DESIGNING A TREATMENT BEAM - Radiation Oncology: A Physicist's-Eye View - page 78

and, very importantly, on the size of the radiation field. 9 So far as the
beam energy is concerned, the Compton interaction probability is
somewhat higher for a low energy photon than for a high energy
photon, and so the secondary photons produced in a high energy
photon beam are more likely to escape the patient and therefore
deposit less dose than the secondary photons produced in a low
energy photon beam. In consequence, higher energy beams have a
lesser fraction of the total dose due to scattered radiation.
So far as the changes in a beam with depth are concerned, there are
two important effects. First, there are increasingly fewer primary
photons at greater depths due to the beam attenuation by Compton
interactions. Second, because secondary photons tend to be emitted
in the forward direction, there is an increasing number of secondary
photons in the beam at greater depths. These two facts together result
in the fractional contribution of scattered

radiation to the total dose being increa-
singly greater at increasingly greater
depths.

The most interesting effect is the in-
fluence of the field size on the amount

of scattered radiation at a given point of
interest. Consider the point, P, in Figure
4.17. It lies on the central beam axis and,
when irradiated by the inner smaller
beam, will receive a certain dose which
has primary and secondary components.
Now let us widen the beam to the size of
the outer beam outline. We have done
nothing to disturb the impact of photons

Figure 4.17. Schematic re-
presentation of the scatter
dose reaching P from dis-
tant primary photons (see
text).

within the smaller beam. Primary

photons within the stippled volume
between the two beams will have no
effect on the dose at P because they are

9

The ' field ' of a beam is, in essence, its cross-sectional shape. More

precisely, its borders form the boundary of the beam's photons in a plane
perpendicular to the central axis of the beam (usually at the beam
isocenter). The ' isocenter ' of a gantry is the point in space about which it
rotates - i.e., the center of the smallest sphere through which the central
axis of the beam passes as the gantry rotates through its full angular range.