Clinical Radiation Generators
F. MEGAVOLTAGE THERAPY
X-ray beams of energy 1 MV or greater can be classified as megavoltage beams. Although the term strictly applies to the x-ray beams, the γ-ray beams produced by radionuclides are also commonly included in this category if their energy is 1 MeV or greater. Examples of clinical megavoltage machines are accelerators such as Van de Graaff generator, linear accelerator, beta- tron and microtron, and teletherapy γ-ray units such as cobalt-60.
Cooling water Target
X-rays Focusing
coil
Filament Electrons
Input line voltage Figure 4.3. Diagram of a resonant transformer unit.
42 Part I Basic Physics
4.2. VAN DE GRAAFF GENERATOR
The Van de Graaff machine is an electrostatic accelerator designed to accelerate charged particles.
In radiotherapy, the unit accelerates electrons to produce high-energy x-rays, typically at 2 MV.
Figure 4.4 shows a schematic diagram illustrating the basic principle of a Van de Graaff generator. In this machine, a charge voltage of 20 to 40 kV is applied across a moving belt of insu- lating material. A corona discharge takes place and electrons are sprayed onto the belt. These elec- trons are carried to the top where they are removed by a collector connected to a spherical dome.
As the negative charges collect on the sphere, a high potential is developed between the sphere and the ground. This potential is applied across the x-ray tube consisting of a filament, a series of metal rings, and a target. The rings are connected to resistors to provide a uniform drop of potential from the bottom to the top. X-rays are produced when the electrons strike the target.
Van de Graaff machines are capable of reaching voltages up to 25 MV, limited only by size and required high-voltage insulation. Normally the insulation is provided by a mixture of nitro- gen and CO2 or sulfur hexafluoride (SF6). The generator is enclosed in a steel tank and is filled with the gas mixture at a pressure of about 20 atm.
Van de Graaff and resonant transformer (Section 4.1.E) units for clinical use are no lon- ger produced commercially. The reason for their demise is the emergence of technically better machines such as cobalt-60 units and linear accelerators.
4.3. LINEAR ACCELERATOR
The linear accelerator (linac) is a device that uses high-frequency electromagnetic waves to accel- erate charged particles such as electrons to high energies through a linear tube. The high-energy electron beam itself can be used for treating superficial tumors, or it can be made to strike a target to produce x-rays for treating deep-seated tumors.
There are several types of linear accelerator designs, but the ones used in radiation therapy accelerate electrons either by traveling or stationary electromagnetic waves of frequency in the microwave region (~3,000 megacycles/s). The difference between traveling wave and station- ary wave accelerators is the design of the accelerator structure. Functionally, the traveling wave structures require a terminating, or “dummy,” load to absorb the residual power at the end of the structure, thus preventing a backward reflected wave. On the other hand, the standing wave structures provide maximum reflection of the waves at both ends of the structure so that the com- bination of forward and reverse traveling waves will give rise to stationary waves. In the standing wave design, the microwave power is coupled into the structure via side coupling cavities rather than through the beam aperture. Such a design tends to be more efficient than the traveling wave designs since axial, beam transport cavities, and the side cavities can be independently optimized (3). However, it is more expensive and requires installation of a circulator (or isolator) between the power source and the structure to prevent reflections from reaching the power source. For further details on this subject and linear accelerator operation the reader is referred to Karzmark, Nunan, and Tanabe (3).
Charge collector
Dome
Filament
Electrons
X-ray tube
X-rays Charge
sprayer
+ Belt
20−40 kV
Figure 4.4. a Van de Graaff generator.
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CHaPtEr 4 Clinical radiation Generators 43
Figure 4.5 is a block diagram of a medical linear accelerator showing major components and auxiliary systems. A power supply provides direct current (DC) power to the modulator, which includes the pulse-forming network and a switch tube known as hydrogen thyratron. High- voltage pulses from the modulator section are flat-topped DC pulses of a few microseconds in duration. These pulses are delivered to the magnetron or klystron2 and simultaneously to the electron gun. Pulsed microwaves produced in the magnetron or klystron are injected into the accelerator tube or structure via a waveguide system. At the proper instant electrons, produced by an electron gun, are also pulse injected into the accelerator structure. Figure 4.6 shows the time duration of klystron (or magnetron) voltage pulse, microwave pulse, electron gun voltage pulse, and radiation pulse. The pulse duration in each case is the same (~5 ms). The interpulse duration is longer (~5 ms).
The accelerator structure (or accelerator waveguide) consists of a copper tube with its inte- rior divided by copper disks or diaphragms of varying aperture and spacing. This section is evacuated to a high vacuum. As the electrons are injected into the accelerator structure with an initial energy of about 50 keV, the electrons interact with the electromagnetic field of the
2 Magnetron and klystron are both devices for producing microwaves. Whereas magnetrons are generally less expensive than klystrons, the latter have a long life span. In addition, klystrons are capable of delivering higher-power levels required for high-energy accelerators and are preferred as the beam energy approaches 20 MeV or higher.
Wave guide system Electron
gun
Power supply
Magnetron or klystron Modulator
Bending magnet
Treatment head (straight beam)
Treatment head (bent beam) Accelerator tube
Figure 4.5. a block diagram of typical medical linear accelerator.
5µs Timing
pulse Klystron voltage pulse Klystron microwave output pulse Electron gun voltage pulse
0
0
0
0
0
−18 kV
−120 kV Radiation
pulse
5 ms
Figure 4.6. timing diagram for voltage, microwave, and radiation pulses. (From Karzmark CJ, Morton rJ. A Primer on Theory and Operation of Linear Accelerators in Radiation Therapy. rockville, MD: U.S. Department of Health and Human Services, Bureau of radiological Health; 1981, with permission.)
44 Part I Basic Physics
microwaves. The electrons gain energy from the sinusoidal electric field by an acceleration process analogous to that of a surf rider.
As the high-energy electrons emerge from the exit window of the accelerator structure, they are in the form of a pencil beam of about 3 mm in diameter. In the low-energy linacs (up to 6 MV) with relatively short accelerator tube, the electrons are allowed to proceed straight on and strike a target for x-ray production. In the higher-energy linacs, however, the accelerator structure is too long and, therefore, is placed horizontally or at an angle with respect to the hori- zontal. The electrons are then bent through a suitable angle (usually about 90 or 270 degrees) between the accelerator structure and the target. The precision bending of the electron beam is accomplished by the beam transport system consisting of bending magnets, focusing coils, and other components.