Synchronous Nodular Epithelial Hyperplasia and Type A Microthymoma Adjacent to a Thymic Cyst: A Report of a Rare Case

Thymic cysts are uncommon benign lesions of the anterior mediastinum and are frequently detected incidentally on imaging studies. Microscopic thymoma and microthymoma are extremely rare microscopic thymic epithelial proliferations that are usually identified only by histopathological examination. Their relationship to thymic cysts and to each other remains poorly understood. A 54-year-old woman was referred to our institution after an anterior mediastinal lesion was incidentally detected on routine chest computed tomography (CT). Magnetic resonance imaging and fluorodeoxyglucose positron emission tomography/CT findings were consistent with a thymic cyst. Robot-assisted thoracoscopic resection of the cyst with the adjacent thymic tissue was performed for definitive diagnosis and treatment. Histopathological examination revealed a unilocular thymic cyst accompanied by two incidental microscopic thymic epithelial lesions within the adjacent thymic tissue: a 0.6-mm microscopic thymoma and a 2.0-mm Type A microthymoma. Based on their characteristic size and histological features, both lesions were diagnosed without additional immunohistochemical analysis. The postoperative course was uneventful, and the patient remains free of recurrence 18 months after surgery. To our knowledge, the coexistence of microscopic thymoma and Type A microthymoma adjacent to a thymic cyst is exceptionally rare. The close proximity and similar histological features of these lesions suggest that they may represent different stages within a continuous morphological spectrum of microscopic thymic epithelial proliferations rather than entirely distinct entities. Furthermore, this case highlights the potential value of careful pathological examination of thymic tissue adjacent to resected thymic cysts for detecting occult microscopic epithelial lesions and improving pathological assessment.