Biomedical Engineering Reference
In-Depth Information
that delivers most of
the dose within the
target volume and
proton
beam
proton
beam
x-ray
beam
x-ray
beam
relatively little out-
side it. Protons come
much closer than do
external beam pho-
tons to accomplishing
this desirable goal,
as is immediately evi-
dent on comparing the
broad beam dose
Figure 10.1. Side-by-side comparison of (left)
a proton, and (right) a photon posterior-oblique
beam.
distributions of pro-
tons and high energy
photons shown in Figure 10.1. This was first realized by Robert
Wilson in 1946 and, prompted by his seminal article (Wilson, 1946),
protons and other light ions have been evaluated at a number of centers
throughout the world over the past four decades for their promise of
providing superior therapeutic radiation beams.
∗
To what is the dramatic difference, seen in Figure 10.1, between
beams of protons and photons due? Well, protons, being charged
particles, interact with matter very differently than do photons, whose
interactions were discussed in Chapter 4. Their different modes of
interaction result in quite different dose distributions. Now, my alert
readers will already have recognized that the same is true of electrons,
which are also charged particles, and whose interactions with matter
were also discussed in Chapter 4. Indeed, the types of interactions
that protons experience are quite similar to the interactions of
electrons. However, protons are some 1836 times heavier than
electrons and this has the consequence that proton dose distributions
are quite different in practice.
∗
Some of the material in this chapter is adapted, with permission, from the
article “Treating Cancer with Protons” which appeared in the September 2002
issue of Physics Today (pp 45
50) by Goitein M, Lomax AJ, and Pedroni ES.
A good source of information concerning proton beam therapy can be found in
ICRU78 (2007), from which portions of this chapter have been taken with the
permission of the Oxford University Press.
−
