Nuclear medicine employees radioisotopes to diagnose or treat illnesses. Radioisotopes are isotopes of elements which are unstable and prone to radioactive decay during which they may emit alpha, beta and/or gamma radiation. There are over two hundred radioisotopes in commercial use today. Most of these isotopes are manufactured in nuclear reactors by neutron activation. Extra neutrons are inserted into the nucleus. This does not change the element. The other method of production involves inserting protons into the nucleus in a cyclotron. This results in a change in the type of element. Nuclear medicine was developed in the 1950s in the field of endocrinology with the use of iodine-131 to diagnose and treat thyroid disease. Today, over ten thousand hospitals in the world use radioisotopes.
Diagnosis
Ninety percent of the radioisotopes are used for diagnosis, primarily to aid in imaging specific organs. Short lived radioisotope that emit gamma radiation are linked to specific chemical compounds that are involved in particular physiological processes in the body. These tracers are injected, inhaled or swallowed to be taken up by a targeted organ.
In one type of procedure, individual gamma photons are registered one by one to build up a picture of the organ which is then computer enhanced and reviewed for signs of abnormalities. Another procedure called Positron Emission Tomography (PET), an positron emitting isotope created in a cyclotron is injected and accumulates in the targeted organ. Positron are the antimatter version of the electron. When they are emitted, they combine with an electron and both are annihilated releasing a pair of gamma photons. The PET camera detects these gamma photons with great accuracy. PET scans are mostly used for tumor detection but can be used in cardiac and brain imaging. Flourine-18 is the most effective radioisotope for this purpose. Organ malfunction can be indicated by either too much or too little of the isotope being taken up by part of the organ or the whole organ.
Therapy
Cancer cells divide rapidly and are especially susceptible to damage by radiation. A beam of gamma photons from a cobalt-60 radiation source can be directed into the body at a tumor for treatment.
An alternative to an external radiation source is to implant small radiation source directly into the tumor. Iodine-131 is used for treating thyroid cancer with this method. Iridium-192 implants are often used in the head or the breast. The isotopes are in the form a wire that is inserted with a catheter.
Radiation is also used to destroy bone marrow before a bone marrow transplant. Radiation is also used to relieve pain from bone cancer. Strontium-89, samarium-153 and rhenium-186 can all be used for this purpose.
A new procedure called Targeted Alpha Therapy (TAT) has been developed for treating dispersed cancers. It uses radioisotopes which emit alpha particles. Another new procedure called Boron Neutron Capture Therapy (BNCT) relies on brain tumors concentrating boron-10. Once the boron-10 is in place, thermal neutrons irradiate the tumor and cause the boron to emit alpha particles. More esoteric therapies are being developed such as the use of carbon 60 buckyball spheres to carry radioactive particles into tumors.
The use of radioisotopes in medicine has been very successful for diagnostic and treatment of a number of different diseases and will continue to evolve.
