|MadSci Network: Medicine|
Nuclear medicine is usually used in the diagnosis of diseases, not their treatment. But you are very right that some diseases are in fact treated by nuclear medicine. Before I get to them, I would like to talk a little about diagnostic nuclear medicine and about the treatment of diseases by use of radiation. Diagnostic nuclear medicine works by administering radioactive materials to a patient and seeing where they are taken up. These materials are selected because they are concentrated in the area of interest. They also emit radiation so that they can be detected and create an image for evaluation. Common nuclear medicine diagnostic studies are bone scans, heart scans, thyroid uptake and scans and pulmonary scans. These diagnostic studies use small amounts of radiation. Such small amounts produce no detectable harm to cells. But large amounts of radiation can cause harm; they can damage or kill cells. Soon after the discovery of x-rays it became known that high doses of x-rays (about 100,000 times the amount used for diagnostic studies) could be used to kill cancer cells while sparing normal cells. This is possible because 1) cancer cells are more sensitive to radiation than healthy cells, and 2) the radiation beam could be aimed to hit the cancer while avoiding to some degree the surrounding structures. The same line of reasoning applies in the field of nuclear medicine. If small doses of radiation could be used for diagnosis, could larger doses be used for treatment? In some cases, the answer is “yes”. Here are some diseases that are treated by nuclear medicine: Thyrotoxicosis (Grave’s Disease) – In this condition the thyroid is overactive and produces too much thyroid hormone. Radioactive Iodine-131 is given orally and is taken up very strongly by the thyroid. The amount of I-131 given is calculated to be enough to damage thyroid function and so reduce the production of thyroid hormone. Thyroid Cancer – A large dose of I-131 is given. The principle is the same as in the treatment of thyrotoxicosis, but the dose is large enough to kill the thyroid cells and the cancer cells. This also can kill distant metastases. Bone Cancer – Many cancers can spread to the bone. These bony metastases can produce severe and intractable pain. The radioactive materials Strontium-90 and Samarium-153-EDTMP go to the metastatic sites in the bone. They do not kill the cancer; rather, they decrease the sensitivity of the bone to pain. Thus, they are palliative treatments rather than curative treatments. Polycythemia Vera – In this disease there is an increased number of red blood cells and an enlarged spleen. The radioactive element Phosphorus-32 is taken up by the bone marrow. When given in the correct dose it reduces but does not destroy the production of red cells. Malignant Pleural or Peritoneal Effusions – Some cancers produce fluid in the chest (pleural effusions) and in the abdomen (peritoneal effusions.) One way of treating these is to inject a Phosphorus-32 compound. The radiation decreases the production of fluid. This is a palliative treatment. Rheumatiod Arthritis and other joint diseases – Some joint diseases produce thickened joint lining (synovium) which can produce a joint effusion and destroy bone and cartilage. Sometime the synovium can be surgically removed. Sometimes, in a procedure called “radionuclide synovectomy”, a radioactive agent is injected into the joint and used to destroy the overgrown joint lining. Many different elements can be used for this. Liver Cancer –The injection of I-131 containing Lipiodol (an oil-based liquid) through a catheter into the hepatic artery can be used as a palliative treatment for hepatocellular carcinoma. Work is being done on using monoclonal antibodies tagged with radioactive elements. Attempts are sometimes made to use nuclear medicine to treat certain brain tumors and adrenal tumors. And more advances will surely be coming.
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