ABSTRACT
The various analyticalmethods used to compute the dose distribution in patients irradiated
by photon and electron beams have been described in Chapter 26 and Chapter 27. These
methods attempt to account for the effect of irregular patient surface and inhomogeneities
in the body with varying degrees of success. For photon beams, three-dimensional convolution
of the point-spread function or kernelwith the primary photon fluence is the most sophisticated
analytical method to date (see Section 26.3.4). The kernels are derived from Monte-Carlo
(MC) simulation in water (Mackie et al. 1988). However, there is no exact analytical way to
modify these water kernels to account for the non-water patient and thus approximate scaling
methods have been devised. Figure 28.1 is an elegant illustration of the approximations
involved in such kernel scaling. The pattern of electron tracks at an equivalent path length
(indicated by the horizontal dashed line) downstream from the interaction point is clearly
different depending on the order of the varying densities. In the situation on the right, at
the level of the dashed line, the tracks are clearly more spread out than on the left. Therefore,
using a scaled kernel (i.e. stretched according to the tissue density) to calculate the dose at the
level of the dashed line is clearly unphysical.
