Thyroglossal Duct Cyst Imaging 

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.^{[1, 2]}

TDC is the most common nonodontogenic cyst in the neck, representing approximately 70% of all congenital neck abnormalities.^{[3, 4]} 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.^{[5, 6, 7]}

When examined pathologically, TDCs have a variable number of histologic components, including columnar, cuboidal, and/or non-keratinized stratified squamous epithelium.^[8] Ectopic thyroid tissue is present in a proportion of TDCs, with estimates ranging widely, from 1.5 to 62%.^{[3, 9]}

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.^{[3, 10]} 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.^{[11, 12]}

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. 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

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.

Anatomy

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.^[13] 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.^{[14, 15, 16, 17, 18]}

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.^{[19, 20, 21, 22]}

In 80% of cases, TDC carcinoma is of papillary cell origin.^{[23, 24]} 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.^[21] 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.^[25] Regional lymph nodes metastases occur in 8% of cases.^{[3, 20]} Coincident orthotopic thyroid gland carcinoma occurs in 14-25% of cases.^{[9, 26]}

Computed tomography (see the images below) has a high degree of diagnostic accuracy for thyroglossal duct cysts (TDC). The most helpful features in the differential diagnosis are the midline location, most often at or below the hyoid bone, and the intimate relationship of infrahyoid TDCs to the strap muscles. CT better evaluates the potential for thyroglossal duct carcinoma and is thus preferred in adult patients.

Reede et al performed a retrospective analysis of 12 preoperative cases of TDC and 2 cases with postoperative complications and described the typical CT appearance of a TDC as a well-circumscribed, low-density lesion with peripheral rim enhancement. Occasionally, increased attenuation, internal septations, and indistinctness of surrounding tissue planes are seen and are presumably the result of prior infection. Thyroid cartilage erosion and laryngeal extension are both extremely rare.^[27]

Calcifications and soft tissue nodules in the wall of the cyst are important features suggestive of TDC carcinoma.^{[21, 22]} Branstetter et al retrospectively reviewed 6 cases of TDC carcinoma and found that all 6 had either a dense or enhancing mural nodule, calcification within the cyst, or both.^[28]

Magnetic resonance imaging (see the images below) provides a high degree of diagnostic accuracy for thyroglossal duct cysts (TDC), but it is rarely required for the diagnosis. Although TDCs are invariably hyperintense on T2-weighted images, T1-weighted signal intensity is variable. King found that 56% (9 out of 16) of TDCs were isointense or hyperintense on T1-weighted images, while the remaining 44% (7 out of 16) were hypointense.^[29]

In a report of 2 cases of TDC, Blandino found high T1-weighted signal intensity in both lesions.^[30] The high T1-weighted signal intensity is thought to be due to thyroglobulin or hemorrhage. Peripheral rim enhancement is sometimes observed on postcontrast images and is suggestive of previous infection.

On ultrasonography (see the images below), most thyroglossal duct cysts (TDCs) appear as unilocular lesions with thin walls and posterior acoustic enhancement. TDCs are highly variable in echogenicity. Ahuja et al studied 23 clinically diagnosed TDCs in children and found 3 appearances^[31] : 3 lesions (13%) were anechoic, 13 lesions (56.5%) were hyperechoic, and 7 lesions (30.5%) were heterogeneous.

In another study by Ahuja et al, 40 clinically suspected TDCs in adults were found to have variable sonographic appearances ranging from a typical anechoic cyst to a pseudosolid appearance.^[32] Four patterns of TDCs were identified: 11 lesions (28%) were anechoic, 7 lesions (18%) were hypoechoic with low-level echoes, 11 lesions (28%) were hyperechoic, and 11 lesions (28%) were heterogeneous.

Surrounding soft tissue edema, regional reactive lymphadenopathy and a sinus tract to overlying skin may be demonstrated.^[33] Doppler interrogation should be performed on initial ultrasound to identify any solid vascular component.

While most TDCs can be confidently diagnosed by ultrasonography, the presence or absence of functioning thyroid tissue within the TDC may be difficult to ascertain. The same holds true for thyroid tissue extending superiorly or inferiorly from the TDC. A solid nodular component may be difficult to identify, particularly within a uniformly hyperechoic TDC. For this reason, Doppler interrogation should always be performed in cases of suspected TDC.

In adults, the findings of carcinoma may be inevident on ultrasound.

The following entities can have sonographic findings similar to TDC:

• Branchial cleft cysts: these cysts can usually be differentiated from TDCs by their characteristic lateral location in the neck; however, TDCs located off the midline could be mistaken for branchial cleft cysts, as their sonographic appearances are similar^[31]
• Cystic hygromas: these lesions have characteristic cystic spaces of varying sizes with intervening septa, allowing for easy differentiation from TDCs
• Hemangiomas: these lesions are typically hypoechoic, with vascular spaces, and may demonstrate the presence of flow with color flow Doppler; the presence of phleboliths may also aid diagnosis^[34]
• Lipomas: these lesions have a characteristic feathery pattern on ultrasound with multiple bright reflectors^[32]
• Lymph nodes: lymph nodes are hypoechoic, are often multiple, and demonstrate the presence of an echogenic hilus; a suppurated node in the prevascular space might be mistaken for an infected TDC
• Dermoid inclusion cysts: these lesions may be confused with TDC, particularly when they are located in the vicinity of the hyoid bone; however, dermoid cysts lie superficial to the strap muscles and usually have visible fat components on CT.

There is controversy regarding the need for preoperative thyroid scintigraphy in patients with a presumed thyroglossal duct cyst (TDC).^[33] Proponents argue that scintigraphy may be more sensitive than CT or MRI for detection of ectopic thyroid tissue in the neck and that the excision of a TDC in a patient with no additional functioning thyroid tissue might result in permanent hypothyroidism.^[35]

Other experts do not feel that preoperative scintigraphy is necessary, arguing that a properly performed Sistrunk procedure will remove all ectopic thyroid tissue regardless of whether it was identified on preoperative imaging and that scintigraphy results in an unnecessary radiation dose to the neck, especially in pediatric patients.