Two different types of radiotherapy are used in the treatment of thyroid cancer. The choice of which sort of radiotherapy you have depends on which kind of thyroid cancer has been diagnosed.
The one type of radiotherapy is called external beam treatment. This involves a course of treatment, usually as an out-patient, attending five days a week, Monday to Friday, for up to six weeks. At each treatment you lie on a couch under a machine for a dew minutes. The machine treats the area of the thyroid gland, in the front of your neck, with a beam of high energy x-rays , to kill any cancer cells. This treatment does not make you radioactive and you can mix normally with other people throughout your course of radiotherapy.
The second type of radiotherapy uses a radio-active chemical called radio-iodine. The radio-iodine is concentrated in thyroid tissue and is also taken up by some types of thyroid cancer cells. This treatment gives a high dose of radiation to those cancer cells, and normal thyroid cells, but a low dose to all other normal tissues.
The radio-iodine is taken as a capsule and treatment is usually given as an in-patient in hospital. This is because you will be quite radio-active for a few days afterwards as the radio-iodine stays in your body for a little while. This radio-activity will not make you feel ill in any way but it does mean that your visitors will be limited to spending only a few minutes each day with you. Also, young children and pregnant women will not be allowed to visit you.
The hospital team will measure your radioactivity and as soon as this has dropped to a safe level you will be allowed home. Even then you will usually be told not to go back to work and to avoid contact with small children and pregnant women for about two weeks, while the final traces of radioactivity disappear from your system.
Whilst you are in hospital after the radio-iodine treatment drinking lots of fluids can help to flush the radio-iodine out of your body and so you will be encouraged to drink plenty.
Incidentally, the regulations about safe levels of radioactivity after radio-iodine treatment changed in the UK in the spring of 2000, recommending longer times for avoiding contact after treatment than previously.