Thyroglossal Duct Cyst Imaging

​Thyroglossal duct cyst  (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. ^{[1, 2]} 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. ^{[3, 4]}  Approximately 65% of TDCs are located between the hyoid bone and the 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. Cysts in this location may be associated with dysphagia and respiratory arrest. The thyroglossal duct begins to atrophy from the oral side; this may explain the low incidence of lingual TDC.  ^{[5, 6, 7, 8, 9]}

TDCs are present in about 7% of the population worldwide. TDC is rarely detected in infants younger than 1 year. Even though TDC is histologically benign, it is associated with preterm delivery or sudden infant death due to airway obstruction, with a mortality of 30-40%. ^[10] TDCs have an equal preponderance in males and females. Although they are known to be the most common pediatric mass, they also present in adults with varying frequency. These types of cysts are closely associated with the hyoid bone. Between 20 and 25% are present at the level of the suprahyoid, 15-20% are present at the level of the hyoid, and 25-65% are present at the infrahyoid level. ^[1]  

Most TDCs do not cause serious medical problems. Exceptions include the following: (1) very rarely, these cysts become cancerous; (2) some cysts cause dysphagia (problems swallowing food or liquid); and (3) these cysts can become infected, which can be painful. ^[2]  TDC carcinoma is much less common, occurring in roughly 1% of patients with diagnosed TDC. A vast majority of these tumors are papillary-type thyroid cancer. ^{[11, 12, 13]}

Diagnosis and management

TDC is usually diagnosed clinically, but ultrasonography is the preferred imaging technique in children. Diagnosis is difficult, but imaging and fine-needle aspiration (FNA) biopsies can help. Given the rarity of TDC carcinoma, treatment is not well agreed upon and can include the Sistrunk procedure, ^[14] thyroidectomy, nodal dissection, and postoperative radioactive iodine ablation (RAI). ^[15]  

When examined pathologically, TDCs have a variable number of histologic components, including columnar, cuboidal, and/or nonkeratinized stratified squamous epithelium. ^[16]  Ectopic thyroid tissue is present in a proportion of TDCs, with estimates ranging widely from 1.5 to 62%. ^{[17, 18]}  Presentation is typically a painless rounded midline anterior neck swelling or, if infected, a red, warm, painful lump. This lump may move with swallowing and may be classically elevated on tongue protrusion. ^[19]

Symptoms that may be signs of TDC cancer include (1) a noticeable lump in the front of the throat that appears to be getting larger; (2) a noticeable lump in the front of the throat that feels hard; (3) swollen lymph nodes; and (4) difficulty swallowing food or liquid. ^[2]

Given its rarity, there is wide variation in management recommendations for this disease. Extent of surgical management and need for adjuvant therapy, including radioactive iodine ablation (RAI), are particularly debated, with some researchers arguing for aggressive therapy, including RAI, for any patients who undergo concurrent thyroidectomy with the Sistrunk procedure for TDC carcinoma. ^[20]

The management approach to any anterior midline neck lump as recommended by Mimery and coworkers involves a comprehensive workup, including radiologic imaging, routine histopathologic analysis, and early involvement of the subspecialty multidisciplinary team. ^[21]  Complete resection of the cyst and of the duct up to the foramen cecum is curative. The Sistrunk procedure includes resection of the middle third of the hyoid bone and involves a small risk of recurrence (~2.5%). ^[19]

After the Sistrunk procedure is performed, the prognosis is usually excellent. Pain medication or antibiotics may be prescribed postoperatively. Patients can usually return to work or school 1 week after surgery. ^[1]

About 10% of TDCs recur after Sistrunk. The recurrence rate is much higher with simple excision without excision of the middle third of the hyoid bone. In all, 1% of TDCs are malignant, and this is usually diagnosed after surgical removal. ^[1]

Imaging modalities

In children, TDCs are diagnosed clinically. Imaging is used to confirm the clinical diagnosis and to identify the presence of the thyroid gland. Most authorities believe that ultrasonography is the most appropriate initial imaging technique for TDC. Ultrasonography is readily available, inexpensive, and noninvasive, and it does not involve ionizing radiation or sedation, which is particularly important in children.

