Six cases of large thymic cyst are described; five were cystic thymomas and one was nonneoplastic. Two of the thymomas were not only grossly degenerate but showed severe chronic inflammatory change, so that the diagnosis might be missed. (Davis, 1956). Although the clinical diagnosis of thymic cyst must ultimately await histological examination, it is hoped that a review of the subject will help the clinician in the differential diagnosis of mediastinal masses, especialiy as these cysts do not appear to be as rare as the literature suggests.
MATERIALThe records of the Institute of Pathology and Department of Thoracic Surgery at the London Hospital were examined over the 16-year period from 1950 to 1965. Mediastinal tumours and cysts that were surgically treated during this period were reviewed. All cases of lymphoma and retrosternal goitre were excluded. The different lesions encountered are shown in Table I. The tumours classified as neurogenous included neurofibroma, ganglioneuroma, and neurilemmoma or their malignant counterparts. 'Present address: