[Branchial cyst of unusual localization: report of a case and considerations on organogenesis].

We report a case of branchial cyst of unusual location. A asymptomatic 41-year-old man had a nontender deeply located left neck mass. Sonography, CT scan, and MRI showed a cystic lesion posterior to the sternocleidomastoid muscle. The diagnosis of branchial cyst much debated because of this atypical location was confirmed by histologic analysis after surgical resection. During organogenesis, the important caudal proliferation of the second branchial arch generates a transient cavity, the cervical sinus, which finally becomes obliterated. The incomplete obliteration of which can result in a sinus, fistula or cyst. Such cysts typically lie at the level of the mandibular angle, anterior to the sternocleidomastoid muscle. This location has been regarded as a major diagnosis criteria, but it is not absolute. The sternocleidomastoid muscle develops apart from the branchial apparatus, caudally and anteriorly. As a result the cysts which are located on an inferior portion of the cervical sinus can lie posterior to this muscle.