Biomedical Engineering Reference
In-Depth Information
I cannot, however, leave things there. It was recognized in ICRU78
(2007) that the above is a simplification and that there are several
sources of generally small variations of the RBE within a proton
beam around the value of 1.10. Figure 11.9 summarizes schema-
tically where these differences may lie.
RBE rises slightly
(few %) across the
SOBP in some
experiments
RBE rises slightly
(few %) across the
SOBP in some
experiments
RBE in center of
SOBP = 1.10
RBE in center of
SOBP = 1.10
1
1
3
3
DOSE
DOSE
4
4
2
2
≤
10% “blip”
near the end of
the SOBP
≤
10% “blip”
near the end of
the SOBP
1.1
1.1
1.0
1.0
bio-effective
dose
physical
dose
bio-effective
dose
physical
dose
5
elongation
of range by
~ 1-2 mm.
5
elongation
of range by
~ 1-2 mm.
6
6
RBE may vary with
dose/fraction
RBE may vary with
dose/fraction
RBE in entrance
region may be a bit
less (by a few %)
than in the center of
the SOBP
RBE in entrance
region may be a bit
less (by a few %)
than in the center of
the SOBP
7
7
RBE may vary with
tissue/endpoint
RBE may vary with
tissue/endpoint
0
0
DEPTH
DEPTH
Figure 11.9. Schematic diagram suggesting the ways in which proton
RBE may vary from the fixed value of 1.10 recommended
by ICRU78 (2007). The “blip at the distal end of the SOBP is, in reality,
not a separate phenomenon, but a region in which the average proton
energy becomes increasingly low as the depth gets larger
−
so that the
LET (and, hence, RBE) becomes therefore increasingly higher with
increasing depth.
Of all these effects, the greatest uncertainty is in item 6, the variation
of RBE with fraction size. Both on theoretical grounds, and from
extrapolation of
in vitro
experiments, one would expect the RBE to
rise as fraction size is reduced. However, neither the scant
in vivo
data nor clinical experience seem to exhibit such a behavior at
therapeutic dose levels (Paganetti
et al
., 2002).
In the future one hopes and expects that the various effects identified
in Figure 11.9 will be better understood and quantified. If so, and if
international agreement can be reached as to how to estimate them, a
more nuanced RBE estimate will be possible.
P
LANNING
P
ROTON
B
EAM
T
REATMENTS
:
WHAT
'
S DIFFERENT
?
At this point I have devoted many pages to making the point that
proton dose distributions are different from photon dose distributions
because the physics of the interactions is different. And that, with
