Differentiated thyroid cancer (including papillary thyroid cancer and follicular thyroid cancer), especially papillary thyroid cancer requires iodine-131 radiation therapy after surgery. The specific circumstances are as follows: 1. The tumor is known to have distant metastases to the lung, bone and other organs (M1); 2. The tumor is visible to the naked eye intraoperatively and invades the subcutaneous soft tissue, larynx, trachea, esophagus, recurrent laryngeal nerve, prevertebral fascia or encircles the carotid artery and mediastinal vessels (regardless of the size of the tumor, T4); 3. The primary tumor is more than 4 cm in diameter (T3); 4. diameter is between 1-4 cm, but there is proven lymph node metastasis or other intermediate or high risk of recurrence and death. These include: surgical pathology suggesting microscopic tumor invasion of soft tissue around the thyroid, highly invasive histological manifestations (e.g., hypercellular, columnar cell, insular cell, diffuse sclerosis, hypofractionated carcinoma, follicular carcinoma, eosinophilic carcinoma, etc.) or vascular invasion, incomplete tumor resection, and hypothyroglobulinemia.