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.

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A review of the subject was published in 1999 by Wynn A. Volkert and Timothy J. Hoffman.^[1]

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 neuroblastoma
• 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

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 halflife and chemically it behaves like calcium and so is concentrated in bones.^[2] 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. Cesium has a biological halflife of 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. 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.

http://itu.jrc.ec.europa.eu/

Notes:

1. ^ Wynn A. Volkert and Timothy J. Hoffman, Therapeutic Radiopharmaceuticals, Chemical Reviews 99(9) (1999); 2269–2292
2. ^ R. M. Macklis, The great radium scandal, Sci. Am. 296(2) (1993); 94–99