Isotopes are defined as the types of an element, which have the same atomic number and position in the periodic table. They share similar chemical behavior but have different atomic mass and physical properties. Radioactive ones are those that have an unstable number of protons and neutrons. This instability is created by neutron activation, wherein a neutron captured in the nucleus of an atom leads to an excess of neutron-rich nucleus. Cyclotrons are used to manufacture proton rich radioactive isotopes. The nucleus of an isotope emits particles like alpha, beta or positron, and photons like gamma rays, to achieve energetic stability during radioactive decay.
What is Nuclear Medicine?
It is the branch of medicine, which uses radiation to provide information, regarding the functioning of a specific organ in the human body, or the treatment of a disease. This collected data gives an accurate and immediate diagnosis of the patient's illness. Radioactive isotopes are used to form images of the thyroid, bones, heart, liver, and many other organs. They used also have helped in treating diseased organs and tumors.
The most commonly used example of such isotopes is technetium-99, which accounts for 80% of nuclear medicine procedures. In the US alone, over 18 million nuclear medicine procedures are recorded per year.
In the 1930s, scientists used radioactive isotopes to measure the required dose of administered radioactive iodine, when localized in the thyroid. A Geiger counter was used to evaluate the radiation emitted from the neck and make to further diagnosis. The real breakthrough came with the invention of gamma scintillation camera in the 1950s, by Hal Anger, an American engineer. This device helped to make the use of radioactive isotopes in the field of medicine, mainly for diagnoses and treatment of possible illness or diseases.
The first isotopes were used as a tool to diagnose, detect, and treat thyroid disorders like goiter. There was an extensive research conducted in the field of nuclear medicine, which led to many discoveries and inventions of ultra sharp diagnostic methods and imaging systems. There are 5 Nobel prizes awarded for various discoveries and inventions in nuclear medicines. The Positron Emission Tomography (PET) scan was the first diagnostic tool invented by Peter Alfred Wolf, which used radioactive isotopes in medicine. This invention was followed by CT scan (computerized tomography), and MRI (magnetic resonance imaging).
Most of the techniques uses radioactive tracers, which emits gamma rays from within the body. These are short-lived ones that are linked to chemical compounds, and they help in scrutinizing specific physiological processes. The mode of administering these tracers is by injections, inhalation, or oral ways. Single photons are detected by a gamma camera that provides a view of organs from different angles. The image is build up by the camera from the point through which the radiation is emitted. A computer helps in enhancing the image, which is viewed by the physician on a screen, and helps him to detect any abnormality in the organ.
In a PET scan, a positron-emitting radionuclide is introduced by an injection, that is accumulated in the target tissue. With the decay of the radionuclide, the emitted positrons that combine with the nearby electrons, result in the emission of gamma rays that are easily identifiable, by traveling in opposite direction. A PET camera detects these rays, and provides a precise indication of their origin. The most common role of these scans regarding radioactive isotopes is fluorine-18; it is used as a tracer in oncology. It is the most effective non-invasive method to detect and evaluate cancers. This method is also used for cardiac and brain imaging.
PET and CT scans have been combined to make a new procedure that provides 30% better diagnosis. The position and concentration of isotopes in the body can also be detected using these techniques. Thus, organ malfunction can be observed if the isotope is absorbed partially by the organ known as 'cold spot', or in excess called 'hot spot'. When a series of images are taken over a time period, it helps in detecting the malfunctioning by the unusual pattern or rate of isotope movement.
Radionuclide Therapy (RNT)
The use of radioactive isotopes in medicine involves radionuclide therapy. Cancerous cells can be controlled or even eliminated by irradiating the tumor growth region. Teletherapy, also known as external irradiation is carried out by gamma beams emitted from radioactive cobalt-60 source. In developed countries, the use of versatile linear accelerators is being utilized.
Internal radionuclide therapy involves administering small radiation sources like a gamma or beta emitter in the target area. Brachytherapy or short-range therapy mainly utilizes Iodine-131 to treat thyroid cancer. It also helps for treating non-malignant thyroid disorders. In case of brain cancer or breast cancer, Iridium-192 is preferred. These isotopes are produced in a wire form, and introduced through a catheter into the target area. The implantation wire is removed once the appropriate dose has been administered. Advantages of this technique include being more target specific, less exposure of radiation to the body, and being is cost-effective.
A lethal dose of radiation is given to the patient to kill all the defective bone marrow cells, before replacement with healthy ones, in case of treatment of leukemia. Strontium-89 and Samarium-153 are used to provide relief to pain induced by cancer. The new radioactive isotope being used for pain therapy is Rhenium-186.
To control dispersed cancer types, the method called Targeted Alpha Therapy (TAT) is being used. In this technique, a short-range of highly energetic alpha emissions are allowed to enter the targeted cancer cells, after a carrier has taken the alpha-emitting radionuclide to the target area. There are positive results from laboratory studies, which have led a way for clinical trials for diseases like leukemia, cystic glioma, and melanoma.
Radioactive isotopes can be easily detected even if they are present in low concentration. This has helped in the use of these isotopes in medicine, for labeling molecules of biological samples in vitro. There are many tests that help to detect the constituents of blood, serum, urine, hormones, antigens, and drugs by linking them with the isotopes. This type of tests are called radioimmuni-assays.
All organs in the body act differently due to the presence of specific chemicals absorbed by them. This knowledge has helped in developing diagnostic radio-pharmaceuticals, to examine the blood flow to the brain, and functioning of organs like heart, lungs, liver, kidneys, bones (excess growth), etc. It also helps in predicting the effects of surgery and assessing changes since the start of treatment. This non-invasive technology helps in observing the organ functions, and diagnosing abnormalities without the patient experiencing any form of discomfort. The most widely used radioactive isotope is Technetium-99m, which has the ability to disappear without a trace after completion of the test, in a short time. Thallium-201 chloride or Technetium-99 is used in Myocardial Perfusion Imaging for detection and prognosis of coronary artery diseases.
Radiation has the ability to weaken or destroy malfunctioning cells under certain medical conditions. A radioactive element that can generate radiation is localized on the target organ with the help of its usual biological path or attaching an element to a suitable biological compound. Beta-radiation is commonly used to destroy damaged cells. This is known as radionuclide therapy (RNT) or radiotherapy. Iodine-131 is utilized in treating abnormal conditions like hyperthyroidism. Phosphorus-32 is used to control a disease called 'Polycythemia vera', where an excess of red blood cells are produced by the bone marrow. An extensive research is being carried out all over the world, to find out about new ways of incorporating the use of radionuclides in curing numerous diseases.
There are many radioactive neutron rich and proton rich isotopes that are made in nuclear reactors and in cyclotrons. There are many factors that govern the selection of these isotope in medicine. The dosage and half-life requires study of many factors. The use of radioactive isotopes in medicine is increasing day by day with accurate results. It also helps in early diagnosis, and is a mode of treatment for patients, especially for those suffering from cancer and tumors. Before undergoing radiotherapy, make sure you speak to your physician regarding all the matters related to the methods.