Thymic cyst appearing after treatment of mediastinal non-Hodgkin lymphoma

We report the first case of a thymic cyst appearing in the course of treatment for non-Hodgkin lymphoma of the anterior mediastinum. The patient was a 9-year-old child in whom an abnormal contour of the left cardiac border persisted after chemotherapy, suggesting residual disease. The mass was found at thoracotomy to be a benign thymic cyst. The lesion was not present 2 years previously, and most likely represented cystic degeneration of the thymus, secondary to lymphomatous involvement. CT scan was not helpful in distinguishing the cystic lesion from residual lymphoma.