Answer to Question #2913 Submitted to "Ask the Experts"

Q

How are beta radiations used in medical therapy and diagnosis?

A

When incorporated into drugs, beta radiations are used at present to treat hyperthyroidism, thyroid cancer, bone metastasis from any cancer, non-Hodgkin's lymphoma, rare chronic malignancies of the bone marrow producing too many red blood cells (polycythemia rubra vera) and too many platelets (thrombocytemia vera), malignant effusions in the peritoneal and pleural spaces, certain types of arthritis, and primary and metastatic liver cancers. Beta radiation is also used in brachytherapy sources to treat benign eye growths that threaten vision and a wide variety of cancers and some benign growth disorders. Beta brachytherapy sources are also use in the treatment of blood vessels following angioplasty. A great deal of research is being performed at present with a variety of radionuclide-labeled antibodies to treat numerous cancers (such as prostate, malignant melanoma, multiple myeloma, and neuroendocrine tumors). There are no uses of beta emitters for diagnosis. ¹³¹I is still used for diagnosis of certain thyroid conditions, but it is the gamma ray, not the beta particle, that is the useful radiation of the radionuclide. Depending upon the energy, beta particles are absorbed within a few millimeters of tissue and are virtually entirely absorbed within the patient who receives a radiopharmaceutical. The beta particles therefore cannot be used by cameras that detect radiation that leaves the body to form an image of the distribution of the radiopharmaceutical within the body. An exception to the beta radiation in diagnosis statements above is the positively charged beta particles, called positrons. Positrons are produced by the radioactive decay of many radionuclides containing too few neutrons for nuclear stability. Positrons are the antiparticles of electrons or beta particles. Beta particles are just electrons created within the nucleus and expelled to establish a stable nucleus. The positively charged positrons react with electrons as the positrons slow down moving through tissue. The reaction is one of particle-particle annihilation, with the masses of the electron and positron being converted into gamma radiation. The gamma radiation is used to create an image of the localization of the positron-emitting radiopharmaceuticals. This is called "PET" scanning, where "PET" stands for "positron emission tomography." PET scanning is very useful for cancer imaging, for determining the places in the brain in which epilepsy occurs, and in the evaluation of coronary artery disease. Virtually all positron emitters are used for diagnosis, but there is a small amount of research going on applying some to therapy.

Carol S. Marcus, PhD, MD