P3C266: The Value of MRI in Diagnosing Thymic Lesions

Introduction: Though the anterior mediastinum is the most common site for chest mass in childhood, only 1-4% of diagnosed mediastinal masses are thymic cysts. Accurately diagnosing these lesions can be challenging because they are typically asymptomatic and hence ordering the correct imaging study can be crucial.

Case Description: Our patient is a previously healthy 15-year-old female who presented with sore throat for one week and three days of throat swelling, fever, chills, and night sweats. A CT neck at an outside hospital showed 11 x 22 mm pretracheal mass at the level of the thoracic inlet and bilateral cervical lymphadenopathy. Notably, she had a family history of lymphoma.

She was sent in for admission by an outside hospital to evaluate for lymphoma given lymphadenopathy, anterior mediastinal mass, B symptoms of night sweats and fevers, and a positive lymphoma family history. CBC and iron studies were consistent with microcytic anemia and iron deficiency. She had a down-trending LDH, normal uric acid and AFP, and a peripheral smear without blasts or evidence of a lymphoproliferative neoplasm. Her infectious workup was positive for EBV and elevated inflammatory markers (ESR 47). Oncology was consulted and they had no concern for malignancy. ENT was consulted, and due to concern for acute tonsillitis, she was started on Unasyn. MRI Neck was recommended to further evaluate the mediastinal mass. MRI showed tonsillitis without abscess, reactive cervical lymphadenopathy, and ectopic thymic tissue with a central cyst at the pre-vascular upper mediastinum at the level of the thoracic inlet.

Discussion: Mediastinal thymic cysts are rare findings and hence challenging to diagnose. ~60% of patients with thymic cysts are asymptomatic and most are incidentally discovered on imaging in the first two decades of life. If symptoms are present, they are generally cough, dyspnea, and chest pain.

Benign thymic lesions are susceptible to misinterpretation on imaging. In our case, it led to a costly workup and significant emotional distress for the family. On CT scan thymic hyperplasia can be visualized as grossly visible fat or alternatively may show diffuse soft tissue attenuation which can resemble a neoplasm. An MRI has superior utility in distinguishing cystic tissue from solid masses, identifying cystic components within solid masses, and discerning thymic hyperplasia from thymic tumors. Our patient is not the first to have this incidental finding worked up extensively. Previous cases include children with mediastinal masses who underwent thoracotomy for benign ectopic thymic tissue. At times these cysts are surgically removed to confirm diagnosis.

Conclusion: Mediastinal thymic cysts are incredibly rare and typically asymptomatic anatomic anomalies. Given this rarity, when they are incidentally found on imaging, they can lead to expensive, time-consuming, and psychologically stressful work-ups for families. Obtaining an MRI can be essential in accurately identifying thymic lesions.