Thymic Hyperplasia Mimicking Lymphoma in a Pediatric Patient: A Case Report.

Mediastinal masses in children often raise concern for malignant conditions, particularly lymphoma. The thymus, which is normally prominent in infants and young children, begins its physiological involution around the age of six to seven years; however, this process is gradual and can extend into adolescence. Thymic hyperplasia beyond this age is uncommon, and it can mimic a mediastinal tumor on imaging. We report a case of benign thymic hyperplasia in a 10-year-old asymptomatic patient who was initially suspected of having lymphoma based on an anterior-superior mediastinal mass seen on an external CT scan. Imaging showed a homogeneous mass occupying the thymic lodge, associated with bilateral cervical lymphadenopathy. Although the clinical and imaging findings were suggestive of benign thymic hyperplasia, the presence of a large anterior mediastinal mass with bilateral cervical lymphadenopathy warranted exclusion of T-lymphoblastic lymphoma. Therefore, a mediastinal biopsy was performed, confirming the benign nature of the lesion. This report underscores the importance of considering malignancy in the differential diagnosis and using biopsy judiciously to achieve diagnostic certainty when imaging alone cannot fully exclude aggressive pathology.