ABSTRACT
Quantitative characterisation of clinical beams is needed in order to undertake the
calculation of dose distributions within patients. This calculation is a two-step process
where the starting point is the absolute dose calibration in standard conditions, followed
by the derivation of the dose at other points in other conditions. Such derivation requires a
deep understanding of the beam characteristics presented in Chapter 22 and makes use of a
number of quantities and parameters which must be part of a self-consistent system. Many
of the parameters defined and discussed in this chapter were originally intended to describe
the behaviour of orthovoltage photon beams (see Section 21.6) and have been adapted or
expanded to fit the properties of high-energy (megavoltage) radiation. Only the latter will
be considered in what follows. The treatment given here is based on the methodology of
Khan (1984).
