Journal of Proton Therapy
www.protonjournal.org
ISSN 2469-5491
Increasing importance of proton therapy
Waldemar Ulmer
Strahlentherapie Nordwürttemberg and MPI of Physics, Göttingen, Germany
Article history:
Submission: April 23, 2015
First Revision: April 26, 2015
Cite this article as:
Ulmer W. Increasing importance of proton therapy. Jour Proton Ther 2015;1:112.
DOI: 10.14319/jpt.11.2
Acceptance: April 26, 2015
Publication: April 27, 2015
© Ulmer. Published by EJourPub.
Corresponding author:
Waldemar Ulmer, PhD; Strahlentherapie Nordwürttemberg and MPI of Physics, Göttingen,
Germany.
Scientific Note
1. Basic problems: protons versus heavy
carbons
During the past decade, many proton therapy facilities
have been established or are planned to become available
very soon. Thus it is amusing that in the meantime much
more vendors of proton treatment machines offer these
facilities than corresponding machines working either
with bremsstrahlung or fast electrons. Many researchers
in the field of radiotherapy have expressed the opinion
that about 20 % of malign tumors can be better treated
with protons due to the rapid fall-off behind the Bragg
peak than with the conventional radiotherapy with ultrahard photons or electrons. Thus the normal tissue can be
better protected by this behavior, whereas photons may
travel long distances behind the target.
However, this overall situation is not so simple as it may
appear, since there are still some open questions, and it
should be mentioned that three important points have to
be considered in detail:
1. Radiotherapy with ultra-hard bremsstrahlung
between 6 MV and 15 MV has brought some
essential
improvements
due
to
recent
developments and progresses of IMRT/IGRT, Rapid
Arc (VMAT) and Stereotaxy. These novel
techniques have also provided better tumor control
compared to older techniques like conformal radiotherapy or simple opposing fields, and it appears
that these techniques now have most widely
passed.
2. The RBE of radiotherapy with γ-quanta is per
definition '1', where Co60 is used as the reference
standard. On the other hand, research centers
having now for a long period clinical experience
with protons point out that the corresponding RBE
of protons is also only '1' or slightly more, if the
dose per fraction is assumed to be between 1.8 Gy
and 2 Gy or a little bit higher as usual in the
www.protonjournal.org
conventional radiotherapy with photons, e.g.
integrated boosts in IMRT or Rapid Arc (VMAT).
Therefore the principal advantage of proton
therapy would only exist in the improved
protection of normal tissue behind the target due
the mentioned properties of the Bragg peak or
SOBP. However, this aspect does not appear to be
sufficiently explored, since the improved protection
of normal tissue should enable to administer doses
behind the shoulder of radiobiological dose-effect
curves. With regard to protons there is only
experience with high dose Stereotaxy, but this
modality is already established since a long time in
photon-radiotherapy.
3. Since radiotherapy with protons is not the only
modality with regard to the application of
positively charged particles, the nomenclature
'hadron therapy' has been established in order to
denote besides protons also therapy modalities
with neutrons, α-particles, heavy carbons C612, etc.
In spite of the relatively high RBE the neutron
therapy in general failed because of some severe
shortcomings like the control of the beam-line.
However, if one compares the Bragg curve of
protons with that of heavy carbons, one should
expect a Bragg peak about 36 times higher than
that of protons, since according to Bethe-Bloch
equation (BBE) the charge q of the projectile
appears as the square (q2 = 36 for C612 versus q2 = 1
for protons)1, 2. This behavior is, in fact, not true,
and there exists a principal reason for the obvious
discrepancy, as analyzed in previous studies, which
may be verified from Figures 1‒3. Thus according
to these figures based on previous investigations all
positively charged projectile particles show besides
energy transfer to environmental electrons via
collision interaction a concurrence behavior after
sufficient slowing down, namely the capture of
electrons. This behavior implies that the effective
charge qeff of a proton at the Bragg peak is slightly
Ulmer: Increasing importance of proton therapy
2
Volume 1, Issue 1, 2015
Journal of Proton Therapy
www.protonjournal.org
lower than 1, i.e. for mono-energetic protons qeff =
0.995, whereas for heavy carbons q eff amounts to
1.19. When one compares real Bragg curves of
heavy carbons with those of protons, one may
verify that the height of the Bragg peak of heavy
carbons is increased by a factor 1.4 - 1.8, i.e. the
Bragg curve is significantly steeper in this domain
due to its mass factor 12 reducing energy straggling
and lateral scatter drastically.
