Thymoma type b2 is a thymoma whose pathological type is cortical in nature. During surgery, it is often found that a part of b2 type thymoma, which grows through the envelope and invades the surrounding adipose tissue, pleura and pericardium, can be completely resected and may recur after surgery; therefore, it belongs to stage ll thymoma. Another part of b2 thymoma, which grows through the envelope, has infiltration with the surrounding tissues, invades the lung and large blood vessels, and cannot be completely removed; therefore, this part of thymoma would belong to stage lll. The actual pathological type of thymoma is not directly related to the stage, however, it can be determined by observing the pathological type of tumor and its relationship with the surrounding tissues during surgery to determine what stage it belongs to.