A simple TDC will appear on ultrasound as an avascular, anechoic structure with posterior acoustic enhancement. ^[13]

Computed tomography (CT) and magnetic resonance imaging (MRI) are useful for determining the full extent of the lesion and its sometimes complex relationships with surrounding structures such as hyoid bone. ^{[22, 23, 24, 25]}

For adults, CT is the preferred modality for several reasons:

• TDC is less frequently diagnosed in adulthood, so the differential is broader.

• Radiation risk is lower in adults than in children.

• Risk of carcinoma is substantially higher in adults, and CT can better identify the suggestive features of malignancy.

On CT, the cyst usually has a mean attenuation value of less than 20 Hounsfield units (HU). On MRI, water characteristics are seen (dark on T1-weighted images and bright on T2-weighted images). ^[13]

Although rare, F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT can assist in evaluating the extent of malignant involvement of TDC squamous cell carcinoma. ^[26]

Ultrasonography 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 exposure to ionizing radiation. Both CT and MRI usually require sedation and are more expensive to perform than ultrasonography. ^[24]

Thyroid carcinoma in TDC

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 carcinomas. ^{[12, 27, 28, 29, 30]}  However, reports of TDC carcinoma are rare, occurring in about only 1% of cases of TDCs. ^[12]

In 80% of cases, TDC carcinoma is of papillary cell origin. ^{[31, 32]} This is of radiologic significance because the presence of calcifications in papillary carcinoma is one of the key imaging findings indicative of malignancy. ^[29] No cases of medullary carcinoma arising in the TDC have been documented. This is not surprising because medullary carcinomas arise from parafollicular cells originating from the ultimobranchial bodies of the fourth branchial pouch rather than from the thyroid anlage.

Invasion into surrounding soft tissue is seen in only 17% of thyroglossal duct carcinomas. Metastatic disease occurs in 1.3%, which is much lower than the rate of carcinoma arising in the thyroid gland. ^[33] Regional lymph node metastases occur in 8% of cases. ^{[17, 28]} Coincident orthotopic thyroid gland carcinoma occurs in 14-25%. ^{[18, 34]}

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. ^[12]

Surgical intervention is the primary therapy for patients with a large TDC. ^[35]

Tachibana and associates, in a retrospective analysis of the medical records of 20 patients, found that OK-432 (picibanil) sclerotherapy was effective in 5 of the 20 patients and had a lower effective rate in multilocular cysts (9.1%) than in unilocular cysts (44.4%). Five patients were treated with surgery following OK-432 sclerotherapy. There was no significant difference in operating time or amount of bleeding between patients with and without OK-432 sclerotherapy. Results of histologic examination of the cyst wall revealed that 2 patients had stratified squamous epithelium and that 2 showed absence of lymphocyte infiltration. ^[36]

Although ultrasound-guided sclerotherapy has been well established for benign thyroid cysts, its use for various nonthyroid cysts has been challenged. However, it has become an accepted alternative for benign nonthyroid cystic neck masses. ^[37]

Computed tomography (CT) has a high degree of diagnostic accuracy for 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 with strap muscles. CT is the best modality for evaluating the potential for thyroglossal duct carcinoma; it is the preferred modality for use in adult patients. ^{[12, 13, 23]}

On CT, TDCs are thin-walled, smooth, well-defined, homogeneously fluid-dense lesions with an anterior midline or paramedian location. The generally accepted rule is that they should be within 2 cm of the midline. TDCs may show slight rim (capsular) enhancement. The sternocleidomastoid muscle is typically displaced posteriorly or posterolaterally. In some cases, TDCs may be embedded in the infrahyoid (strap) muscles. ^[19] Occasionally, increased attenuation, internal septations, and indistinctness of surrounding tissue planes are seen and are presumably the result of prior infection. Both thyroid cartilage erosion and laryngeal extension are extremely rare. ^[38]

Calcifications and soft tissue nodules in the wall of the cyst are important features suggestive of TDC carcinoma. ^{[29, 30]}

(See the images below.)

