Patient Positioning

Dr Evan de Kock (iThemba Labs), Dr Neil Muller (iThemba Labs, Applied Mathematics)

Background

iThemba Labs, just outside Stellenbosch, has a small 200MeV cyclotron originally designed for Physics experiments. Proton therapy for brain tumors commenced in September 1993, at that time one of only a few proton therapy facilities world wide.
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Robert R Wilson was one of the first to realize the importance of proton therapy, back in 1946. Large scale treatment of patients started in 1990 at Loma Linda University Medical Center.

In order to understand why proton therapy is so effective, one has to understand some basic Physics.

Physics

Since protons are positively charged they can knock negatively charged electrons out of their orbits, ionizing the atoms. It the atoms happen to form part of live tissue, the ionization messes with the DNA of the cells, causing the cells to die. The basic idea of all procedures is to confine most of the damage to the tumor, with as little damage as possible to healthy tissue. It is not possible not to damage healthy tissue but since it has the ability to recover, and recover more efficiently that tumors, repeated treatment will eventually kill the tumor while giving healthy tissue time to recover between treatments.

What makes proton therapy so effective is the possibility of localizing the damage. The idea (my understanding!) is as follows: As a proton passes through the tissue it interacts with the electrons, knocking them out of orbit. Thus some of its energy is transferred to the electron, causing it to slow down. As it slows down, the interaction with the electrons is amplified, causing it to knock out even more electrons, causing it to slow down even more. Since this is an exponential effect, most of the energy is released over a very small regions, as shown in the figure.

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Knowing the depth of the tumor inside the body, one can therefore arranges that the protons enter the body with such energy that the Bragg peak coincides with the position of the tumor.

Patient Positioning

The problem is to align the tumor with the proton beam. It should be clear that the alignment needs to be precise, certainly for brain tumors where the damage to healthy tissue can be critical. And at iThemba proton therapy treatment is currently restricted to brain tumors.

Since protons have a positive charge it is possible to change its direction by applying a magnetic field. 200 MeV protons are fast, moving at about half the speed of light, however, and this means that a really powerful magnetic field is required. Modern treatment facilities specially designed for medical purposes do exactly this, using a number of 2500kg magnets, costuming something in the order of 3000 amperes each. This is housed in three story, 200 ton gantries, substantial setup.

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Power consumption is huge, in the order of a million watt, in order to produce a 0.2 watt output over about 0.3 s during treatment. This in not an energy effective procedure.

Aligning the proton beam is not an option at iThemba Labs, instead the patient is aligned using a visual system.

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For each patient a tight-fitting, custom-made mask with a number of markers is provided. A CT scanner is used to determine the position of the tumor with respect to the markers. The patient, wearing the mask, is then fixed to a mechanical chair and a setup of nine calibrated cameras find the markers in the images (since they are chosen to be reflective this is not too hard). From this their 3D coordinates with respect to a world coordinate system (think proton beam coordinate system) are calculated. This means that the position of the tumor is known sit respect to the proton beam, and since the desired position is known, the mechanical chair can be automatically manipulated into position, amounting to a control problem.

The mathematics to do this is well-known, the real challenge is to achieve the necessary accuracy and all sorts of additional safeguards are built into the system to ensure this.

The video shows the mechanical chair being manipulated into position using the markers on the mask fixed to the chair.

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