Medical Linear Accelerator (LINAC) Components

A LINAC accelerates electrons using microwave radiofrequency energy. Electrons can be used directly for superficial treatments or directed onto a target to generate high-energy X-rays for deeper treatments.

The electron gun releases electrons into the accelerating structure. It controls the supply of electrons that will be accelerated into a treatment beam.

A magnetron or klystron provides or amplifies high-power microwaves. These electromagnetic fields transfer energy to electrons inside the waveguide.

A magnetron generates microwave power. Some medical LINAC designs use it as the radiofrequency source.

A klystron amplifies microwave power supplied by an oscillator. It is used in some LINAC designs to deliver high radiofrequency power.

A vacuum structure containing microwave electric fields that accelerate electron bunches. The electrons gain kinetic energy as they travel through the waveguide.

The radiofrequency fields organise electrons into bunches and accelerate them at suitable phases of the microwave field.

In some LINAC designs, magnetic fields bend and focus the accelerated electron beam towards the treatment head. Other designs use a straight-through beam path.

High-energy electrons strike a high-atomic-number target, generating mainly Bremsstrahlung X-rays. The resulting photons are shaped and monitored before reaching the patient.

The target is removed from the electron path in machines designed for electron treatments. Electron scattering and collimation components spread and shape the beam for relatively superficial treatment.

A high-Z target converts some of the incident electron kinetic energy into X-ray photons through electron deceleration near nuclei. Much of the energy becomes heat.

A shaped metal filter can reduce the forward-peaked photon intensity and create a more uniform profile across the central part of the field at a specified depth. Some treatments use flattening-filter-free beams.

FFF photon beams omit the conventional flattening filter. They have a non-flat profile and may support higher dose rates; treatment planning and calibration account for their properties.

A fixed collimating structure in the treatment head that limits the maximum angular spread of radiation leaving the target region.

Movable dense metal jaws define a rectangular field and reduce radiation outside the chosen opening.

An MLC contains individually movable high-density leaves that shape the radiation field to match the treatment target and can move during modulated treatments.

Chambers in the treatment head measure beam output and help control delivered monitor units. Safety systems can terminate irradiation if monitored parameters are outside tolerances.

Monitor units are machine output settings linked to delivered dose through machine calibration and treatment-plan calculations. One MU is not universally equal to one cGy under all conditions.

The gantry rotates the beam around a defined isocentre, allowing irradiation from different directions. The patient is positioned relative to this geometric reference.

Megavoltage photons have a build-up region and can treat deeper targets. Electron beams deliver relatively high dose near the surface and fall off more rapidly beyond their useful range.

For megavoltage photon beams, secondary-electron build-up means the surface dose can be lower than the dose at dmax. Surface dose varies with field size, energy and accessories.

LINACs use interlocks and monitoring to prevent or interrupt irradiation when important machine or treatment conditions are not satisfied.

Higher-energy photon beams generally penetrate more deeply and have a deeper depth of maximum dose, but the final dose distribution depends on beam geometry, tissue and planning.

A LINAC combines electron acceleration, beam production, collimation, monitoring and accurate patient positioning to deliver a prescribed radiotherapy treatment.