Tube stand

The tube is mounted on a mechanism commonly referred to as the tube stand (Figure 8.2). The design of the tube stand enables manipulation of the beam direction by the machine operator in order to direct it at the patient’s lesion. There are both floor mounted and ceiling mounted stands. The design allows translational and rotational movement of the beam and there may be scales of distance and angle which enable the set-up position or changes to it to be recorded and monitored. Electrical or mechanical brakes are fitted to the rotational axes and translational runners in order that the position can be reliably fixed in position prior to treatment and monitored during treatment.

Figure 8.2 Orthovoltage machine on a stand.

High voltage circuits

On early machines, step up transformers were used to provide high voltages to excite the x-ray tube. The transformers operated at mains frequency and were very bulky due to the large iron core required.

Modern units employ switched mode power supply techniques, to improve voltage stability and make savings in size and efficiency. The power supply comprises of an input rectifier to convert mains AC to a DC voltage. A switching inverter converts the DC to a high frequency (25   kHz) pulsed waveform that is stepped up to 10   kV via a high frequency transformer. Finally, the waveform is rectified and passed through a number of cascaded multiplying stages (Cockcroft-Walton) to produce a DC voltage in the 100 to 300   kV range.

The extra high tension (EHT) voltage is monitored and a control signal fed back to alter the pulse width of the waveform leaving the inverter stage. In this way, a stable voltage is maintained across the x-ray tube.

The intensity of the x-ray beam produced at a particular kilovoltage depends upon the number of electrons emitted from the filament, this tube current is a function of the filament temperature and hence the filament current. The filament drive voltage is controlled electronically to respond to changes in the AC supply and stabilize the beam current. A schematic is shown in Figure 8.3. Typically, for voltages in excess of 200   kV, two high voltage power supplies are used in series.

Figure 8.3 Schematic of HT generator with feedback and connection to controller.

The HT generator is controlled externally so that safe operating parameters can be set for the particular tube in use. The generator status can also be fed to the operating system to indicate faults, to link into the interlock system to ensure safe treatment and to assist in fault diagnosis.

The high voltage and filament power supply is connected to the x-ray tube by means of a high voltage cable. In order to prevent voltage flash over in the connectors, care must be taken to keep all parts clean and use high voltage insulating grease during assembly and reassembly following maintenance inspection.

Collimation

The anode construction provides the initial collimation of the beam as the x-rays come off the target almost omnidirectionally (Figure 8.4). This is referred to as a hooded anode and the aperture defines the maximum size of conical x-ray beam that could be provided. The collimation is completed by use of an applicator which is fixed directly below the tube aperture and provides the following:

Figure 8.4 Anode construction, hood and omnidirectional emission with applicator to collimate beam and set SSD and define field size.

Beam energy

The x-ray beam from any bremmstrahlung target consists of a range of energies up to the maximum accelerating potential produced by the machine. Hence, for a 120   kV machine, 120   keV will be the accelerated electron energy and, in turn, the maximum x-ray photon energy will be 120   keV. This spectrum of energies is modified using a filter to remove lower energy x-rays and so reduce the dose to the skin. The beam that emerges from the tube through the x-ray window has undergone what is called inherent filtration. However, additional filtration is chosen to provide a beam with the desired depth dose penetration. The energy characteristics of the beam are often referred to as its quality and are dependent upon both the accelerating potential and filtration. The beam quality is normally measured using thicknesses of aluminum or copper and specified in terms of the half-value layer (HVL) which is the thickness of the layer of metal required to reduce the intensity of the beam by half. Typically, when this is measured it is done in a narrow beam, obtained by additional collimation to a broad clinical beam. The measuring ionization chamber is also placed at sufficient distance from the machine, floor and walls to ensure there is no additional scattered radiation. In practical terms, the penetration of the beam is also due to the source to surface distance and the field size. Depth dose data indicating the penetration of different HVL beams can be compared in the British Journal of Radiology, Supplement 25.

