Biomedical Engineering Reference
In-Depth Information
If the penetration of protons is selected by placing material in the
beam just upstream of the patient (termed a degrader), rather than by
changing the beam energy, then the preceding comments about the
Bragg peak width and the peak-to-plateau ratio no longer hold. The
depth
dose distribution of the degraded beam in water is essentially
the same as that of the un-degraded beam, but shifted towards smaller
depths by an amount equal to the water-equivalent thickness of the
degrader in the beam.
−
The spread-out Bragg peak (SOBP)
It was the dose distribution of a monoenergetic proton beam such as
that shown in Figure 10.5 that so attracted Robert Wilson's attention
(Wilson, 1946) and led him to suggest that a beam of protons would
deposit almost all of its energy within a deep-seated tumor, none
beyond it, and very little proximal to it. However, as we have just
seen, the Bragg peak is very narrow. Few tumors are that small in
extent. Most tumors used to be described, before the advent of more
quantitative imaging methods, variously as plum-sized, orange-sized,
and so forth. That is, they are likely to extend at least many
centimeters in depth and sometimes more than 10 cm. To treat such
tumors, the extent of the high dose region needs to be much greater in
depth than is provided by a single Bragg peak.
As Wilson observed, an extension in depth can be achieved by
delivering not just one, but many Bragg peaks, each with a suc-
cessively slightly differ-
ent range (i.e., energy).
These peaks should not
all be equally weighted.
Rather, the more proximal
a peak is, the less weight it
should have. This is illus-
trated in Figure 10.9. The dis-
tal region of near-constant
high dose is referred to as
the
spread-out Bragg peak
,
abbreviated as SOBP. Just
how this non-uniformly
weighted stacking of beams
is accomplished is dis-
cussed below.
Figure 10.9. Illustration of how a
spread-out Bragg peak (
top curve
)
is made up of near-monoenergetic
proton beams of successively lower
energy and weight. Figure courtesy of
B. Gottschalk, HCL, USA (Gottschalk,
2004).
