The thymus gland degenerates at the age of pre-puberty, usually at the age of 13 or 14, and after puberty, the thymus gland is basically degenerated, usually by the age of 18. The thymus gland is usually not visible during the CT chest examination, but if the occupancy of the anterior mediastinum can be seen after the age of 18, and the occupancy is larger than 1 cm, surgery should be performed, because a thymoma larger than 1 cm cannot be diagnosed as benign or malignant, because a thymoma larger than 1 cm may break through the envelope and grow, which means a malignant thymoma, and surgery should be performed. Treatment should be performed by surgery. If a breakthrough is found, postoperative radiotherapy should be performed.