 

Magnetic resonance imaging (MRI) provides a high degree of diagnostic accuracy for TDC, but it is rarely required for diagnosis. ^{[12, 13]} Although TDCs are invariably hyperintense on T2-weighted images, T1-weighted signal intensity is variable. High T1-weighted signal intensity on MRI is thought to be due to thyroglobulin or hemorrhage. Peripheral rim enhancement is sometimes observed on postcontrast images and is suggestive of previous infection. ^[39]

(See the images below.)

 

Ultrasonography is the ideal initial imaging choice for diagnosing TDC and evaluating for the presence of healthy thyroid tissue. Ultrasonography is readily available, inexpensive, and noninvasive. It does not require ionizing radiation or sedation, which is important in treating children. CT and MRI can be used to evaluate TDCs and the presence of normal thyroid tissue, but ultrasonography alone is usually sufficient. ^{[1, 40]}

If normal thyroid tissue in the inferior neck is absent, the patient and/or the parents should receive counseling regarding the possibility of lifelong thyroid replacement therapy after surgery. ^[1]

On ultrasonography, most TDCs appear as unilocular lesions with thin walls and posterior acoustic enhancement. TDCs are highly variable in echogenicity. ^{[12, 13]}  

In a study by Park et al of 2885 patients who underwent thyroid or neck ultrasonography, 126 (4.4%) were shown to have a thyroglossal duct cyst, with common features being suprahyoid location, size approximately 1 cm, and a flat-to-ovoid or round shape. ^[40]

(See the images below.)

Unless infected, TDCs are painless, fluctuant cystic structures that splay the strap muscles with posterior acoustic enhancement. The fluid is usually hypoechoic or anechoic on ultrasonography, and the walls are thin without internal vascularity. In some cases, there may be internal complexity. The internal fluid may contain debris and septa or internal echoes from proteinaceous material, even in the absence of infection, particularly the case in an adult patient for whom cysts may be complex heterogeneous masses. If infection is associated, there may be thickened/irregular walls, increased blood flow, and surrounding inflammatory change. A soft tissue lesion associated with the cyst may be ectopic thyroid tissue or rarely represents carcinoma (usually of papillary origin). ^[19]

Surrounding soft tissue edema, regional reactive lymphadenopathy, and a sinus tract to overlying skin may be noted. ^[41] Doppler interrogation should be performed on initial ultrasonography to identify any solid vascular component.

Although 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. ^[25]

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

The following entities can have sonographic findings similar to TDC:

• Branchial cleft cysts: These cysts usually can 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. ^[42]

• Cystic hygromas: These lesions have characteristic cystic spaces of varying size with intervening septa, allowing easy differentiation from TDCs.

• Hemangiomas: These lesions typically are hypoechoic, with vascular spaces, and may reveal the presence of flow with color flow Doppler imaging; the presence of phleboliths may aid diagnosis. ^[43]

• Lipomas: These lesions have a characteristic feathery pattern on ultrasound with multiple bright reflectors. ^[44]

• Lymph nodes: Lymph nodes are hypoechoic, are often multiple, and reveal 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.

Controversy surrounds the need for preoperative thyroid scintigraphy in patients with a presumed TDC. ^[41] Proponents argue that scintigraphy may be more sensitive than CT or MRI for detection of ectopic thyroid tissue in the neck and that excision of TDC in a patient with no additional functioning thyroid tissue might result in permanent hypothyroidism. ^[45]

Bhatt and coworkers conducted a retrospective observational study of 26 patients with midline neck swelling who underwent technetium-99m (^99mTc) pertechnetate thyroid scintigraphy (TS) and compared TS findings with those of ultrasonography. Patients were injected with 1-5 MBq/kg of ^99mTc pertechnetate. Ultrasonography and TS showed similar results in detecting thyroid tissue in normal locations, but TS was better in detecting topically located thyroid tissue. ^[46]

Some experts believe that preoperative scintigraphy may not be 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 for pediatric patients.