Control of output

The stability of the output from the unit is directly related to the electrical stability of the tube voltage and current. Stabilized electrical supplies as described above ensure that this is achieved consistently. Typically, a timer is used to control the amount of dose that is delivered. The timer is normally a countdown device and starts to operate when the treatment is initiated and the voltage and current are supplied to the tube. After the set time has elapsed, the voltage and current are switched off and treatment terminates. A timed exposure requires that the output dose rate is stable and that is dependent upon the electrical control stability mentioned above. There is always an inherent increase in the dose rate as the voltage and the current ramp up to their operating value and this effect is normally taken into account using a transit time to accommodate the underdose which this would otherwise lead to. The transit time is added to the calculated time based upon a nominal dose rate to produce a total set time.

Most modern deep x-ray (DXR) and SXR units utilize a full field ionization monitor chamber through which the beam passes. By measuring the charge released against the absolute dose under particular set-up conditions, it is possible to control the output from the machine by a system which terminates the beam when the accumulated charge equals the required dose. The units of charge from the ionization chamber are referred to as monitor units and the specific dose delivered to any individual patient is achieved by calculating the required number of monitor units to be set.

Skin and eye shielding

Beam collimation is provided on orthovoltage machines with a range applicators which have regular field sizes. These applicators are usually chosen by the user when the unit is purchased. The choice is made to provide a nominal range of sizes for coverage of typical lesions. Often, it is necessary to treat lesions of non-standard size and irregular shape. It is common practice to manufacture a lead cut-out to define the treatment field exactly to the area specified by the radiotherapist. The cut-out is used in conjunction with one of the regular sized applicators to irradiate the treatment area and shield the surrounding normal skin from the rest of the beam. For lesions on the face and in close proximity to the eyes, a lead mask is manufactured based upon a plaster cast of the patient. As well as defining the treatment field and shielding the normal skin and the eyes, the lead mask can also be used to provide direction and localization of the beam (Figure 8.5).

Figure 8.5 Irregular lead cut-out and mask.

To provide effective shielding, the thickness of lead for the cut-out has to be chosen taking into account the energy of the beam. This can be done from tabulated data of attenuation in lead against field size and HVL (beam energy). It is always prudent, however, for the Radiotherapy Physicists to undertake measurements and verify the shielding that is being obtained with the chosen thickness of lead.

When the treatment area impinges onto the eye, it is possible to insert eye shields to provide some protection to the lens. There are commercial lead and tungsten shields available. The problem encountered with eye shields is the contribution from scatter which reaches into the region under the shield from the surrounding field.

Calibration of dose output

As described above, there are two types of exposure control utilized on orthovoltage equipment. For a great many years, it has been done utilizing a timer. However, in more recent years, exposure is controlled by a full field ionization monitor chamber mounted in the radiation beam.

For timer control, the machine operator requires to know the dose rate of the machine and, for ionization chamber control, the response of the chamber must be directly related to an absolute dose. The ionization chamber response is normally quantified in monitor units and calibrated to be dose per monitor unit. Hence, for both systems of control, it is necessary to measure the absolute dose delivered by the beam under a very specific set-up, often referred to as the calibration conditions. The time to be set or the monitor units to be set are then determined using relative factors from the calibration set-up. For example, the specific dose rate may be measured for a 5   cm diameter applicator at 30   cm source to surface distance (SSD). In order to calculate a time to set, relative factors for other applicators, difference in SSD and changes in the irradiated area are used.

The absolute dose is measured in accordance with a protocol or Code of Practice. This ensures uniformity of practice between institutions. It will utilize the absolute dose calibration of the ionization chamber being used for the measurement and this will be traceable to a Standards Laboratory. The calibration can be done in air with the use of mass absorption coefficient ratios to determine the dose to water or tissue. Alternatively, the measurement can be done directly at depth in water. For superficial kV units with HVL values up to 8   mm Al, it is typical to determine the surface dose rate as this is where the dose is to be applied. However, for deep x-ray units with HVL values higher than 8   mm Al, it can be preferable to quote the dose deeper than the surface and closer to the target. In the latter case, it is preferable to calibrate at depth in water.

