Biomedical Engineering Reference
In-Depth Information
the accuracy of the dose distribution delivered to the patient. There
are three caveats. First, activity is carried away from the site of
creation by blood flow so that, by the time of imaging, the PET image
may be washed out and not accurately reflect the dose distribution.
Second, the amount of activation depends on the proton energy. In
particular, there is no activation at the very end of range of the
protons - although this problem can be mitigated by modeling. And,
third, the spatial resolution of PET scans (of the order of a few
millimeters) is not quite good enough to detect dose perturbations
within small volumes which, as we have seen in Figure 11.4, may
occur in distances as small as a millimeter.
Finally, proton radiography, although it has so far not been used
in practice, has the potential to verify proton dose algorithms and
even, perhaps, be used to modify a patient's plan on a day-to-day
basis (Schneider and Pedroni, 1995). Proton radiography involves
measuring the residual energy of protons exiting the patient (or a
phantom) using a position-sensitive range telescope of some kind.
Since proton beams used for treatment normally do not exit the
patient, one has to add energy to the beam for the purpose of making
(low dose) transmission measurements. One can then determine the
water-equivalent path length through the patient. This is not the same
as that to the distal target volume surface, but good agreement with
the calculated exit energies would build substantial confidence in the
treatment delivery.
D
OSE
D
ISTRIBUTIONS
A
CHIEVABLE
W
ITH
P
ROTONS
Scattered beams
We have seen that even a single proton beam (e.g., Figure 10.1, left
panel), in contradistinction to a single photon beam (e.g.,
Figure10.1,right panel), can provide an acceptable treatment.
However, just as for photons, the use of multiple cross-firing proton
beams focused on the target volume reduces the dose delivered
outside the target volume - at the cost, of course, of spreading the
inevitable energy over a larger volume. Figure 11.10 schematically
illustrates the difference between protons and photons for one-, two-
and four-field approaches designed to deliver 60 Gy (RBE) to the
target volume. (The top row of this figure is identical to the sketches
in Figure 1.3 of Chapter 1.) No matter how many fields are used, the
proton dose outside the target volume is substantially lower than the
photon dose - with the exception of the proton's lack of skin-sparing
