Multilocular thymic cysts with pseudoepitheliomatous hyperplasia.

Six cases are described of benign thymic cysts of the anterior mediastinum showing focal pseudoepitheliomatous hyperplasia of the lining epithelium. The patients' ages ranged from 11 to 54 years; five cysts occurred in males and one in a female. Histologically, the lesions were characterized by exuberant proliferation of the cyst lining epithelium that grew as sheets and tongues of atypical squamous cells with large, hyperchromatic nuclei, prominent nucleoli, and scattered mitotic figures. The walls of the cyst adjacent to the areas of epithelial proliferation showed abundant hemorrhage, necrosis, and severe inflammatory changes. All cases were treated by local surgical excision. There was no evidence of recurrence or metastases over a follow-up period of up to 8 years (average follow-up, 4 years). It is proposed that pseudoepitheliomatous hyperplasia may develop in thymic cysts as an expression of regeneration of the lining epithelium in response to the inflammatory, hemorrhagic, and necrotizing changes which often accompany these lesions. This should not be mistaken for malignancy, and should be distinguished from the exceptional cases of true thymic neoplasms seen in association with thymic cysts.