While kilovoltage machines are suitable for superficial treatments and diagnostic imaging, they are limited by the high skin dose arising from their low energy photons. Photons with energies over 1 MeV are desirable in the setting of deeply seated tumours, as they allow significant skin sparing. Unfortunately, the voltages required to accelerate the electrons to the desired speeds are unrealistic for application in an x-ray tube, and therefore another method of megavoltage photon generation was required.
This was first accomplished on a worldwide basis by using cobalt-60, a radioactive isotope that decayed over several years. It releases two photons of energies 1.17 and 1.33 MeV. Techniques using isotopes were limited by the energies released by isotopes, and there were also radiation issues associated with their use.
The linear accelerator is a device that uses an electromagnetic field to accelerate electrons close to the speed of light. By bombarding a target with these electrons, photons with energies of 18 MeV (and above) are realisable. Like the conventional x-ray tube, the electrons generate bremsstrahlung radiation in a tungsten target; however their highly increased energies means that the design of the machine is significantly different.
The slang term/abbreviation for a linear accelerator is 'Linac'.
Components of a Linear Accelerator
The linear accelerator generates photons in several steps:
- Electrons must be generated and guided into the accelerating waveguide - the electron generation component
- Electrons must be accelerated close to the speed of light - the electron acceleration component
- Electrons must be transported to the x-ray target - this is beam transport
- Bremsstrahlung occurs in the target, generating a photon beam
- The beam must be modified for clinical use in the treatment head. This includes collimation, flattening filters and ionisation chambers.
Several accessories are required for the linear accelerator to work:
- A radiofrequency generator that produces the electromagnetic wave in the acceleration component
- A pulse modulator which generates timed pulses of energy to the electron gun and the RF generator
- A control panel to operate the linear accelerator