10.1
Basic properties of MV photon beams
10.2
Previous chapters considered the production of megavoltage (MV) beams, the acceleration of electrons to energies >6 MeV, the subsequent production of bremsstrahlung photons, and the collimation of the beam to a shape that suits clinical requirements. Here, we consider the properties of the MV photon beam in a patient. Figure 10.1.1 shows the dose distribution of one such beam entering straight down onto a water phantom. (Note that “phantoms” are devices used to make measurements of the beam. They can be tanks of water, pieces of plastic, or other devices.) Figure 10.1.1 shows two plots of data through the dose distribution: a percent depth dose (PDD) (Figure 10.1.1b, taken along line B) and a profile (Figure 10.1.1c, taken along line C). These are useful ways to characterize a beam and will be used extensively in this and subsequent chapters.