Gamma Radiation

Gamma radiation is similar to light and x-rays. This type of radiation is produced mainly by sealed sources or from nuclear medicine radiopharmaceuticals. Patients who have received large doses of radioactive materials that emit gamma rays may be a source of exposure to nurses and other personnel. For example, some therapy procedures use Iodine-131(131I) to treat thyroid cancer or Grave’s disease. 131I MIBG is used to treat neuroendocrine tumors.

Beta Radiation

Beta radiation is electrons with a range of energies. This type of radiation is less common in the medical setting because beta particles are far less penetrating than gammas, and generally will be stopped by about one-half of an inch wood, plastic, water, tissue…etc, depending on the energy.

Applications may include Yttrium 90 (90Y) for cases where it is not possible to surgically remove hepatic tumors. The 90Y is delivered by loading the yttrium into tiny resin microspheres. The spheres are injected via microcatheter into the common hepatic artery. A patient who has received a radiopharmaceutical that gives off only beta radiations does not become an external radiation hazard to nurses or others. The patient’s body provides natural shielding of the beta particles. Universal precautions such as gloves are appropriate if there is contamination of bedding or dressings, due to urine or perspiration.

Positron Radiation

Isotopes used in positron emission tomography (PET) scans, such as 18F,  11C, 15O or 13N decay by positron emission. A positron is the anti-particle of a beta particle, and is emitted by a proton-rich nucleus. The collision of an electron and a positron yields two 0.511 MeV gamma rays. Positron gamma radiation can penetrate through inches of iron, concrete, wood, plastic, water, etc.

Patients administered positron emitters such as the typical PET/CT radiopharmaceutical 18F-FDG (fluorodeoxyglucose used in PET) are a source of exposure to nurses and other personnel.

A strong advantage to positron emitters is their very short “half-life” or, the time it takes for the isotope to decay and disappear. The tracer 18F has a two hour half-life. Most patients need to wait about an hour for the drug to be taken up in the body, and the PET/CT setup and scan can also be about an hour, which means, by the time a 18F patient leaves nuclear medicine the isotope has already been reduced by half due to physical half-life decay alone. Drugs also leave the body physiologically, usually through urine. The physical and biological half-lives work together to remove radioisotopes from the human body.

Radioactive Decay

Radioactive decay is the process that changes an unstable atom to a more stable atom. The concept is important, especially for medically used radioisotopes. Radioactive material disappears, or decays, at a predictable rate. Medical isotopes are chosen and used in humans because of the quick decay properties of the isotope.

Decay means a sample of radioactive material with a specific number of atoms will undergoes radioactive transformation. Over time there will be progressively smaller numbers of atoms that were originally radioactive.  When half of the original atoms have decayed, the material is said to have gone through a “half-life.” During the next half-life, half of the remaining atoms will continue to decay; leaving one-fourth of the original and so on.

Some elements, such as Cesium-137 (137Cs) have a very long half-life (30 years), so they essentially maintain a significant level of radioactivity over a human life span.  Others, such as Flourine-18 (18F) and Iodine-131 (131I), have fairly short half-lives, approximately 2 hours and 8 days respectively, and therefore, the numbers of radioactive atoms diminish relatively rapidly.  Nuclides which are used for diagnostic purposes, scans, or images have short half-lives. For example, a commonly used nuclide, Technetium-99m (99mTc) has a half-life of 6 hours. The nuclide used in liver cancer therapy for radioembolization is 90Y and has a half-life of 64 hours.