Most thymomas are low-grade malignant. Only thymoma type A is benign, growing within the envelope, and can be completely cured after surgery without radiotherapy. Type A thymomas are medullary thymomas and spindle cell thymomas, which are benign. Other thymomas such as type AB thymoma and type B thymoma, of which type B thymoma includes type B1, B2 and B3 thymoma, are potentially malignant and are determined to be benign or malignant based on whether they infiltrate and adhere to surrounding tissues during surgery. type C thymoma is thymic carcinoma and is definitely malignant thymoma. Therefore, thymoma type AB, thymoma type B and thymoma type C, all require radiotherapy after surgery.