Ectopic Thymic Cyst Presenting as Lateral Neck Mass in a Young Boy: A Case Report

The thymus is derived from ectoderm and endoderm of third pharyngeal pouches. It descends down along the cervical region to anterior mediastinum and involutes by increasing age. The ectopic thymic tissue in neck is a rarity and few mentioned in literature are described as sporadic case reports. Ectopic thymic cyst accounts for less than 1% of all cystic neck masses in children and even more rare in adults. Worldwide literature states that there are about 150 cases of congenital ectopic thymic tissue. It can occur anywhere along the path of its descent, from mandible to mediastinum. This entity being unusual is infrequently considered in the differential diagnosis of cervical cystic masses. Histopathology forms the mainstay in confirmation of diagnosis. Here, we report a case of 12-year-old boy who presented with right-sided neck mass. The excised specimen revealed presence of lymphoid tissue with Hassalls corpuscles in the wall of cyst. This hallmarking feature is the pointer to arrive at a correct diagnosis of thymic cyst and to exclude its closest differential at this anatomical location including branchial cyst, cystic hygroma, lymphadenopathy and epidermoid cyst. The literature is reviewed and differentials of ectopic thymic cyst in cervical region are being discussed.