[Multilocular thymic cyst associated with rheumatoid arthritis].

A 66-year-old woman had been receiving treatment for rheumatoid arthritis( RA) since her 28 years of age. Chest computed tomography( CT) taken during follow-up showed an anterior mediastinal tumor of 35×20×65 mm in size as a region of heterogeneous internal density. Accumulation of 18F-fluorodeoxyglu cose was identified on positron emission tomography( PET)[ maximum standardized uptake value( SUV max) 8.7]. Thymic epithelial tumor( thymoma or thymic cancer) with cystic degeneration was initially suspected, so total thymectomy was performed. A multilocular, non-invasive tumor mass was completely resected. Pathological diagnosis was lymphoid follicular hyperplasia with concomitant multilocular thymic cyst( MTC). Surgery resulted in alleviation of RA symptoms and a decreased inflammatory response. Inflammation due to autoimmune disease is believed to play an important role in the formation of MTC. In the present case, surgery may have inhibited the progression of RA.