A case of cystic thymoma

Abstract

We report a rare case of thymoma presenting with cystic changes. A 71-year-old man presented with an anterior mediastinal tumor on computed tomography (CT) during postoperative follow-up for gastric cancer. Six months later, the tumor enlarged, and he was referred to our department. Radiographic examinations revealed that the inside of the tumor had a homogeneous density equivalent to water. We suspected a thymic cyst with a malignant component, and performed video-assisted thoracic surgery to remove it. During surgery, the tumor showed no invasion to the adjacent organs, and was removed easily. The tumor, measuring 55×40×20 mm, had a unilocular cyst containing clear yellowish fluid, and was pathologically diagnosed as a type B1 thymoma. It is generally thought that cystic changes of a thymoma are due to hemorrhage and necrosis occurring within it, and whose contents are necrotic tissue or clots. In contrast, a cystic thymoma containing clear fluid, like in the present case, may arise due to mechanisms different from necrosis. We consider the possibility that a cystic thymoma may originate from a thymic cyst and, thereafter, invade the cyst wall to grow.