A Linear Accelerator, or Linac, is the standard method of producing photons and electrons for radiation therapy treatments. Linacs account for the majority of treatment machines in Australia. Linacs are intricate computer driven machines that deliver precise, known doses of radiation to treat patients.
Linacs produce high energy electron or photon beams by means of an accelerating structure. Electrons are injected into one end of this structure and are accelerated as they travel towards the other end. If an electron beam is required, these electrons are then passed through several components before exiting the machine
When a photon beam is required, the electrons are directed towards a high density target material. These electrons are stopped in this material and result in the production of photons. These photons pass through several components in the machine to turn them into a therapeutic beam, before exiting the linac and entering the patient.
Linacs are mounted isocentrically and the machine can be split up into four major subsections: the stationary gantry stand/support, Treatment Table and Control Console.
GANTRY:
The gantry rotates around a central position called the isocentre. The Linac can be rotated into position about the horizontal gantry axis for use in treatment.
GANTRY STAND:
The Gantry stand houses electronic and other systems.
TREATMENT TABLE:
The treatment table is used to precisely position the patient under the treatment machine.
TREATMENT CONSOLE:This is positioned outside of the treatment room. The control console uses CCTV to maintain visual (and audio) contact with the patient whilst staff are outside of the room.
INJECTION SYSTEM:
The injection system is the source of electrons which are fed into the accelerating waveguide. The electrons are generated by a heated filament cathode, focused into a pencil beam by a focusing electrode and accelerated towards a perforated anode through which they drift into the accelerating waveguide.
RF POWER GENERATING SYSTEM:
RF (radiofrequency) power is used in the accelerating waveguide to accelerate electrons to the desired kinetic energy. The RF power generation system consists of two major components; an RF power source and a pulsed modulator. The RF power source is either a magnetron or a klystron.
ACCELERATING WAVEGUIDE:
The basic accelerating waveguide structure is a stack of cylindrical cavities connected by an axial hole. Electrons from the injector system pass through the waveguide and gain energy from the electric field in each cavity. The electric field is established by the RF pwer generation system
AUXILLARY SYSTEM:
The linac auxiliary system consists of several services which indirectly support electron generation. The Linac auxillary system comprises several systems depending on machine design:
• A vacuum pumping system to produce a vacuum of approximately 10-6 torr in the accelerating guide and RF generator.
• A water cooling system used for cooling the accelerating guide, target, circulator and RF generator.
• Shielding against leakage radiation.
