We report the case of a 66-year-old man with a cervical neck mass located behind the left sternocleidomastoid muscle. To exclude malignancy, a full workup, including clinical, radiological, and cytological examination, was performed but failed to provide a definitive diagnosis. Histological analysis following excisional biopsy revealed a benign epithelial cyst, consistent with an atypically located branchial cyst. We describe an approach to the management of these neck masses and discuss several theories of the etiology of branchial cysts and how they may come to be abnormally located. 1. Case Presentation A 66-year-old man was referred to our department with a 2-day history of a painless left cervical neck mass. He denied any other symptoms and reported that he only occasionally drank alcohol and had stopped smoking some 30 years ago with a total of 10 pack years. Clinical examination revealed a well-defined, painless neck lump, posterior to the sternocleidomastoid muscle close to the mastoid. The lump was some 5 centimeters in size, roughly oval, and not fixed to adjacent structures. There were no surrounding skin changes or other associated findings. Intraoral examination and transnasal fiber-endoscopy were normal. Cervical ultrasound showed an irregularly walled mass, 5？cm in diameter, with hyperechogenic reflections in an echo-poor center. The other neck structures were sonographically normal. Magnetic resonance imaging (MRI) showed a cystic lesion with an irregular wall lateral and posterior to the sternocleidomastoid muscle (neck level five; see Figures 1 and 2). Repeated fine-needle aspiration cytology (FNAC) revealed squamous epithelial cells without signs of malignancy, consistent with a branchial cleft cyst. However, given the patient’s age and the atypical localization, the differential diagnosis included a metastasis of a well-differentiated squamous cell carcinoma (SCC). We therefore recommended the patient to undergo panendoscopy and extirpation of the mass with intraoperative frozen section analysis of the specimen. The patient agreed with neck dissection in case of malignancy. Panendoscopy revealed no extra findings, and both intraoperative frozen section and definitive histology of the excisional biopsy confirmed a branchial cleft cyst. The patient recovered well after surgery and was discharged from follow up 12 months later. Figure 1: Axial slides of T1-weighted MRI showing a hyperintense cystic lesion behind the left sternocleidomastoid muscle in the neck level five. Figure 2: Axial slides of T2-weighted MRI showing a hyperintense
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