It has been experimentally and clinically proven that thyroid cancer is not sensitive to chemotherapy drugs, which means that chemotherapy is not effective for thyroid cancer. Thyroid cancer is sensitive to radiation therapy, especially radionuclide therapy (commonly known as internal radiation therapy). Its principle is to take advantage of the biological property of thyroid tissue and differentiated thyroid cancer cells to take up 131 iodine, and use the biological effect of ionizing radiation emitted from 131 iodine to destroy the residual thyroid tissue and metastatic cancer cells, so as to achieve the treatment purpose. However, not every thyroid cancer patient needs radiotherapy. So what circumstances require post-surgical radiotherapy? It is also known as the indications for the application of radiotherapy: 1, incomplete excision of differentiated thyroid cancer (papillary, follicular and medullary carcinoma) with residual thyroid tissue; 2, iodine 131 absorption rate > 1%; 3, isotope scan of thyroid bed with residual thyroid shadowing. For those who are too old and have other diseases, end-differentiated carcinoma or distant metastases such as bone, external radiation therapy such as cobalt-60 can also be used. Precautions before taking 131 iodine 1. No iodine-containing contrast examination should be done within one month before taking 131 iodine. Such as iodine oil imaging, angiography, biliary tract imaging. Do not take Chinese medicine, potassium iodide or Lugol’s solution. 2. Do not rub iodine, consume seafood such as kelp, nori, jellyfish, seaweed, sea fish, sea shrimp, sea clams, etc., or foods with high iodine content such as cabbage and lettuce within two weeks before the examination. 3. Stop using Wassup tablets, iodine-containing throat tablets, grass coral, watermelon cream, cortisone, sulfonamides, sex hormones, contraceptives, anti-tuberculosis drugs, thyroxine tablets, etc. two weeks before the examination. 4. 4. Breastfeeding women should stop breastfeeding and stay away from infants and children after taking 131 iodine. 5. Patients should have an empty stomach at the time of consultation and should not eat until two hours after taking 131 iodine. 6. Take rest and avoid overexertion after taking 131 iodine. 7. Avoid taking iodine-containing drugs, seafood and iodized salt for one month after taking 131 iodine to avoid reducing the therapeutic effect. 8. 8. Keep a distance of more than one meter from family members for 3 days, sleep in separate beds for 2 weeks, and avoid contact with infants and children for 1 month. Because 131 iodine is radioactive, it can have an adverse effect on the development of the thyroid gland in infants and children. 9. Flush the urinal after urination and defecation to avoid radiation of radioactive iodine from the urine and defecation to family members.