Why protons?
Protons are positively charged particles – the nucleus of the
hydrogen atom. Free protons are produced by ionisation of hydrogen atoms
(the electrons are displaced from the atom-shell). The free protons
are then accelerated to high speed. This is done in proton cyclotrons
or synchrotrons by strong electrical fields (in so called cavities).
In radiotherapy they enter the human body at a pre-selected energy level
and continue in a straight line up to a precisely calculated depth.
While moving, they release very little energy. Toward the end of their
trajectory they slow down, coming to rest at the Bragg peak (called
after the physicist of that name), where they release most of their
energy. Behind the Bragg peak the dose reduces to nil after a few millimeters.
This physical profile is the reason for using protons in radiotherapy.
It permits deep-seated tumors to be treated without overshooting the
mark.
How radiation
therapy works
If a charged particle, such as a proton, passes through a cell, or comes
to rest in it, the cell nucleus will be damaged by the energy which
the proton deposits (the dose). Under certain circumstances, however,
the cell is capable of repairing this damage. The challenge of radiation
therapy is to administer the dose in such a way that tumor cells have
no chance of repairing themselves, and, without exception, die off.
Healthy cells, on the other hand, should suffer no major damage and
be able to recover.
The Proton pencil
beam
As protons are elementary particles which carry a positive charge they
can be deflected and focused in magnetic fields, and the beam can be
shaped as desired. The PSI facility uses a proton beam as thin as a
pencil. In contrast to photons, which are generally used today for radiation
therapy, protons have a quite specific and precisely limited depth of
penetration into the body.
The Bragg peak
Photons deliver the largest dose immediately after penetration into
the body. Thus the healthy tissue is unnecessarily strongly irradiated.
The range of the protons depends on their initial speed and on the material
in which they are absorbed. Between the surface of the body and the
point where they stop, the material absorbs only a relatively low dosage,
causing the velocity of the protons to fall continuously. At the end
of their range they stop moving and release their maximum energy dosage.
This generates a dosage peak, the Bragg peak. Beyond this point the
dosage drops to zero within a few millimeters.
The figure shows the dosage curve for a monoenergetic thin pencil beam
of protons. This is compared with a depth dose curve of a photon beam
(the modality used today in hospitals for radiation therapy), with characteristic
exponential decrease of the dose with depth. Through the weighted superposition
of proton beams of different energies (Bragg peaks with different proton
ranges ) it is possible to deposit a homogenous dose in the target region
using only a single proton beam direction. The resulting (range-modulated)
proton beam distribution is called Spread Out Bragg Peak (SOBP). The
picture shows that the protons deposit a substantially smaller dosage
than photons. Behind the target volume, the tissue is still essentially
irradiated, with protons it’s absolutely not.
How much is a
radiation dose?
The radiation dose is a measure of the energy absorbed in a material,
such as body tissue. The biological effect of radiation however, depends
not only on how much energy is deposited in the cells, but also how
it is deposited. In each case, it is the energy dose which is measured
in Gray (Gy). A typical therapy dose for the destruction of a tumor
amounts to approximately 60 to 70 Gy. It is transferred in individual
fractions in several successive days (app. 30 fractions in total)
In order to further improve
treatment PSI has developed the spot-scanning
technique which works in complement to the physical advantages of
protons.
