development of thymus must be reviewed. Thymus arises from the ventral saccules of the third and fourth branchial pouches during the 6th week of fetal life. Proliferation of the endodermal cells within the outpouching gives rise to paired solid masses which later fuse in midline by the 7th week of gestation and are covered by a mesenchymal capsule which is in close proximity to the pericardium. By the 8th week the tyhmus descends to its location in the superior mediastinum. Ectopic thymus tissue can be found along the line of descent and this accessory tissue undergoes hyperplasia during the first decade of life or after vaccination and infections (1, 2). Ectopic thymus can either be cystic or solid. Solid ectopic thymus as seen in our patient, constitutes 10% of all ectopic thymus masses. The embryologic or clinical differences between solid and cystic types are not known. Multiple combinations of solid and cystic ectopic thymus have been described and include accessory cervical thymus, cervical thymic cyst, completely undescended thymus, persistent thymic cord, cervical extension of mediastinal thymus or ectopic thymus out of the normal decent path of the tyhmus (1, 4). Although the mass tends to be asymptomatic, ectopic thymic mass can present with stridor, choking spells, dyspnea, dysphagia and difficulty feeding in the infant (1, 2). The differential diagnosis of a cervical mass in an infant includes thyroglossal duct cyst, branchial cleft cyst, abberant thyroid, cervical lymphadenopathy, benign tumors (dermoid, epidermoid, hemangioma and lymphangioma), and malignant tumors (2, 4). CT scan findings of an ectopic thymic mass usually reveals a homogeneous mass that lies anterior to the sternocleidomastoid muscle and often extends posteriorly to the retropharyngeal space. A distinctive feature is that it has no mass effect on the adjacent structures when presenting in infancy (2). On MR scans, the solid thymic mass is again homogeneous. It is slightly more intense than muscle on T1-weighted images and isointense to fat on T2 similar to the appearance of normal mediastinal thymus (5). Since these lesions can undergo malignant degeneration (4), the management of solid ectopic cervical thymus is surgical, by complete excision. However, accurate diagnosis is mandatory before removal so that the presence of a normal mediastinal thymus can be confirmed in order to prevent the risk of immuno-incompetence that might follow after complete removal of all thymic tissue in the infantile period.

Differential Diagnosis List

Final Diagnosis

Ectopic cervical thymus

URL:	https://eurorad.org/case/859
DOI:	10.1594/EURORAD/CASE.859
ISSN:	1563-4086