Biomedical Engineering Reference
In-Depth Information
thought to minimize the total integral dose delivered to the patient,
sharpen the lateral fall-off of the resulting dose distribution and can
be more quickly calculated and delivered due to the significantly
reduced number of Bragg peaks that need to be delivered. However,
one would expect that, if this is truly the optimal technique, that
optimization programs used for IMPT would converge to it and there
would be no need to “guide” the solution towards the distal-edge
tracking geometry.
Figure 11.15 presents a side-by-side comparison of photon and proton
beam dose distributions.
photons
photons
protons
protons
(a)
(a)
(b)
(b)
single
beam
single
beam
(c)
(c)
(d)
(d)
IMRT
IMRT
Figure 11.15. Example of a meningioma treated by photons (
left
side
) and protons (
right side
). The three volumes are the GTV,
CTV and PTV. Panels (a) and (b) show single left posterior
oblique fields, and panels (c) and (d) show the full IMRT plan for
(c) photons and (d) protons. Figure courtesy of A. Lomax, PSI,
The largest difference, as already emphasized, is that photons deliver
a substantial excess “dose bath” outside the target volume. This point
is made again, even more explicitly, in Figure 11.16 which shows a
section of a plan to treat a large Ewings sarcoma with either IMPT or
IMXT. It is hard to imagine anyone wanting voluntarily to receive
