Biomedical Engineering Reference
In-Depth Information
Figure 4.14. Depth dose of photon beams produced from linacs.
Left:
varying linac energies, 10
10 cm
2
field.
Right:
varying field
sizes of a 10 MV linac. Data courtesy of Varian Medical Systems.
⋅
Distribution of dose laterally
The discussion to this point has dealt with the depth
dose distribution
along, say, the central axis of a photon beam. It is now time to see
what happens along a direction within a plane normal to the central
axis dose. Figure 4.15 presents a sequence of lateral profiles, taken at
some depth within the irradiated material, as we “turn on” various
physical effects.
−
Figure 4.15. Schematic representation of the cross-field profile of a
photon beam: (a) point source with perfect collimation; (b) includes
finite radiation source and electron transport at the lateral beam edges;
(c) includes contribution of scattered radiation (see text).
An ideal beam, with a point source, perfect collimation, and which
deposits all its energy at the site of an interaction would give rise to
the lateral profile shown in Figure 4.15a. This is a step-function-like
dose distribution whose width is equal to the projected width of the
collimators.
However, two processes cause the edge of the beam to be blurred out
- creating what is called a beam
penumbra.
The first effect is purely
geometric and is a consequence of the fact that the radiation source is
