True thymic hyperplasia associated with a unilocular thymic cyst: an unusual combination not previously reported.

A mediastinal tumor mass in a 53-year-old woman with the sole complaint of persistent cough was studied and found to be a true thymic hyperplasia associated with a unilocular thymic cyst, felt to be of the congenital type. The bilobed thymus obtained was macroscopically and microscopically normal except for the size (12 x 12 x 3.5 cm) and weight (97 g). It showed normal thymic tissue segregated in lobules separated by adipose septa and with good corticomedullary separation. There were numerous Hassall corpuscles in the medullary portion. The thymus-to-adipose ratio was approximately 80:20. In the anterior aspect of the thymus, there was a 5-cm unilocular cyst, filled with a clear watery fluid, and with smooth and gray white inner surfaces. Microscopically, the cyst was lined by a cubic and low cylindrical nonciliated epithelium with normal-appearing thymic tissue in the cyst wall. This very unusual combination of true thymic hyperplasia and unilocular thymic cyst has not been previously reported in the literature.