Thyroglossal duct cyst (TDC) is usually diagnosed clinically, but ultrasonography is the preferred imaging technique in children. The 2 most common complications of TDC are infection and malignancy, the latter of which occurs in 1-4% of cases. Carcinoma occurs in approximately 1% of TDCs. TDCs are the most frequently encountered neck mass in children, with 50% presenting by 10 years of age.
TDC is the most common nonodontogenic cyst in the neck, representing approximately 70% of all congenital neck abnormalities.
It occurs as a result of anomalous development and migration of the thyroid gland during the fourth through eighth weeks of gestation. It is a cystic remnant along the course of the thyroglossal duct between the foramen cecum of the tongue base and the thyroid bed in the visceral space of the infrahyoid neck.
When examined pathologically, TDCs have a variable number of histologic components, including columnar, cuboidal, and/or non-keratinized stratified squamous epithelium.
Ectopic thyroid tissue is present in a proportion of TDCs, with estimates ranging widely, from 1.5 to 62%.
TDC typically presents in children and young patients, with an average age at presentation of 6 years. About 50% of patients present before 20 years of age, but a significant percentage (15%) present after 50 years of age.
Renard et al reviewed 146 cases of thyroglossal duct carcinoma and found that the average age was 39 years (range, 6–81 years). The squamous type of thyroglossal duct carcinoma tends to arise in an older age group, with an average age of 54 years at presentation.
In children, TDCs are diagnosed clinically. Imaging is used to confirm the clinical diagnosis and identify the presence of the thyroid gland. Most authorities feel that ultrasound is the most appropriate initial imaging technique for TDC. Ultrasound is readily available, inexpensive, non-invasive, and does not involve ionizing radiation or sedation, which is particularly important in children.
A simple TDC will appear as an avascular, anechoic structure with posterior acoustic enhancement on US.
CT and MRI are useful for determining the full extent of the lesion and its sometimes complex relationship to surrounding structures such as the hyoid bone.
In adults, CT is the preferred modality for several reasons:
TDC is less frequently diagnosed in adulthood, so the differential is broader
The radiation risk is lower in adults than in children
The risk of carcinoma is substantially higher in adults, and CT can better identify the suggestive features of malignancy
On CT, the cyst is usually less than twenty Hounsfield Units. On MRI, water characteristics are seen (dark on T1-weighted images and bright on T2-weighted images).
Ultrasound may not depict the deep extent of hyoid and infrahyoid TDCs, and it cannot reliably assess the region of the tongue base in the setting of suprahyoid TDCs.The thyroglossal duct is intimately related to the hyoid bone, and CT depicts this relationship best in hyoid lesions. MRI is preferred for lesions at or near the tongue base. The major disadvantage of CT is ionizing radiation. Both CT and MRI usually require sedation, and are more expensive to perform than ultrasound.
Approximately 65% of TDCs are located between the hyoid bone and thyroid gland. Fifteen percent occur at the level of the hyoid bone and 20% occur above it. Lingual TDCs are rare, accounting for 1-2% of all TDCs. In a series of 300 TDCs only 2 (0.67%) were in the region of the foramen cecum.
Cysts in this location may be associated with dysphagia and respiratory arrest. The low incidence of lingual TDC may be because the thyroglossal duct begins to atrophy from the oral side.
Thyroid carcinomas in TDCs
Virtually every type of thyroid carcinoma has been identified within a TDC, including, in descending order of frequency, papillary, mixed follicular-papillary, squamous, follicular, anaplastic and Hurthle cell carcinoma.
However, reports of TDC carcinoma are rare, occurring in about only 1% of TDCs.
In 80% of cases, TDC carcinoma is of papillary cell origin.
This is of radiologic significance, because the presence of calcifications in papillary carcinomas is one of the key imaging findings indicative of malignancy in TDC.
There are no documented cases of medullary carcinoma arising in TDC. This is not surprising, because medullary carcinomas arise from parafollicular cells originating from the ultimobranchial bodies of the fourth branchial pouch rather than the thyroid anlage.
Invasion into surrounding soft tissue is seen in only 17% of thyroglossal duct carcinomas. Metastatic disease is present in 1.3%, which is much lower than the rate from carcinoma arising in the thyroid gland.
Regional lymph nodes metastases occur in 8% of cases.
Coincident orthotopic thyroid gland carcinoma occurs in 14-25% of cases.
Ultrasonography is the imaging test of choice. TDC carcinoma may appear as a cystic mass with a solid component on the wall (mural mass), sometimes with evidence of microcalcification, or as a tumor invading the cyst wall. Carcinoma in the TDC typically presents as a midline cystic neck mass.
Surgical intervention is the primary therapy for large TDCs. Recently, the effect of ethanol sclerotherapy on TDCs was investigated.
In a small study of 8 patients, under sonographic guidance, ethanol was slowly instilled into the thyroglossal duct cavity after the cyst fluid was aspirated. The procedure was performed in an outpatient setting. Although ultrasound-guided sclerotherapy has been well established for benign thyroid cysts, it has been challenged for various nonthyroid cysts; however, it has become an accepted alternative for benign nonthyroid cystic neck masses.