Any thymoma larger than 1cm requires surgery. The reason for this is because in thymoma larger than 1cm, it may develop into an infiltrative thymoma. When it is less than 1cm, it has little effect on the surrounding tissues and can be treated under close observation; for thymoma larger than 1cm, surgery is required for any thymoma larger than 1cm because there is no guarantee that it will not develop into an infiltrative adenoma. Thymoma is divided into benign thymoma and malignant thymoma. Benign thymoma grows within the peritoneum and has no infiltration relationship with the surrounding tissues, and it will not recur and metastasize after resection. Malignant thymoma grows through the pericardium, infiltrates with the surrounding tissues, and invades the pleura, lungs, and blood vessels without complete resection, so it may recur and metastasize after resection, and it needs radiotherapy and chemotherapy after surgery.