[Malignant transformation of a branchial cleft cyst].

Branchial cleft cyst is the most common birth defect involving the neck. Malignant transformation is known, however, differentiating from a neck metastasis of a squamous cell carcinoma of unknown primary is challenging. Even though there are strict criterias, the diagnosis of this entity remains controversial. We present the case of a 69-year-old woman, who presented with a swelling under the left side of the mandible. After diagnostic workup, fine-needle aspiration biopsy raised the suspicion of a cystic squamous cell carcinoma metastasis, therefore we performed panendoscopy and modified radical neck dissection. The pathological examination confirmed branchial cleft cyst carcinoma. After surgery, the patient received adjuvant radiation and chemotherapy. During the case workup, we present the difficulties of the diagnostic process, differential diagnostic problems, and the review of the international literature. In the case of a solitary, cystic mass on the neck without a primary tumor, we should consider the possibility of a branchiogenic carcinoma. Orv Hetil. 2023; 164(10): 388-392.