A “no-touch” neck “mass” in a child
How to cite this article: Kanamathareddy HV, Irodi A, Mathai J, Gurijala PR. A “no-touch” neck “mass” in a child. Case Rep Clin Radiol 2023;1:84-7.
Aberrant cervical thymus is a rare cause of pediatric neck masses related to the embryological development. In most of the cases, this condition is asymptomatic and essentially benign in nature. Here, we describe the sonological and magnetic resonance imaging (MRI) features of cervical thymus in a 2-month-old baby who presented with the left submandibular swelling. The unique imaging findings on ultrasound and MRI paralleling that of the normal thymus tissue were helpful in confidently establishing the diagnosis. Therefore, the radiologist must be aware of the condition and the typical normal appearance of thymus which can obviate the need for biopsy or surgical intervention.
Aberrant cervical thymus
Child neck mass
Cervical thymus is a rare cause of neck mass in pediatric age group. It can occur anywhere along the embryological pathway of thymus descent from angle of the mandible to superior mediastinum.[1,2] Here, we present a case of 2-month-old baby boy whose mother perceived a submandibular swelling at 15 days of age. High resolution ultrasound and magnetic resonance imaging (MRI) were done to localize the lesion, study its size, extent, and composition which confirmed the diagnosis of aberrant cervical thymus and prevented further intervention and surgery.
A 2-month-old baby boy was taken to a pediatrician by his mother who perceived a bulge in the left submandibular region of the neck which was initially noticed at 15 days of age and apparently increased in size since then.
On clinical examination, there was a 3 × 2 cm sized non-tender soft palpable mass in the left submandibular region at the angle of the mandible. There were no signs of inflammation or any skin changes. No restriction of the neck movements was noticed.
High resolution ultrasound showed a well-defined oval shaped mass just inferolateral to the submandibular gland and anteromedial to the sternocleidomastoid muscle. It showed multiple echogenic linear structures and foci under hypoechoic background, which was reminiscent of the echo pattern of normal thymic tissue [Figure 1a]. The thymus in the superior mediastinum was then imaged through the intercostal spaces which showed a similar echopattern as the lesion in the submandibular space [Figure 1b]. A provisional diagnosis of aberrant left cervical thymus was made. Other differentials include lymph nodes, ectopic thyroid, and fibromatosis colli.
MRI of the neck and upper thorax was performed to confirm the diagnosis and to see for any other ectopic thymus tissues in the neck. MRI revealed a well-defined homogeneous lesion in the left submandibular region and a small similar signal intensity lesion in left lower neck, medial to left common carotid artery along the thymopharyngeal tract with no obvious communication between the two. The signal intensity of both the lesions was similar to that of normal thymus in anterior mediastinum, which was isointense to muscle on axial T1W images, hyperintense to muscle on axial T2W images and coronal fat suppressed T2W images [Figure 2].
The diagnosis of cervical thymus was made based on the location and imaging characteristics of the mass.
As our patient was asymptomatic with typical imaging features consistent with cervical thymus, parents were reassured and explained that no specific intervention was required and the child was kept on clinical follow-up half yearly with high resolution ultrasound of neck.
Thymus is a specialized primary lymphoid organ of the immune system. Embryologically, it is developed from the ventral wings of third and fourth pharyngeal pouches on each side and it is pulled inferomedially forming thymopharyngeal duct.[1-3] During its descent, the tract obliterates and the thymus reaches its normal position in anterior mediastinum. However, in this process, the thymic remnant can occur anywhere along the tract from angle of the mandible to superior mediastinum.
Aberrant cervical thymus is a rare cause of neck masses in children, most of them being otherwise asymptomatic as in our case. Very small fraction of patients (6 in 100) can have symptoms related to compression of trachea or esophagus such as stridor, dyspnea, and/or dysphagia. It has a male preponderance and occurs relatively more commonly on the left side than on the right side. Small percentage of them, 5–7% are seen in midline or the pharynx. The mass could be because of the hyperplasia of the undescended thymus which is associated with absent thymus gland in its normal position or hyperplasia of sequestrated thymus remnants along with well-developed normal orthotopic thymus gland as in our case.
Aberrant thymus gland can be solid or cystic. Solid lesions represent early stage and cystic lesions represent late stage in thymic development. Other ectopic thymic masses include thymic cysts, thymopharyngeal tract or cyst, mediastinal thymus with cervical extension, and ectopic thymus out of the normal descent pathway of thymus. Very rarely, ectopic thymic tissue may undergo hyperplasia and malignant transformation such as thymic carcinoma or lymphoma with paucity of the literature.
Differential diagnosis of the neck masses in infants includes lymphadenopathy, venolymphatic malformations, dermoid, ectopic thyroid or thyroglossal cysts, branchial cysts, fibromatosis colli, and other metastatic lesions.[5,6] High resolution ultrasound is the first modality of choice particularly in children due to the lack of ionizing radiation and need for sedation or contrast administration.[6,7] The location the neck mass with typical thymus echopattern on ultrasound will confidently establish the diagnosis. According to a study by Han et al. on ultrasound and histopathological thymus anatomy, multiple echogenic linear structures in thymus represent connective tissue septa and accompanying blood vessels. They have concluded that f are unique to make the diagnosis and biopsy may not be required. Zielke et al. also concluded that the neck mass can be confirmed by ultrasound as aberrant thymus based on the unique imaging features. MRI is adjuvant in diagnosis of the cervical thymus by demonstrating the same signal to that of the orthotopic mediastinal thymus in all the sequences. It is also useful to rule out other ectopic or aberrant thymic tissues elsewhere in the neck.
In the past, the diagnosis was made on biopsy and surgery. With the introduction of high resolution ultrasound and MRI, the diagnostic algorithm has shifted from histopathology to radiological diagnosis alone.
Cervical thymus although a rare entity should be kept in the differential diagnosis of asymptomatic neck masses in children. The unique imaging findings on ultrasound and MRI paralleling that of the normal thymus tissue should aid to confidently establish the diagnosis. The radiologist must be aware of the condition and the typical normal appearance of thymus which can obviate the need for biopsy or surgical intervention.
Aberrant cervical thymus should be included in the differential diagnosis of unilateral neck mass in children
High resolution ultrasound remains the first choice of imaging due to lack of ionizing radiation, ready availability, and non-sedative technique
The unique imaging features on ultrasound include well defined, hypoechoic lesion with multiple linear hyperechoic foci and septae, which parallels the echo-pattern of normal thymus
MRI shows the signal characteristics of the lesion similar to that of normal thymus in all the sequences and additionally helpful in identifying other lesions along the thymic tract
Radiologists should be aware of this condition to avoid unnecessary investigations and interventions.
Q1. Ectopic thymus presents as
Either solid or cystic mass.
Answer Key: c
Q2. Normal embryological development of thymus
Caudal and medial migration
Caudal and lateral migration
Caudal and posterior migration.
Answer Key: a
Q3. First investigation of choice in pediatric neck masses
Answer Key: b
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