Principles of Radiation Oncology (updated 08/06) 1. Orthovoltage vs. megavoltage x-rays. MM Orthovoltage: 200-500 kV Megavoltage: 1-25 MV Orthovoltage: Lower energy x-rays generated by bombarding a metal target with high-energy electrons. Maximum dose desposited at skin surface – significant effects on skin, but hard to treat deep tumors. Also risk of bone damage/necrosis. Best for: Superficial tumors that do not involve bone; i.e., skin, nasal cavity Megavoltage: External beam delivered via medical linear accelerator of Cobalt60 unit. Maximum dose beneath skin; spares skin. Photos traverse entire tissue thickness but deposit progressively less with increased depth. 2. Describe the “Photoelectric effect” and “Compton effect” in ionizing radiation.MM Photoelectric effect – predominant in low-energy photons In the photoelectric effect, one of the inner electrons of the atom absorbs the energy of the gamma ray, and is ejected from the atom, again leaving a positively charged ion and a free electron. Following this, it is often the case that one of the outer electrons ‘falls’ down to fill the vacancy. As a consequence, an X-ray is emitted from the atom. Photoelectric effect involves the interaction of the photon with the tightly bound inner electrons and is proportional to the cube power of the absorbing matter's atomic number. This interaction is responsible for the different radiographic densities seen on diagnostic radiographs. Compton effect- predominant in mid-energy photons In the Compton effect, gamma rays are scattered from the outer electrons of the atoms, transferring energy to the electrons and in the process reducing the energy of the gamma ray. If enough energy is supplied during scattering, the outer electron will be removed from the atom, leaving an ion and giving rise to a free electron. Compton effect involves interaction with outer electrons that are bound more loosely. This effect is related to electron density and, therefore, results in much more uniform tissue absorption than lower energy photons. In radiation therapy, Compton effect predominates; therefore, the contrast observed on therapy port films is inferior to diagnostic radiographs. In the photoelectric effect, a low energy photon strikes an electron. If the photon has the same energy as the binding energy of the electron (the energy that holds the electron in its orbit), the photon will give all its energy to the electron and disappear. The electron is knocked out of the electron shells, forming an ion pair. Therefore, in the photoelectric effect reaction, the photon disappears and an ion pair is formed. In Compton scattering a medium energy gamma interacts with an orbiting electron near the nucleus imparting some of its energy to the electron. When this occurs, the electron that absorbs the energy leaves the atom to form an ion pair, and, because it has a significant amount of kinetic energy, produces ionization the same as a beta particle does. In addition, because the energy of the original gamma photon was not all absorbed the lower energy photon continues on to cause other interactions. Therefore, the eventual result of a Compton scattering reaction is that a mid energy range photon results in the production of an ion pair, and the photon continues at a reduced energy to undergo another interaction. 3. Effects of radiation on a cellular level.MM Direct cell damage – DNA is damaged via direct damage to the nucleus Indirect cell damage – radiation strikes the cytoplasm surrounding the nucleus rather then the nucleus itself. The cytoplasm is compose primarily of water and is the intercellular fluid described in the previous section. When radiation interacts with a water molecule, certain free radicals can be formed. The free radicals are chemically reactive, and they can cause the cell to become chemically imbalanced; the result is cell damage. The effect is caused indirectly, the chemical changes brought about by the formation of the free radicals are what ultimately cause the cell damage. If the damage is so great that the cell cannot repair itself, the result is the same as in direct cell damage, the cell dies 5. How do dose-response curves differ for radiosensitive and radioresistant tumors? Explain therapeutic ratio. MM The biologic effects of radiation on both tumors and normal tissue structures are dosedependent. A dose-respone curve, or the plot of response vs. dose given is typically sigmoidal. Radiosensitive tumors typically have a steeper dose-response curve, and will be shifted left of normal tissue (respond at lower doses than normal tissue will be hit and result in complications); radioresistant tumors will be shifted right (respond at higher doses than normal tissue will be complicated). Therapeutic ratio: the chance that the tumor will be killed versus the chance of a normal tissue complication. Radiation oncologists attempt to increase this ratio by many methods including fractionization in which repeated small doses of radiation are less damaging to a sensitive cell than a single fraction of equivalent total dose.