Biomedical Engineering Reference
In-Depth Information
at the beam entrance. While the numbers in Figure 11.10 are very
approximate, detailed treatment planning comparisons fully bear this
point out. The integral dose delivered outside the target volume is
typically between two or more times lower for protons than for
photons (Lomax
et al.
, 1999).
Figure 11.10. Schematic diagram comparing 1-, 2- and 4-beam
treatments of photons (
top
) and protons (
bottom
). In both cases, the use
of multiple fields reduces the dose outside the target volume while
spreading it out. The integral dose outside the target volume due to
protons is, in this very crude example, 2.5 times less than from photons.
A practical example of the
dose distribution of a treatment
of a base of skull sarcoma,
using scattered protons, is
shown in Figure 11.11. The
white diamonds show the
intersection of the target
volume (CTV) with the
coronal plane. The colored
lines show the field outlines as
they intersect the plane. This
looks like an attractive dose
Figure 11.11. Coronal section of a
proton treatment of a base-of-skull
sarcoma employing six non-coplanar
beams (doses in Gy (RBE)).
