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The protons originate at the ion source that sits within the COMET cyclotron. In it, hydrogen atoms are continuously, in fractions of a second, split into negatively charged electrons and positively charged protons. An electric field draws the protons into the cyclotron, where they travel around a circular path 630 times. Then they are guided into the beam path and bundled together with the help of magnets.
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At the treatment stations of the Centre for Proton Therapy at PSI (shown here, Gantry 3), tumours can be irradiated from any direction. Therefore the gantry or its irradiation head must be rotated around the patient. The enormous rotating body of the gantry weighs 220 tons and is up to
10.5 metres wide. This mass is due to a rotating steel framework, nine magnets for directing the proton beam, the vacuum tube in which the protons are bundled, devices
for radiation diagnostics, and a 1.5-metre-wide cable carrier, among other things. The rotation itself is accomplished by two electric motors with ten horsepower each.
10.5 metres wide. This mass is due to a rotating steel framework, nine magnets for directing the proton beam, the vacuum tube in which the protons are bundled, devices
for radiation diagnostics, and a 1.5-metre-wide cable carrier, among other things. The rotation itself is accomplished by two electric motors with ten horsepower each.
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The protons fly at two-thirds of the speed of light through the 50-metre-long beam path to the gantry and are fired in a targeted way at the tumour. How deeply they penetrate the body is determined by their energy. Wedges of graphite ensure the right energy level. The thicker the layer of graphite that
the protons have to penetrate, the greater the braking effect, and the sooner the protons release their energy in the tissue. How far these wedges are inserted into the beam path is determined by a dosimetrist during the therapy planning.
the protons have to penetrate, the greater the braking effect, and the sooner the protons release their energy in the tissue. How far these wedges are inserted into the beam path is determined by a dosimetrist during the therapy planning.
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The proton beam scans the tumour layer by layer. In the body, the protons transfer most of their energy precisely within the cancerous tissue. Protons have a high energy density. This leads to direct damage to the genetic material DNA, which
is located in the cell nucleus and contains all the information that is crucial to the cell's survival. Likewise, the absorbed energy produces highly reactive compounds, so-called free radicals. These free radicals, too, damage the cell's DNA .
is located in the cell nucleus and contains all the information that is crucial to the cell's survival. Likewise, the absorbed energy produces highly reactive compounds, so-called free radicals. These free radicals, too, damage the cell's DNA .
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