Unsealed source radiotherapy
Unsealed source radiotherapy relates to the use of soluble forms of radioactive substances which are administered to the body by injection or ingestion. Such substances are typically used for their biological properties, which are similar to their non-radioactive parent substance.
A review of the subject was published in
1999by Wynn A. Volkert and Timothy J. Hoffman. [Wynn A. Volkert and Timothy J. Hoffman, "Therapeutic Radiopharmaceuticals", Chemical Reviews 99(9) (1999); 2269–2292]
iodineis an element selectively taken up by the thyroid gland in healthy people. Thyroid disease (both benign conditions like thyrotoxicosisand 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 protectionregulations governing the use of these sources.
Other unsealed sources include:
* I-131-MIBG (
metaiodobenzylguanidine) for the treatment of phaeochromocytomaand neuroblastoma
* P-32 for overactive bone marrow (the main place of use of
phosphorusis the bone marrow)
* Sr-89 & Sm for secondary cancer in the bones (
strontiumand samariumbehave just like calcium)
* Y-90 (
yttrium) for radiosynovectomyin the kneejoint Radium-226 and caesium-137 are two classic examples of isotopes which are unsuitable for use in this type of radiotherapy.
* Radium has a very long physical
halflifeand chemically it behaves like calcium and so is concentrated in bones. [R. M. Macklis, "The great radium scandal", Sci. Am. 296(2) (1993); 94–99] It will remain within the body far too long and it would continue to irradate the bone marrow for the rest of the life of any patient.
* Caesium acts like potassium and enters all the cells of the body, thus it does not concentrate in a single organ. As a result it is not possible to deliver a high dose to a single part of the body using
cs-137. Caesium has a biological halflifeof between one and four months in humans.
Experimental antibody based methods - alpha emitters
At the ITU work is being done on Alpha-Immunotherapy, this is an experimental method where antibodies bearing alpha isotopes are used.
Bismuth-213 is one of the isotopes which has been used. This is made by the alpha decay of Ac-225. The generation of one shortlived isotope from longer lived isotope is a useful method of providing a portable supply of a shortlived isotope. This is similar to the generation of technetium-99m by a technetium cow. The actinium-225 is made by the irradiation of radium-226 with a cyclotron.
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