Unsealed source radiotherapy
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|Unsealed source radiotherapy|
Unsealed source radiotherapy (also known as unsealed source radionuclide therapy (RNT) or molecular radiotherapy) uses solid, liquid, gaseous or other forms of radioactive substances called radiopharmaceuticals (essentially a radioactive drug). These are introduced into the body by various means (injection or ingestion are the two most commonplace) and localise to specific locations, organs or tissues depending on their properties and administration routes. This includes anything from a simple compound such as sodium iodide that locates to the thyroid via trapping the iodide ion, to complex biopharmaceuticals such as recombinant antibodies which are attached to radionuclides and seek out specific antigens on cell surfaces.
As such, this is a type of targeted therapy which uses the physical, chemical and biological properties of the radiopharmaceutical to target areas of the body for radiation treatment. The related diagnostic modality of nuclear medicine employs the same principles but uses different types or quantities of radiopharmaceuticals in order to image or analyse functional systems within the patient.
Compare this with sealed-source therapy (brachytherapy) where the radionuclide remains in a capsule or metal wire during treatment, is never allowed to disperse within the body & hence needs to be physically placed precisely at the treatment position.
Treatments with sodium (I131) iodide (radioiodine)
This is the most common RNT worldwide & uses the simple compound sodium iodide with the iodine-131 isotope of iodine. The patient (human or animal) may ingest an oral solid or liquid amount or receive an intravenous injection of a solution of the compound. The iodide ion is selectively taken up by the thyroid gland. Both benign conditions like thyrotoxicosis and certain malignant conditions like papillary thyroid cancer can be treated with the radiation emitted by radioiodine. Iodine-131 produces beta and gamma radiation. The beta radiation released damages both normal thyroid tissue and any thyroid cancer that behaves like normal thyroid in taking up iodine, so providing the therapeutic effect, 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 radioiodine treatment the urine will be radioactive or 'hot', and the patients themselves will also emit gamma radiation. Depending on the amount of radioactivity administered, it can take days to weeks for the radioactivity to reduce to the point where the patient does not pose a radiation hazard to bystanders. There are strict radiation protection regulations governing the use of all radiopharmaceuticals.
Other unsealed source therapies
Other types of RNT include:
|131I-MIBG (metaiodobenzylguanidine)||for the treatment of phaeochromocytoma and neuroblastoma|
|32P as sodium phosphate||for overactive bone marrow||the main place of use of phosphorus is the bone marrow|
|223Ra chloride, 89Sr chloride & 153Sm EDTMP||for secondary cancer in the bones||radium and strontium mimics calcium in the body; samarium is bound to the tetraphosphonate EDTMP and as before, phosphates are taken up by osteoblastic (bone forming) repairs that occur adjacent to some metastatic lesions|
|90Y colloidal suspension||radiosynovectomy in the knee joint|
|90Y resin or glass spheres (SIRT - selective internal radiation therapy)||for primary and metastatic liver cancer||Intra-arterial interventional radiology procedure|
Experimental antibody based methods - alpha emitters
At the Institute for Transuranium Elements (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 short-lived isotope from longer lived isotope is a useful method of providing a portable supply of a short-lived isotope. This is similar to the generation of technetium-99m by a technetium cow (US term) or generator (UK/Euro term). The actinium-225 is made by the irradiation of radium-226 with a cyclotron.