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Other types of accelerators.

In the following section, some other machines are presented. They are used only for treating a few percent of patients. We don’t pay attention to machines no longer used (betatron, e.g.), or not expected to be important in the future.

Cyberknife (Accuray Inc., Sunnyvale, California engedélyével)
Microtron’s principle of operation

Cyberknife. Practically, it’s a combination of a linac and a robotic arm, the system being supported by imaging devices. The doubled frequency compared to standard linacs leads to the size reduction of resonating cavities and the waveguide.
Mikroton. The microtron is a circular accelerator with only one resonating cavity. Electrons passing through this cavity are forced to an orbit in a homogenous magnetic field, and they are passing through the cavity repeatedly. The radius of the orbit is increasing with the speed of the electrons, and if appropriate energy is reached, the beam extracted by using a deflection tube. The beam can be used as electron beam with appropriate energy, or with appropriate target (Bremsstrahlung) an X-ray beam is produced. The improved version uses a multiple cavity system (race track microtron), with the same principles of operation. Its significance can be best characterized with the figures: while about 500 linacs are installed, only 1-2 microtrons are produced in a year.

Tomotherapy device and binary MLC
Chinese cyber knife

Tomotherapy. The principle of operation is the same as that of the spiral-CT; the radiation source is a low energy linear accelerator. It is adjusted with a binary MLC.
Gamma knife. Several number of Co-60 sources are applied on a spherical heavy metal segment containing radial boring for each source thereby collimating the individual beams (Leksell) or a limited number of radial collimated sources are mounted on a movable arch (Chinese solution). Both systems permit the exact irradiation of small volumes.
In medical practice the most important circular accelerators are the cyclotrons. They are used for the production of radioisotopes with short half-life (used in nuclear medicine, positron emission tomography, PET) and in radiation therapy (proton therapy and neutron therapy). For the latter purpose nuclear reactions of accelerated heavy ionizing particles (proton, deuteron, \alpha) ) are used.
The device contains two semicircular direct current magnets, and a short metallic cylinder divided in two sections (high frequency field connected between them). The particles injected from a source in the device’s centre, are accelerated by the electric field only between the magnets, and the magnetic field is forcing them onto a circular orbit. In the gap, they will receive again an increment of energy, and so on. The particles radius increases with the speed, and after an appropriate energy reached, the particles are deflected. If neutrons are required, then deuterons are accelerated to 15-50 MeV, and collided to some type of a low atomic number target, for example, beryllium. The peak of the energy-spectrum of neutrons generated in a nuclear reaction is between 6-20 MeV, depending on the energy of the colliding deuterium. The beam’s depth-dose curve looks like that of the cobalt sources. The only radiobiological advantage of neutrons is the oxygen effect is practically missing. (See in the radiobiology chapter).
The mono-energy particles’ depth-dose curve seems to be very attractive: Near to the surface only quarter of the maximum value, and (depending on energy) it increases suddenly at a greater depth (Bragg peak), and it falls to zero immediately. The problem is that the peak’s FWHM (full-width half-max) is 2-3 cm, so in clinical practice, it’s significantly less than the linear size of the irradiating area. So several beams have to be superimposed to raise the Bragg peak (e.g. replacing human tissue with filters), and with this, the benefits of the low surface dose can be completely lost.


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