Biomedical Engineering Reference
In-Depth Information
Figure 10.19. Schematic diagram, not to scale, of a passive scattering nozzle
(see text).
N.B.
monitoring devices are omitted in this diagram.
the protons lie in the near-flat region at the center of the scattered
beam, and are therefore useful.
Double-scattering
systems are now in widespread use. These can be
of several designs. In perhaps the most sophisticated approach, the
first scatterer is a simple piece of material of uniform thickness,
usually of high atomic number (since this minimizes energy loss for a
given degree of scattering), and the second scatterer, located some
distance downstream from the first, is shaped so that it preferentially
scatters the center of the beam more than the outside. This system
transmits a substantial fraction of the beam (up to about 45%) which
is uniform enough to use for treatments (Gottschalk, 2004). In an
additional refinement, the range modulator (see below) and first
scatterer can be combined, using both high and low atomic number
materials (see inset in the lower left of Figure 10.19), in order to
achieve a constant level of energy loss throughout the useful beam
(Gottschalk, 2004).
Double scattering systems have a couple of disadvantages that must
be overcome: (1) because they spread out the beam in two separated
scatterers, they produce a beam with a much larger effective source
size than is produced by a single scattering system and consequently
have a larger penumbra; and (2) to obtain a flat dose distribution, the
beam must be very well centered on the contoured second scatterer.
