This chapter provides a brief overview of the principles of radiation therapy. The topics to be discussed include the physical aspects of how radiation works ionization, radiation interactions and how it is delivered treatment machines, treatment planning, and brachytherapy.
Recent relevant techniques of radiation oncology, such as conformal and stereotactic radiation therapy, also will be presented. These topics are not covered in great technical detail, and no attempt is made to discuss the radiobiological effects of radiation therapy. It is hoped that a basic understanding of radiation treatment will benefit those practicing in other disciplines of cancer management.
This chapter does not address principles of radiobiology, which guide radiation oncologists in determining issues of treatment time, dose, and fractionation or in combining radiation with sensitizers, protectors, and chemotherapy or hormones. Ionizing radiation is energy sufficiently strong to remove an orbital electron from an atom. This radiation can have an electromagnetic form, such as a high-energy photon, or a particulate form, such as an electron, proton, neutron, or alpha particle.
High-energy photons By far, the most common form of radiation used in practice today is the high-energy photon. Photons that are released from the nucleus of a radioactive atom are known as gamma rays. When photons are created electronically, such as in a clinical linear accelerator, they are known as x-rays. Thus, the only difference between the two terms is the origin of the photon. Inverse square law The intensity of an x-ray beam is governed by the inverse square law.
This law states that the radiation intensity from a point source is inversely proportional to the square of the distance away from the radiation source.
