A case of a shrunken multilocular mediastinal cyst that developed into thymic carcinoma with lung metastases 13 years later.

Multilocular thymic cysts (MTC) are acquired multilocular cysts caused by inflammation. The rarity of such lesions and a lack of recognition make diagnosis and treatment difficult. Herein, we present our experience with a multilocular mediastinal cyst that resulted in the development of thymic cancer with metastasis over a period of 13 years. Computed tomography findings revealed an anterior mediastinal mass that was suspected to be an MTC in a 49-year-old man. The mass shrank gradually over a period of 7 years; however, growth was observed at 10 years after initial detection. At 13 years after detection, thymic carcinoma with multiple lung metastases was diagnosed. Resection was recommended during the follow-up period, but the patient refused treatment. A multilocular wall and location are factors that indicate MTC. However, even if a definitive diagnosis is not made, resection of multilocular anterior mediastinal cysts should be considered as determining the preoperative diagnosis is difficult. Nevertheless, our case suggests that the coexistence of tumors with cysts is possible, and the potential for malignant tumor development exists.