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.
How radiation works
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. In other words, the dose at 2 cm will be one-fourth of the dose at 1 cm.
Electron volt Photon absorption in human tissue is determined by the energy of the radiation, as well as the atomic structure of the tissue in question. The basic unit of energy used in radiation oncology is the electron volt (eV); 103 eV = 1 keV, 106 eV = 1 MeV.
Three interactions describe photon absorption in tissue: the photoelectric effect, Compton effect, and pair production.
Photoelectric effect In this process, an incoming photon undergoes a collision with a tightly bound electron. The photon transfers practically all of its energy to the electron and ceases to exist. The electron departs with most of the energy from the photon and begins to ionize surrounding molecules. This interaction depends on the energy of the incoming photon, as well as the atomic number of the tissue; the lower the energy and the higher the atomic number, the more likely that a photoelectric effect will take place.
An example of this interaction in practice can be seen on a diagnostic x-ray film. Since the atomic number of bone is 60% higher than that of soft tissue, bone is seen with much more contrast and detail than is soft tissue. The energy range in which the photoelectric effect predominates in tissue is about 10—25 keV.
Compton effect The Compton effect is the most important photon-tissue interaction for the treatment of cancer. In this case, a photon collides with a “free electron,” ie, one that is not tightly bound to the atom. Unlike the photoelectric effect, in the Compton interaction both the photon and electron are scattered. The photon can then continue to undergo additional interactions, albeit with a lower energy. The electron begins to ionize with the energy given to it by the photon.
The probability of a Compton interaction is inversely proportional to the energy of the incoming photon and is independent of the atomic number of the material. When one takes an image of tissue using photons in the energy range in which the Compton effect dominates (~25 keV—25 MeV), bone and soft-tissue interfaces are barely distinguishable. This is a result of the atomic number independence.
The Compton effect is the most common interaction occurring clinically, as most radiation treatments are performed at energy levels of about 6—20 MeV. Port films are films taken with such high-energy photons on the treatment machine and are used to check the precision and accuracy of the beam; because they do not distinguish tissue densities well, however, they are not equal to diagnostic films in terms of resolution.
Pair production In this process, a photon interacts with the nucleus of an atom, not an orbital electron. The photon gives up its energy to the nucleus and, in the process, creates a pair of positively and negatively charged electrons. The positive electron (positron) ionizes until it combines with a free electron. This generates two photons that scatter in opposite directions.
The probability of pair production is proportional to the logarithm of the energy of the incoming photon and is dependent on the atomic number of the material. The energy range in which pair production dominates is ≥ 25 MeV. This interaction occurs to some extent in routine radiation treatment with high-energy photon beams.
With the advent of high-energy linear accelerators, electrons have become a viable option in treating superficial tumors up to a depth of about 5 cm. Electron depth dose characteristics are unique in that they produce a high skin dose but exhibit a falloff after only a few centimeters.
Electron absorption in human tissue is greatly influenced by the presence of air cavities and bone. The dose is increased when the electron beam passes through an air space and is reduced when the beam passes through bone.
Common uses The most common clinical uses of electron beams include the treatment of skin lesions, such as basal cell carcinomas, and boosting of areas that have previously received photon irradiation, such as the postoperative lumpectomy or mastectomy scar in breast cancer patients, as well as select nodal areas in the head and neck.
MEASURING RADIATION ABSORPTION
The dose of radiation absorbed correlates directly with the energy of the beam. An accurate measurement of absorbed dose is critical in radiation treatment. The deposition of energy in tissues results in damage to DNA and diminishes or eradicates the cell’s ability to replicate indefinitely.
Gray The basic unit of radiation absorbed dose is the amount of energy (joules) absorbed per unit mass (kg). This unit, known as the gray (Gy), has replaced the unit of rad used in the past (100 rads = 1 Gy; 1 rad = 1 cGy).
Exposure To measure dose in a patient, one must first measure the ionization produced in air by a beam of radiation. This quantity is known as exposure. One can then correct for the presence of soft tissue in the air and calculate the absorbed dose in Gy.
Percentage depth dose The dose absorbed by tissues due to these interactions can be measured and plotted to form a percentage depth dose curve. As energy increases, the penetrative ability of the beam increases and the skin dose decreases.