A Hidden Rarity: Diagnosing and Managing Cervical Thymic Cysts in Pediatric Patients.

Congenital thymic cysts are rare benign neck masses found primarily in the paediatric population, comprising only 0.3% of congenital neck cysts in children. These cysts arise from embryonal thymic tissue along its descent path, with mediastinal extension seen in approximately 50% of cases. Due to their rarity, thymic cysts are often misdiagnosed as branchial or lymphangiomas. A 10-year-old boy presented with a progressively enlarging, painless neck mass. Imaging studies, including ultrasound and CT scan, revealed a fluid-density lesion in the parapharyngeal space extending to the mediastinum. Fine needle aspiration cytology initially suggested an infected branchial cyst. Surgical excision was performed, revealing a 9.5 × 2.5 × 1.5 cm cystic mass. Histopathology confirmed the diagnosis of a cervical thymic cyst, with features including Hassall's corpuscles and normal thymic tissue. Thymic cysts, often misdiagnosed due to their rarity, should be considered in the differential diagnosis of lateral neck masses in children. While typically found on the left side, they can occur elsewhere. Imaging is crucial for preoperative diagnosis, but surgery poses challenges due to the cyst's proximity to vital structures and due to mediastinal extension. Complete excision is advised; however, in young children, if mediastinal thymic tissue is absent, total removal should be avoided to prevent immune dysfunction. Greater clinician awareness is essential to reduce misdiagnosis and ensure proper management.