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.