5.1
Interaction of low-energy photons
5.2
The interaction of photons and particles with matter is fundamental to radiation therapy physics, as photons and particles are used to treat and image patients. Before considering this topic in detail, it is useful to examine the broader context of photons and particles in radiation therapy. Figure 5.1.1 provides an overview of the treatment approaches for various targets (shown in red) when using external beam radiation therapy. Superficial targets are often treated directly with electron beams (see Chapter 15). Targets located deeper within the patient may be treated with high-energy photons (denoted by the symbol γ) or protons (see Chapter 24). There are also other more exotic modalities such as heavy ions or fast neutrons, which are not shown.