The production of x-rays was first performed by Roentgen in 1895, using a device that accelerated electrons between an anode and a cathode. The basic principles of x-ray production today are the same, although refinements have been made to produce a clinically useful beam.
A superficial radiation therapy unit generates x-rays at kilovoltage energies (50 – 150 keV). The core part of the machine is the x-ray tube, which is in a vacuum. The vacuum tube consists of:
- A heated cathode which emits electrons. A focussing cup directs emitted electrons towards the positively charged anode.
- A copper anode with a tungsten target.
- A beryllium window in the path of the x-ray beam. This filters out the lowest energy photons to harden the beam. This allows selection of beam energy, coupled with the voltage applied between the anode and cathode
- A glass window through which the x-rays emerge.
The cathode and anode have a high voltage applied between them, which accelerates the electrons towards the anode, increasing the likelihood of brehmsstrahlung occurring.
A cooling system must be present to disperse the heat generated in the target, as most energy is lost as heat in the target.
A control system and power system must also be present for the machine to function.
When treating a superficial lesion, an applicator is used. These are either cones or cylinders, ranging in size from 1 – 10 cm. Custom cut-outs may be used in the larger cones to define an irregular area. The side of the cylinder or cone is made up of tungsten or some other high atomic number number, preventing scatter of electrons to regions outside the area of treatment.
Orthovoltage Unit
Orthovoltage units use x-rays with energies of 150 - 500 keV. They have a similar design to standard x-ray tubes, but also include:
- Extra shielding around the target anode to absorb the higher energy scattered photons.
- Increased voltage between the cathode and anode to increase the energy of generated photons
- Jaws may be used to alter the beam shape and size as it emerges from the tube
Filtration
To control the energy spectrum produced by a kilovoltage machine, filters are placed in the path of the beam. These filters selectively attenuate the desired part of the beam spectrum; this usually hardens the beam (removes low energy photons). For a full discussion on filtration, see the filtration topic.
