What is a proton?
A proton is one of the basic building blocks that make up the nucleus of an atom.
It is a stable particle that has a positive charge of +1. A single proton surrounded
by an electron makes an atom of hydrogen, the simplest of all elements. Almost
all the mass of a hydrogen atom is associated with the proton.
What is an antiproton?
An antiproton is the symmetric mirror particle of the proton. It has the same
mass and size as the proton but a negative charge of –1. It also differs
in its magnetic properties from the proton not in its magnitude but only in
its sign. Just like the proton the antiproton is intrinsically stable but cannot
exist in the presence of normal matter as it would immediately annihilate,
converting
its mass (and the mass of the particle it annihilates on) into pure energy.
How do you produce antiprotons?
Antiprotons do not occur naturally on Earth, but can be produced from energy
according to Einstein's famous equation relating mass and energy, E = mc2.
In the laboratory we make them by accelerating protons to energies higher than
2
times mc2 (m = mass of a proton) and directing these protons into a heavy target
such as iridium. In these collisions many different sub-atomic particles are
produced, including some antiprotons. These antiprotons can be collected using
magnetic and electric fields to keep them away from “normal” matter
and used for fundamental experiments or medical and other applications.
What is the "Annihilation Event?"
Antiprotons are stable particles when they are alone, but not if they come in
contact with ordinary matter. When matter and antimatter collide, there is a
burst of energy as the entire mass of antimatter and matter is converted back
into energy. Most of the energy is released in form of high energy gamma rays,
neutrinos, and pions (sub-nuclear particles that hold the nucleus together).
When the antiproton annihilates on a nucleus of an atom like oxygen, the oxygen
nucleus may break up into fragments. These nuclear fragments deposit their energy
very close to the point of annihilation, and it is this energy deposition which
is of most interest for us in the context of Antiproton Cancer Therapy.
How is Antiproton Cancer Therapy different from Radiation Therapy?
Antiproton Cancer Surgery (ACT) differs significantly from radiation therapy
using X-rays because the energy deposition is very much localized around the
annihilation point. X-ray therapy deposits an almost constant amount of radiation
along its path in the body before, at, and beyond the tumor. Heavy charged particles
(including protons, carbon ions, and antiprotons) deposit their energy at a well
defined depth in the body doing little damage to the tissue before the tumor
and no damage at all behind the tumor. For the same level of lethality within
the tumor Antiproton Cancer Therapy will do significantly less damage to the
healthy tissue compared to X-rays, but also compared to protons, and carbon ions.
Therefore Antiproton Cancer Therapy should be considered an extension of particle
beam therapy using ordinary protons or heavy ions. We believe that ACT will combine
the best characteristics of proton and heavy ion therapy compared to conventional
x-ray therapies and add additional advantages by having an even higher ratio
of damage to the tumor compared to damage to healthy tissue. In addition, the
annihilation event often produces fragments heavier than protons and therefore
has a higher biological efficiency, enabling the treatment of radio-resistant
tumors.
Does antiproton therapy work for all cancers?
Antiprotons can be used to deliver lethal energy to a localized region within
the body. That means it can be a more biologically effective way of delivering
radiation to a solid tumor surrounded by healthy tissue or located in close proximity
of sensitive portions of the body, like the spinal cord. Because of its higher
lethality, antiprotons are expected also to be more effective against radiation
resistant tumors.
How is Antiproton Cancer Therapy projected to work?
Antiprotons have the unique ability to deliver radiation to the tumor site (white,
in the diagram below) while minimizing collateral damage to healthy tissue (yellow).
The following graphic illustrates the collateral damage to healthy tissue caused
by x-ray, proton, and antiproton radiation relative to cell lethality placed
on the tumor cells.
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| What are the main differences between proton-, ion-, and antiproton
therapy? |
X-ray
Conventional single-field X-rays deliver roughly equal levels of cell lethality
to the entrance path,tumor, and exit path.
Particles
Heavy charged particles (protons, ions, antiprotons) place more cell lethality
on the tumor with less damage to the entrance pathand almost no damage on
the exit path.
What are the main differences
between proton-, ion-, and antiproton therapy?
Protons have been used on about 50,000 patients. The relative biological
effectiveness is the same as for X-rays and all the advantage comes
from the different dose
delivery profile. Carbon ions have been found to have a higher biological
efficiency towards the end of range and therefore offer better chances
to treat “radiation
resistant” tumors. Antiprotons
How efficient are antiprotons in comparison to other particle types?
An international collaboration of scientists has measured the enhanced cell
lethality at the end of range of an antiproton beam of 50 MeV energy relative
to the collateral
damage (=cell lethality in the entrance path of the antiproton). For this
specific beam the penetration into the target was only15 mm and the ratio
between damage
at the end of range to the damage at the surface for antiprotons to protons
to X-rays was found to be 10 : 2.5 : 1. This experiment was only a first,
but very
promising, step on road to the development of antiproton cancer therapy.
Many more studies on the detailed biological effects are necessary to cast
a final
vote. It is this type of leading edge research the Advanced Cancer Therapy
Foundation wishes to support.
Are there other useful aspects of ACT
besides a better “therapeutic ratio”?
Yes, real-time imaging of the delivered dose is an example of another aspect
of ACT that is absolutely unique. The decay of neutral pions in the antiproton
annihilation will generate high energy gamma rays which can be used to produce
a real-time image of the radiation dose delivered. This will allow the clinician
to watch in real time where exactly in the body he is delivering the lethal
radiation of antiproton therapy. This is especially important in cases where
critical organs
close to the tumor need to be spared.
What will be the source of antiprotons for ACT?
Antiprotons in sufficient quantities for these tests are currently produced
for fundamental physics research at CERN in Geneva, Switzerland and at the
Fermi
National Accelerator Laboratory, near Chicago in the US. But only CERN makes
antiprotons available at energies appropriate for medical applications. A
future facility has been approved at GSI in Darmstadt, Germany and will deliver
higher
intensities of antiproton beams than CERN starting in 2010. GSI was the leading
institution in developing carbon ion therapy, the most advanced particle
beam therapy available today, and could easily become again the incubator
for the
next generation advanced particle beam therapy. If the promised advantages
of ACT translate directly into clinical advantages, as is expected, a dedicated
facility will have to be built.
When could ACT be available to patients?
This research is in its very early stages and many questions will have to
be answered before patient treatments can be considered. The goal of the
Advanced
Cancer Therapy Foundation is to enable this research and make this development
possible and to assist researchers to achieve the ultimate goal, patient
treatment with antiprotons, as quickly as possible.
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