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Linear accelerator

Accelerating electrons is theoretically easy, but in practice, it became only available when high-power (above 2 MW), high frequency devices were developed. During the Second World War in Europe the high-power high frequency oscillator, the magnetron has been developed, and in the USA, the klystron invented, which is suitable for high-frequency amplifying. Both were a military secret, so until the end of the war, the medical application was out of question. The medical accelerators are working on 2,97 GHz.
In the magnetron the central cylindrical cathode is surrounded by the anode block made of copper, with a cylindrical cavity between them. In the anode block, the resonant cavities have a circular layout. The magnetron is placed in a homogenous magnetic field, perpendicular to the plane of the figure. The electrons, emitted from the central hot cathode, are moving to the anode on a complex way by the effect of DC pulses and the magnetic field. In case of resonance, high power, high frequency oscillation created, which can be coupled to the accelerator tube through the waveguide with the appropriate antenna. Usually a few hundred 2-5 \mu s wide pulses/second are created.

Section of Magnetron
Section of klystron
Block diagram of Linac

The klystron is not a high frequency generator, it is a microwave amplifier. It has two cavities (buncher and catcher) connected by a tube. The low level microwave to be amplified enters on the cathode side, and it modulates the electron beam velocity, so the electrons arrive to the second cavity sorted in compact bunches. In the second cavity the electron bunches are decelerated resulting in a high-power microwave, which has the same frequency as the input signal. 5-30 MW power can be reached with this unit.
In radiotherapy linear accelerators requiring lower power, operated only at 6 MV or below, exclusively magnetrons are used, and above 15 MV, almost all companies are using a klystron.
The most important units of the linear accelerator are illustrated on the block diagram: 1. pulsed power supply, 2. control console, 3. klystron, 4. wave guide, 5. circulator, 6. electron gun, 7.accelerator structure, 8. magnet and treatment head, 9. vacuum system, 10. automatic frequency control (AFC) system, 11. pressure system, 12. cooling water system (upper left modulator = modulator cabinet; middle: állórész = stand; right: C kar = gantry). The injection system is the source of electrons (electron gun) and the accelerated electrons are drifting through the anode into the accelerating wave guide. The electrons arrive in bunches in the proper time, and accelerated in the waveguide by the transmission of RF power. The length of the waveguide depends on the technique of acceleration. In the travelling wave devices, an electric field parallel to the waveguide is used. The slow bunches only take a short way in unit time, and the cavities close to the electron gun are relatively short. Later as the bunches are accelerating practically to the speed of light, longer cavities are required. At the end of the tube, the energy has to be absorbed or fed back to the input end of the waveguide. The well-defined electron beam energy is the advantage of this system. The disadvantages are the long waveguide, which makes the keeping of the bunch’s convergence more complicated, and only rotating drum suspension available.
Modern radiotherapy accelerators are usually standing-wave type (except one manufacturer’s devices) equipments. In these machines, the accelerator guide is about 1.7 times shorter than in the moving wave types. Further significant decreasing of length was possible by moving of the non-accelerating coupling cavities to side. To these, only a little more decreasing factor is the energy attached form the side in the wave guide. The result is a short accelerating tube (with the electron source), which can be directed to the isocenter using 6 MV, and a bending magnet is not necessary, in the 10-25 MV range it fits into a C-arm. The disadvantage is a wider electron spectrum.
The direction of the electron beam (as it exits the window of the accelerator tube) to the isocenter (if necessary) is possible in two ways. In moving-wave devices several magnets required to use the “slalom technique”. The electron beam is “slaloming” in the field of the first two magnets while the third one, which is hardly more than 90° bending magnet (the 90° magnet is not for focusing the beam, but spreading it), is directing the properly focused beam to the isocenter. In standing-wave machines achromatic, 270° magnet is applied, which has an appropriate magnetic field and it can focus and direct also a wider spectrum to the isocenter.
The beam can be used in two ways. If we want to apply it in an electron therapy, than the narrow beam is usually spread by two scattering foils. If bremsstrahlung radiation is required, then suitable target (e.g. tungsten target) is used. The target is not designed to produce a homogenous irradiation on the body surface, but at a depth of 10 cm and in a large (40x40 cm²) field. On the surface we observe the effect of the “overflattening” filter.
The accelerator is supported with a very complex latch system. Besides the direct safety latches, this is controlling the stability of the accelerator’s physical parameters. The ionisation chamber system is the most important of these. This controls not only the beam’s symmetry, homogeneity, dose rate, but also the dose delivery. The collimator system is added to form the proper field size.
The field shape can be modified with shielding materials (e.g. blocks), and the dose distribution can be changed with wedges. The latter is replaced in modern accelerators by a software: the collimator is moving (dynamic wedge), or an appropriate combination of 60° wedged field and an open field are resulting in the required wedged field. The following figure shows an X-ray unit (with EPID) combined with a linac.


Linac + on board imager (OBI) (Varian, Palo Alto, California engedélyével)

Developing the Multi Leaf Collimator (MLC) was a significant improvement. With this, a conformal shaping can be provided. The MLC can be an independent beam limiting device, or it replaces one of the collimator pairs. On modern accelerators two types of MLC are used. One type is used in the conventional radiotherapy (52-120 leaves, large fields, up to 40 cm), and the other type is the \muMLC used in fields below 10 cm, but in fine steps (stereotactic irradiation). The MLC is an essential device of conformal irradiation and intensity-modulated radiotherapy (IMRT).

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