The dynamical behavior of heavy carbons from C6+ to C+
and finally to the neutral carbon, where all electronic
shells are filled, can be characterized as a cascade
procedure. Behind the Bragg peak the effective charge q eff
tends to 0 for all positively charged hadrons and the LET
significantly decreases to finally assume 0!
However, a comparison of a well-know proton Bragg curve
with Figure 2 (heavy carbon) yields a large tail in Figure 2
resulting from fragments of numerous nuclear reactions.
The angular distribution of such a tail of secondary
fragments represents an additional aspect of heavy carbon
radiotherapy, which is often not accounted for. If we
further consider the high costs of this modality compared
to the proton modality, it appears that the benefit of heavy
carbons is not yet clearly determined, and only in Japan
and Germany installations of this therapy modality exist.
MeV/nucleon).
Figure 3: Effective charge qeff(E) of carbon ions in
dependence of the initial energy E0 for the cases E0 = 200,
300 and 400 MeV/nucleon. Independent of the initial energy
of the carbon ion the effective charge is decreasing to finally
become 0. Protons and α-particles behave in the same
manner.
2. Sophisticated therapy planning systems
According to the LET of protons in the Bragg peak region
one might expect a similar high RBE, which is, however,
not true. Thus the electron capture effect, which yields to a
decrease of LET behind the peak, may be one reason. This
effect implies that RBE can only refer to an overall
behavior. In more sophisticated planning systems the
dynamical behavior of LET before and behind the Bragg
peak should be accounted for. A first study of biological
effects of protons and secondary particle has been
published elsewhere.3
A secondary property of proton irradiation, which appears
to be underrated in planning systems and even in the
Monte-Carlo code GEANT44, represents the release of
neutrons. Thus very low proton energies are able to
perform a nuclear reaction of the form (restriction to the
oxygen nucleus):
p  O816  neutron  F916
Figure 1: LET for mono-energetic protons (dots) and overall
stopping power S(z) of carbon ions 400 MeV/nucleon.
(1)
This reaction results from an exchange of a π - meson
between oxygen and the positively charged proton. The
exchange interaction occurs due to the Pauli Principle. The
reaction isotope F916 undergoes a ß+ decay.
In general, the released neutron does not travel in forward
direction. However, compared to heavy carbons, the
release of neutrons and other secondary particles does not
represent a severe obstacle in proton radiotherapy.
Conflict of Interest
The author declares that he has no conflict of interest. The
author alone is responsible for the content and writing of
this article.
References
Figure 2: Measurement (HIMAC) and theoretical
calculation of the Bragg curve of carbon ions (290
© Ulmer. Published by EJourPub.
Journal of Proton Therapy (ISSN 2469-5491)
1.
Ulmer W. Notes of the editorial board on the role
of m medical physics in radiotherapy. Int J Cancer
Ther Oncol 2013;1:01014.
Ulmer et al.: Increasing importance of proton therapy
3
Volume 1, Issue 1, 2015
Journal of Proton Therapy
www.protonjournal.org
2.
3.
4.
Ulmer W. The Role of Electron capture and energy
exchange of positively charged particles passing
through Matter. J Nucl and Particle Phys 2012;
2:77-86.
Paganetti H. Nuclear interactions in proton
therapy dose and relative biological effect
distributions origination from primary and
secondary particles. Phys Med Biol 2002;4:74762.
Ulmer W. A new calculation formula of the
nuclear cross-section of therapeutic protons. Int J
Cancer Ther Oncol 2014; 2:020211.
© Ulmer. Published by EJourPub.
Journal of Proton Therapy (ISSN 2469-5491)