General layout and components

The major core sections of a linear accelerator serve the purpose of producing a high energy electron beam. These components consist of an electron source, a source of radiofrequency (RF) electromagnetic waves and an accelerating waveguide. These core sections can now be found in some custom machines, such as Tomotherapy, and the principles of operation are identical. In this section, however, the description will be with regard to the isocentric gantry mounted machines which are now in widespread use throughout the world.

Besides the major core components, a modern medical linear accelerator consists of a gantry assembly in order to direct the beam into the patient and a radiation head which enables beam shaping. For x-ray beams, the target, which is bombarded by the electron beam to produce the x-rays, is contained in the radiation head. Steering and stability of the electron beam requires focusing, bending and steering coils. High voltage and high current sources are also needed along with vacuum pumping systems and cooling systems in order to create a stable machine environment for production of the beam.

Figure 8.6 illustrates the typical layout of the major components of a linear accelerator.

Figure 8.6 Typical layout of the major components of a linear accelerator. (A) General; (B) traveling wave guide;(C) standing waveguide.

The gun filament assembly produces electrons by raising tungsten to a sufficiently high temperature through electrical heating. There are two types of waveguide: standing waves and traveling waves. Although this affects the waveguide structure, both types use electromagnetic waves at a radiofrequency of approximately 3   GHz. In the traveling type, the electrons are carried along on an accelerating wave while the standing type utilizes the electric component of the wave to exert an accelerating force on the electrons.

The source of the radiowaves is either a klystron or a magnetron. The klystron utilizes a low power RF signal from a small cavity oscillator. This is applied to a high power electron stream in the klystron and results in a high power RF wave. By contrast, the magnetron is a multiple cavity device which produces a high power RF wave directly. Waveguide sections transport the RF from the magnetron or klystron to the accelerating waveguide section.

Because electrons are charged particles, there is a tendency for an electron beam to disperse as it travels along the accelerating waveguide. Fortunately, this can be countered by using the interactive force which is applied to any charged particle as it passes through a magnetic field. The magnetic fields that are used to counter this dispersion are produced by focusing coils. These coils are wound around the accelerating waveguide and produce a magnetic field flux parallel to the direction of the electron beam.

This interactive force between a magnetic field and the traveling electrons is further utilized with magnetic fields at right angles to the direction of the electron beam. One of these fields is generated by the bending magnet which bends the electron beam round into a trajectory appropriate for the radiation head. This magnet also plays a crucial role in the selection of the beam energy. By altering the strength of the magnetic field, which can be controlled by the electrical current flowing in the bending magnet coil, the appropriate energy selection can be made.

The other use of magnetic fields at right angles to the electron trajectory is to steer the beam into and out of the accelerating waveguide. It is important to maximize the number of electrons that are accelerated and therefore ensuring the most efficient trajectory of the beam along the guide minimizes the losses experienced. These magnetic fields are produced by the steering coils and control of the electric current within them can be used to adjust the trajectory for efficiency and correct beam alignment through the radiation head.

Beam production is not an energy efficient process and there is a lot of energy dissipated within the machine as heat. The stability of beam production relies on the stability of component dimensions, such as, for example, the RF cavities in the waveguide. These can be subject to expansion and contraction with heating. Hence, there is a need for a great deal of water cooling on the machine; the target, all the magnetic coils, the accelerating waveguide, the RF source and large electrical devices such as transformers. Adequate and stable cooling is essential for effective beam production.

The other essential ancillary aspect of the linear accelerator is the need for the accelerating waveguide to be under high vacuum. For some machine designs, the waveguide is factory sealed while for others vacuum pumps work continuously in order to maintain the high vacuum required.

The gantry construction of the standard medical linear accelerator is referred to as being isocentric. In effect, this means that all the main axes of rotation concerning the gantry, the radiation head and the patient couch intersect approximately though the same point in space referred to as the isocentre This is illustrated in Figure 8.7

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