Ectopic thymus

Ectopic thymus is a condition where thymus tissue is found in an abnormal location. It is thought to be the result of either a failure of descent or a failure of involution of normal thymus tissue.
Signs and Symptoms.
Ectopic thymus most often does not cause symptoms. It is most frequently discovered as a mass or swelling in the neck of infants and children. However, when symptoms do occur they are most commonly due to compression of nearby structures such as the trachea and esophagus. This can lead to hoarseness, stridor, difficulty breathing and/or difficulty swallowing.
Cause.
During embryological development, the thymus is formed from the third and fourth pharyngeal pouches. It descends along a pathway from the mandible to its final resting place of the mediastinum. When the thymus tissue fails to descend appropriately or fails to involute, thymus tissue remains in various locations along this pathway. Locations that solid thymus tissue has been reported include near the thyroid (most common), within the thyroid, the base of the skull, and within the pharynx or trachea.            
Diagnosis.
Ultrasound is the recommended diagnostic modality used to diagnose cervical ectopic thymus. The thymus has a unique appearance on ultrasound which allows for specific diagnosis. Upon ultrasound, ectopic thymus appears hypoechoic with characteristic linear echogenic foci. However, MRI can and has been utilized as well to better characterize and identify the location of the ectopic thymus. On MRI, ectopic cervical thymus appears as a homogeneous mass which is isointense to muscle on T1-weighted scans and hyperintense on T2-weighted scans. Biopsy or histological examination upon resection can also be used to make a definitive diagnosis.
An appropriate differential diagnosis depends upon location of the ectopic thymus. For cervical ectopic thymus the differential diagnosis should include additional causes of neck masses. This includes common causes of neck masses in children such as thyroglossal duct cyst, branchial cleft cyst, dermoid cyst, inflammatory lymphadenitis, sternocleidomastoid (SCM) tumor of infancy, a salivary gland infection or benign tumor. Rare causes of neck masses in children include lymphoma, rhabdomyosarcoma, thyroid nodules and thyroid cancer.
Treatment.
If the patient is asymptomatic and the mass is identified based upon radiologic findings, biopsy and/or resection may be avoided. Surgical removal of the mass is the definitive treatment for ectopic thymus tissue that is causing symptoms. It has been reported that the ectopic thymus tissue can transform into cancerous tissue. However, due to most diagnosed ectopic thymus tissue being resected due to this concern, the natural progression is not well explored. The data supporting malignant transformation is limited and ectopic thymus tissue that is not causing problems can likely be left to involute. Given the thymus's role in the body's adaptive immune system, it should be confirmed that the patient has a mediastinal thymus prior to surgery in order to prevent the potential for future immune deficiencies.
Prognosis.
Following surgical removal of the ectopic thymus, there have been no reported recurrences.
Epidemiology.
Ectopic thymus is rarely reported in the literature. The prevalence of ectopic thymus reportedly ranges from 1 to 90%. This variation in prevalence is largely dependent upon the method of investigation used and how extensive the workup is. With most ectopic thymus tissue being asymptomatic, it is likely the prevalence is higher than typically reported.