Thymoma arising within a unilocular thymic cyst during follow-up: a report of a resected case.

Introduction and importance: We report a case of a thymoma that developed in the cyst wall during follow-up of a unilocular thymic cyst. Case presentation: A 70-year-old woman was found to have abnormal findings on a computed tomography (CT) scan of the chest. The chest CT revealed a well-circumscribed, hypodense mass in the left anterior mediastinum. During follow-up 5 years later, a small nodule emerged within the cystic lesion. Surgical resection was performed to establish a definitive diagnosis, which confirmed a type A thymoma, classified as pathologic stage I, arising within a thymic cyst. Clinical discussion: Observation is acceptable in cases of congenital cysts arising in the mediastinum. However, when irregularities or nodules are present in the cyst wall, as in this case, surgical intervention should be considered. Conclusion: This case highlights the occurrence of thymoma within the wall of a unilocular congenital thymic cyst during follow-up. The possibility of such transformation should be considered when planning surveillance, determining its duration, and treating a unilocular thymic cyst.