Thymoma with intracystic dissemination arising in a unilocular thymic cyst

A 33-year-old asymptomatic woman was referred to our hospital for evaluation of an abnormal shadow on a chest radiograph. A chest-computed tomogram revealed a cystic, partially solid tumor in the right inferior mediastinum. Following an initial diagnosis of cystic teratoma, surgery was performed. The tumor, a well-encapsulated, thin-walled cyst located within the right inferior lobe of the thymus, was extirpated by partial resection of the thymus. The cut surface revealed a unilocular cyst containing multiple grayish-white tumors on the inner cyst walls. Some tumors detached from the wall owing to loose connections. Microscopic findings revealed that the cyst was lined with squamous epithelium. The tumor on the cyst wall was diagnosed to be a type B1 thymoma. Based on these findings, we diagnosed this case as a thymoma originating from the wall of a thymic cyst, accompanied by intracystic dissemination from the tumor.