What are the different therapeutic radionuclide emissions?
For radiopharmaceutical therapy to deliver radiation dose preferentially to sites of disease, the radiation must be delivered in a very small area around the location of the radiopharmaceutical. Thus, the gamma and x-ray emissions that are able to travel long distances and are very useful for imaging deliver their radiation dose throughout the body and so add to normal tissue radiation dose and decrease therapeutic ratio. Many therapeutic isotopes also have gamma or x-ray emissions, and their benefit (the ability to image the treatment dose) must be weighed against their downside (adding to normal tissue radiation dose). The primary therapeutic emissions are:
- • Beta particles : These are electrons that are emitted from nuclei and typically travel distances on the order of millimeters in tissue. The relative effectiveness of beta radiation is about equal to that of photon radiation. Beta emitters have been the most commonly used therapeutic radiopharmaceuticals.
- • Auger electrons : When the energy from radioactive decay displaces an inner-shell electron, an outer-shell electron may drop in to takes its place, releasing energy. This energy is sometimes sufficient to eject an electron from the atom, and this electron is called an Auger electron. These generally have a very short path length in tissue, on the order of nanometers, and so must be in or very near the cell nucleus to have the potential to deliver deadly radiation to the cell. Although their short path length is thought to convey potential advantages, Auger emitters are rarely used clinically.
- • Alpha particles : Some large isotopes decay by alpha emission wherein a particle comprised of two neutrons and two protons is ejected from the nucleus. Alpha particles travel on the order of microns (generally one to four cell diameters) and have a mass about 7300 times higher than beta particles. Thus, the amount of energy they deposit over their very short path length is incredibly high, and they, therefore, have a relative effectiveness in causing cell death about 20 times higher than photons or beta particles.