Two European discoveries in the late 1800s led to future radiation treatment of human malignancies. While studying the penetrating power of cathode ray emission in Germany, Wilhelm Roentgen discovered x-rays on November 8, 1895. In France, the Curies isolated radium from uranium ore in 1898. Soon thereafter, Robert Abbe of New York City introduced radium for medical therapy, and Howard Kelly of Baltimore pioneered radium treatment of cervical cancer. Since then, radiation therapy has evolved to become a major modality in the treatment of many cancers, particularly those of the female reproductive tract.
Radiation therapy is used for definitive or palliative treatment of cancer and may be defined as therapeutic delivery of radiation to a target tissue, which results in tissue damage. Radiation causes breaks in DNA and generates free radicals from cell water that may damage cell membranes, proteins, and organelles. Such radiation may be electromagnetic or particulate, both of which transfer energy to the electrons or nuclei of the target atoms.
Electromagnetic radiation is energy that is transmitted at the speed of light through oscillating electric and magnetic fields. The energy contained in these fields can be described as discrete units known as photons. The energy of each photon is proportional to the frequency of the wave associated with that photon. Because radiation with a shorter wavelength has greater frequency, it carries greater energy per photon, allowing deeper tissue penetration. The most clinically relevant forms of electromagnetic radiation are x-rays and gamma rays. For therapeutic applications, x-rays are mechanically produced by linear accelerators that accelerate electrons to very high energies. These electrons then strike a target within the accelerator, usually tungsten, to produce a beam of x-rays that is targeted at the patient. Gamma rays are produced by the decay of radioactive substances. Currently, the most commonly used radioisotopes for gynecologic cancer treatments are cesium-137 and iridium-192.
Particulate radiation uses subatomic particles (electrons, neutrons, protons), instead of photons, to deliver the dose of radiation. Compared with electromagnetic radiation, particle-beam therapy enables more precise dose localization and better depth-dose distribution.
Interaction of Photons with Matter
The first step in the absorption of an incident photon with matter is the conversion of the energy of that photon into the kinetic energy of an electron, or electron–positron pair. Depending on the energy of the photon, this conversion takes place either through the photoelectric effect, the Compton effect, or pair production. In the lower range of energy transfer, the photoelectric effect predominates, whereas in the transfer of higher levels of energy, the Compton effect and pair production are more prevalent.
Photoelectric effect: an incident low-energy photon (0.5–100 kV) interacts with a tightly bound inner shell electron of the target tissue. The energy is completely absorbed by this electron, which is ejected from the atomic orbit with kinetic energy equal to the photon energy. The product electron then ionizes the surrounding ...