Biomedical Engineering Reference
In-Depth Information
various sliver thicknesses. It is noteworthy that even a 1 mm thick
sliver results in a non-trivial
circa
20% reduction of the central axis
dose in the region of the proton beam's end of range.
One needs to be able to compensate for, or at least take into account,
inhomogeneities, and the results shown in Figure 11.4 are particularly
alarming because they suggest the high level of spatial resolution
needed. One millimeter is near the limit of resolution of CT scanners,
and this means that inhomogeneities which can affect the dose
distribution may escape detection or, at the least, may be inadequately
measured.
Dose perturbations due to complex inhomogeneities
So far, I have focused on inhomogeneities of simple shape since they
exhibit the behaviors of protons in a pure form. In practice, of course,
the patient usually presents a complex pattern of inhomogeneities;
this is perhaps most extreme in the region of the base of skull where
protons may be directed along extended bone surfaces, or through
complex bone/tissue/air structures such as the petrous ridge or
paranasal sinuses. In consequence, a complex combination of range
penetration perturbations and scattering-induced dose non-
uniformities takes place. The results of such complex situations are
very hard to calculate analytically, although the preceding discussions
of inhomogeneities gives some insight into the extent of the possible
dose perturbations. Monte Carlo calculations are presently the only
way to get a reasonably reliable estimate in the case of highly
complex geometries. To be accurate, though, a very fine calculational
grid must be used.
Figure 11.5 shows the degradation of the terminal region of two
different proton beams - a monoenergetic beam delivering a single
Bragg peak (upper panel), and a spread-out Bragg peak (lower panel)
- which was passed through a water-filled human skull. The depth
dose was measured in a water tank placed downstream of, and close
to, the skull. Measurements were made at three points, identified in
the left panel of Figure 11.5: (A) downstream of a relatively
homogeneous region of the skull; (B) downstream of a fairly
inhomogeneous region of the skull; and (C) downstream of a highly
inhomogeneous region of the base of skull.
These data demonstrate that the distal portion of the dose distribution
can be very substantially affected by complex inhomogeneities.
The
distal fall-off of both the Bragg peak and the SOBP is not simply
