2017 Sec 1 Green Book

patient group. However, CT occasionally is contributive as a fast and readily available imaging technique in the emergency setting, and is helpful for lesions that involve bony structures and for the acute diagnostic work-up of infections/abscesses in the neck according to the American College of Radiology (ACR) Appropriateness Criteria. 9 Basic knowledge of the normal contents of each anatomi- cal space is essential in evaluating the various pathologies and masses that can arise within each space. The goal of this review is to identify the characteristic lo- cation and imaging features, differential diagnosis, and dif- ferentiating features of the most frequent congenital and ac- quired cervical mass lesions (lumps and bumps) in the pediatric population. Thyroglossal duct remnants are reported in 7% of the population. 10 These anomalies are the most common congen- ital anomalies of the neck, representing 70% of the congenital neck masses (second most common neck mass after cervical adenopathy in the pediatric age group). 6 The formation of the thyroid gland begins with an endodermal thickening in the floor of the primitive pharynx-tuberculum impar in the third embry- onic week. From this site, the thyroid diverticulum develops and its opening forms the foramen cecum. Due to progressive growth/elongation of the embryo, the diverticulum descends caudally into the neck and forms the thyroglossal duct. This duct is a temporary structure coursing from the foramen ce- cum at the tongue base, descending in the anterior midline, looping posterior to the hyoid bone, and continuing its descent anterior to the thyrohyoid cartilage and trachea to the level of the thyroid. The descent of the thyroglossal duct occurs before formation of the hyoid bone, and therefore remnants of the duct can become trapped within the hyoid bone. 6,11 The duct generally involutes around gestational week 8–10. Thyroglossal duct cysts are diagnosed in 40% of surgically removed neck masses in the pediatric population. 6 The lesion manifests from infancy until young adulthood. Thyroglossal duct cysts are often asymptomatic until enlargement or inflam- mation occurs associated with infection or trauma. The cyst Congenital Cystic Masses Thyroglossal Duct Anomalies

arises from a remnant of the thyroglossal duct. Any portion of the duct, from the foramen caecum to the pyramidal lobe of the thyroid gland, may persist and cause cyst formation due to se- cretions from the epithelial lining of the duct. 1,12 In pathologic specimens of excised thyroglossal duct cysts, thyroid tissue is reported in up to 62% of cases. 13 As a result of the course of the duct, thyroglossal duct cysts are primarily located in the mid- line (75%) or just off-midline (25%). Paramedian location occurs more often on the left. The vast majority of the cysts are located at or about the level of the hyoid bone, and a minority has an infrahyoid position or is located in the suprahyoid neck. 1,3 The thyroglossal duct cysts located at the level of the tongue base can be difficult to differentiate from vallecular cyst occurring at the exact same location based on imaging appearance alone. The imaging features of thyroglossal duct cysts comprise a well- defined, thin-walled cystic structure in the typical midline or paramidline location. Heterogeneous cyst content may reflect proteinaceous material/hemorrhage or infection as evidenced by debris and fluid levels both on US and MR imaging. In par- ticular, T1 hyperintensity may indicate prior hemorrhage, and this can be further verified with SWI. Noncomplicated cysts do not demonstrate wall thickening or postcontrast enhancement (Fig 1). The cyst wall thickness and degree of contrast enhance- ment of the cyst wall may vary with inflammation/infection. US enables dynamic evaluation, a characteristic imaging upward movement of the cyst with tongue protrusion or swallowing due to the origin of the duct at the foramen cecum is typically observed. 12 The majority of the cysts show a close relation to the hyoid bone. At the level of the hyoid bone, the cyst may demonstrate a tail “diving” into the hyoid bone, reflecting en- trapment of duct remnants in the hyoid bone. To minimize the risk of recurrence after resection, removal of the central portion of the hyoid bone is typically included in the curative excision of the cyst and/or sinus tract (eg, Sistrunk procedure, modified Sistrunk procedures). 14 Factors influencing the recurrence rate after the Sistrunk procedure (2.6%) are, eg, postoperative infec- tion, the presence of multiple tracts and distorted anatomy due to preoperative infection. 15,16 Infrahyoid cysts favor an off-midline position, deep to or embedded within the strap muscles (Fig 2) with a tail toward the midline (Fig 2). 3 Evaluation of the presence of a normal thyroid gland in the normal location is important in the preoperative

Fig 1. Sagittal contrast-enhanced T1-weighted (A) and sagittal fat-suppressed T2-weighted (B) MR images of a child with a suprahyoid thyroglossal duct cyst. The images reveal a well-defined, thin-walled cystic structure in the typical midline location at the level of the foramen cecum at the tongue base. No cyst wall enhancement is encountered on postcontrast imaging. Differentiation between a thyroglossal duct cyst located at the tongue base and a vallecular cyst is not possible based on imaging appearance alone.

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