ABSTRACT
Charged particles are fundamental to the medical use of radiation. Even if the primary
radiation is a photon beam, it is the charged particles, known as secondary radiation in such
cases, that cause the biological effect, whether it be cell killing or other changes that may
eventually induce cancer. In fact, charged particles are often termed ionising radiation, and
photons (and neutrons) termed non-ionising or indirectly ionising. Furthermore, a precise
knowledge of the spatial distribution of the absorbed dose is crucial to radiotherapy treatment
planning and delivery (and in certain cases, to radiation protection considerations), and this can
only be obtained if the transport of the energy by the charged particles (overwhelmingly
electrons) can be modelled. In many cases, the ranges of, for example, the Compton electrons
(see Section 4.3.2) generated by megavoltage x-ray beams are appreciable (up to several cm)
and must, therefore, be accurately modelled. The generation of x-rays, i.e. bremsstrahlung, is a
charged-particle interaction. Alternatively, radiotherapy is sometimes delivered by primary
charged particle beams, usually megavoltage electrons, where electron interactions with
matter are obviously crucial. However, increasingly, proton beams are coming into therapeutic
use (see Chapter 46), and even so-called heavy ions such as carbon are used (see Chapter 49).
Mention can also be made of unsealed source therapy (Part K) with, for example, b-emitting
radionuclides; these electrons (or positrons) can also have ranges up to a centimetre. The
subject of radiation dosimetry (see Chapter 6 and Part D) depends on an intimate knowledge
of the interactions of both non-ionising and directly ionising (i.e. charged) particles (e.g. the
Bragg-Gray cavity principle) as will be made clear in Chapter 6. At the microdosimetric level, a
fundamental understanding of the action of radiation on cells can only come through studying
the track structure of particle tracks in relation to the relevant targets (i.e. the DNA in the cell
nucleus). Again, this requires knowledge of the charged-particle interactions. Perhaps the only
use of radiation in radiotherapy that does not rely heavily on charged particle interactions is
imaging by diagnostic x-rays.
