In Chapter 16 of
Intermediate Physics for Medicine and Biology,
Russ Hobbie and I discuss the
Bragg peak.
Protons are also used to treat tumors (Khan 2010, Ch.
26; Goitein 2008). Their advantage is the increase of stopping power at low energies. It is possible to make them
come to rest in the tissue to be destroyed, with an enhanced
dose relative to intervening tissue and almost no dose distally
(“downstream”) as shown by the Bragg peak in Fig.16.47.
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Energy loss versus depth for a 150 MeV proton beam in water, with and
without straggling (fluctuations in the range). The Bragg peak enhances
the energy deposition at the end of the proton range. Adapted from Fig.
16.47 in Intermediate Physics for Medicine and Biology. |
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William Henry Bragg
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Sir William Henry Bragg (1862 – 1942) was an English scientist who shared the
1915 Nobel Prize in Physics with his son
Lawrence Bragg for their
analysis of crystal structure using X-rays. In 2004, Andrew Brown and Herman Suit published an article commemorating “
The Centenary of the Discovery of the Bragg Peak” (
Radiotherapy and Oncology, Volume 73, Pages 265-268).
In December 1904, William Henry Bragg, Professor of
Mathematics and Physics at the University of Adelaide and
his assistant Richard Kleeman published in the Philosophical
Magazine (London) novel observations on radioactivity.
Their paper “On the ionization of curves of radium,” gave
measurements of the ionization produced in air by alpha particles, at varying distances from a very thin source of
radium salt. The recorded ionization curves “brought to light
a fact, which we believe to have been hitherto unobserved. It
is, that the alpha particle is a more efficient ionizer towards the
extreme end of its course.” This was promptly followed
by further results in the Philosophical Magazine in 1905.
Their finding was contrary to the accepted wisdom of the
day, viz. that the ionizations produced by alpha particles
decrease exponentially with range. From theoretical considerations,
they concluded that an alpha particle possesses a
definite range in air, determined by its initial energy and
produces increasing ionization density near the end of its
range due to its diminishing speed.
Although Bragg discovered the Bragg peak for alpha particles, the same behavior is found for other heavy charged particles such as protons. It is the key concept underlying the development of
proton therapy. Brown and Suit conclude
The first patient treatment by charged particle therapy
occurred within a decade of Wilson’s paper [the first use of protons in therapy, published in 1946]. Since then, the
radiation oncology community has been evaluating various
particle beams for clinical use. By December 2004, a
century after Bragg’s original publication, the approximate
number of patients treated by proton–neon beams is 47,000
(Personal communication, Janet Sisterson, Editor, Particles) [over 170,000 today].
There have been several clear clinical gains. None
of these would have been possible, were it not for the
demonstration that radically different depth dose curves
were feasible.
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