Since their discovery by Roentgen more than a century ago, x-rays have played a major role in modern medicine. The first recorded use of x-rays for the treatment of cancer occurred within 1 year of their discovery. Subsequently, there has been intensive study of x-rays and other ionizing radiations, and their clinical application to cancer treatment has become increasingly sophisticated. This chapter and Chapter 16 review the biological effects of ionizing radiation and the application of that knowledge to cancer treatment.
The present chapter begins with a review of the physical properties of ionizing radiations and the effects of energy deposition in cells. The molecular and cellular processes that ensue will be described, as well as response pathways that influence radiosensitivity and proliferation.
15.2 INTERACTION OF RADIATION WITH MATTER
15.2.1 Types of Radiation, Energy Deposition, and Measurements of Radiation Dose
X- and γ-rays constitute part of the continuous spectrum of electromagnetic (EM) radiation that includes radio waves, infrared (heat), ultraviolet (UV), and visible light (Fig. 15–1). All types of EM radiation can be considered as moving packets (quanta) of energy called photons. The amount of energy in each individual photon defines its position in the EM spectrum. For example, x- or γ-ray photons carry more energy than heat or light photons and are therefore at the high-energy end of the EM spectrum. Individual photons of x- or γ-rays are sufficiently energetic that their interaction with matter can result in the complete displacement of an electron from its orbit around the nucleus of an atom. Such an atom (or molecule) is left with a net (positive) charge and is thus an ion; hence the term ionizing radiation. Typical binding energies for electrons in biological material are in the neighborhood of 10 eV (electron volts). Thus, photons with energies greater than 10 eV (ie, x- or γ-rays) are considered to be ionizing radiation, whereas photons with energies of 2 to 10 eV are in the UV range and are nonionizing. An interaction that transfers energy, but does not completely displace an electron, is called an excitation because it leaves the atom or molecule in a higher-energy state.
EM spectrum showing the relationship of photon wavelength in centimeters (cm) to its frequency in inverse seconds (s−1) and to its energy in joules (J) and electron volts (eV). The various bands in the spectrum are indicated. Slanted lines between bands indicate the degree of overlap in the definition of the various bands.
UV radiation is split into 3 general classes, UV-C, UV-B, and UV-A, corresponding to wavelengths of 200 to 290 nm, 290 to 320 nm, and greater than 320 nm, respectively. UV-C and UV-B irradiation can be absorbed by DNA in cells leading to the ...