Unsealed source radiotherapy relates to the use of soluble forms of radioactive substances which are administered to the body by injection or ingestion. There is one common feature to all these substances, and that is the biological role of the non-radioactive parent substance.

For example, iodine is an element selectively taken up by the thyroid gland in healthy people. Thyroid disease (both benign conditions like thyrotoxicosis and malignant conditions like papillary thyroid cancer) can be treated with radioactive iodine (iodine-131) which is then concentrated into the thyroid. Iodine-131 produces beta and gamma radiation. The beta radiation released destroys thyroid tissue, and any thyroid cancer that takes up iodine whilst most of the gamma radiation escapes the patient's body.

Most of the iodine not taken up by thyroid tissue is excreted through the kidneys into the urine. After radioactive iodine treatment, the urine will be radioactive or 'hot', and the patients themselves will also be radioactive. Depending on the amount of radioactivity administered, it can take days to weeks for the radioactivity to reduce to the point where the patient is not a radiation danger to bystanders. There are strict radiation protection regulations governing the use of these sources.

Other unsealed sources include:

• I-131-MIBG (metaiodobenzylguanidine ) for the treatment of phaeochromocytoma and nueroblastoma
• P-32 for overactive bone marrow (the main place of use of phosphorus is the bone marrow)
• Sr-89 & Sm for secondary cancer in the bones (strontium and samarium behave just like calcium)
• Y-90 (yttrium) for radiosynovectomy in the knee joint

There are problems with using radium-226 and caesium-137. Radium behaves like calcium and so is concentrated in bones (Macklis RM, The great radium scandal, Sci.Am.93 Aug 296(2);94-99), while caesium acts like potassium and enters the cell. If either radioactive substance is ingested accidentally then the radioactive substance is locked inside the body. The half life of radium-226 is about 1600 years, a lot longer than the lifespan of any cancer and any patient!