SUMMARY:Third branchial cleft cysts (BCCs) are rare entities that represent abnormal persistence of the branchial apparatus. On CT examination, these cysts appear as homogeneous low-attenuation masses with well-circumscribed margins; on MR imaging, they demonstrate variable signal intensity on T1-weighted images and are hyperintense relative to muscle on T2-weighted images. Definitive treatment is surgical excision. We present a case of a third BCC and describe its diagnosis and treatment. Most cases of third branchial cleft cysts (BCCs) are diagnosed in childhood and show a marked preference for the left side (97%).1 Prenatal diagnosis is uncommon. Here, we present an example of this rare anomaly that was diagnosed prenatally. The embryologic development and radiologic evaluation of third BCCs are discussed. Case ReportA female neonate was delivered by planned cesarean delivery at 34 weeks' gestation on the basis of the presence of a neck mass resolved by prenatal ultrasonography, consistent in location with a type 3 BCC. At delivery, physical examination revealed no tracheal abnormalities, fistulas, or neck masses, but intraoral examination showed fullness of the posterior oropharynx on the left side. MR imaging examination demonstrated a large cystic structure in the retropharyngeal space, extending from the nasopharynx through the thoracic inlet (Fig 1). On direct laryngoscopy, palpation of the neck produced clear fluid from an opening at the inferior aspect of the left pyriform sinus, and a left type 3 BCC was diagnosed. Cauterization of this opening was performed; however, postoperatively the mass failed to resolve. On day of life (DOL) #22, intraoperative sonography-guided drainage of the cyst was performed for decompression and produced straw-colored fluid and air. Definitive resection was completed on DOL #36. The surgical procedure included a left thyroid lobectomy and excision of a 3 ϫ 3-cm cystic mass adherent to the left thyroid lobe with a tract entering the pyriform sinus. Pathologic evaluation was consistent with a BCC (Fig 2). DiscussionMost cystic lesions in infants and children are congenital or developmental in origin and reflect aberrancies in embryogenesis. The differential diagnosis most commonly includes thyroglossal duct (TGD) cysts, lymphatic malformations (LMs), and BCCs. The appropriate radiologic evaluation for these masses depends on their location, extent, and presumed consistency (cystic vs solid). To determine consistency, ultrasonography offers many advantages: it does not require sedation, does not expose the patient to radiation, and can determine easy establishment of whether the lesion is solid or cystic. CT scanning and MR imaging are preferred when the lesion is extensive or when it crosses multiple anatomic spaces. 2 A TGD cyst usually presents as a palpable, nontender midline neck mass that elevates with swallowing or protrusion of the tongue. Ultrasonography is the imaging technique of choice for these lesions, which will have a variable appearance (anechoic, homogeneou...