A medical linear accelerator, or linac, is a particular type of machine that produces high-energy x-ray or electron beams for use in radiation therapy. This method of treatment is commonly referred to as “external-beam radiation therapy” because the radiation beams are generated at a distance outside the patient’s body.

Examples of different medical linear accelerators (linacs).
Image courtesy of Varian Medical Systems, Inc. All rights reserved.
Image used with permission from Accuray Incorporated.
Image used with permission from Accuray Incorporated.
Image courtesy of Elekta AB.

How does a linac work?

Electron acceleration

First, a tungsten filament is heated to high temperature so that it continuously emits electrons. Some of these electrons are then injected into a long tube called a waveguide where they are accelerated to high (megavoltage, or MV) energies by electromagnetic waves at microwave frequencies. As the electrons accelerate down the waveguide, they are also focused into a narrow beam by magnets. The beam of high-energy electrons emerging from the end of the waveguide is then steered in the direction of treatment delivery.

Before any radiation reaches the patient, however, the narrow electron beam must be modified to produce the type of treatment beam needed. A linac can deliver either an electron beam or a photon beam at a given time; it can’t deliver both types of beams at once.

Electron beam generation

If the patient is receiving electron-beam therapy—typically prescribed for certain types of lesions within a few cm of the skin surface—the narrow electron beam is sent through a series of metallic foils which scatter the electrons into a wide beam, and the wide beam is then shaped (or “collimated”) by special devices attached to the linac head so that only the part of the patient requiring therapy is irradiated.

Photon (x-ray) beam generation

If the patient is receiving photon-beam therapy, the narrow electron beam is instead directed at a thick metal target to produce a shower of x-rays. (The energies of these x-rays are 15-150 times higher than in a typical chest x-ray exam.) The wide beam of x-rays is then shaped down by a series of thick moveable blocks inside the linac treatment head in order to irradiate only the part of the patient requiring therapy. The x-ray beam is often then further shaped by a system of thin metal blocks called multi-leaf collimators (MLCs). The MLCs block radiation to prevent healthy tissue near the tumor from being irradiated.

This illustration shows the mechanisms inside a linac that produce and shape photon (x-ray) beams for radiation treatment.
Image courtesy of Varian Medical Systems, Inc. All rights reserved.

Radiation delivery

Most modern-day linacs are mounted on a gantry that can rotate in a full circle around the patient. Electron-beam treatments are generally delivered with the gantry in a fixed position whereas photon-beam treatments may be delivered either with the gantry at a fixed position or as it rotates. Most linacs can deliver photon and electron beams of various energies, which enables the doctor to choose a beam with the penetrating power most appropriate for a treatment. Photon beams penetrate deeper into the patient than electron beams, and higher energies penetrate deeper than lower energies.

What is the treatment process with a linac?

In the majority of cases, linacs are used to deliver a computerized treatment plan which is prepared in advance. After a medical doctor (radiation oncologist) prescribes a course of radiation treatment, a dosimetrist or medical physicist creates a plan on a computer to meet the prescription. The radiation oncologist then examines and approves the plan, and a medical physicist reviews and performs safety checks for the plan. Once the plan is deemed satisfactory it is transferred to the linac through a computer network. The patient is set up on the treatment table by radiation technology therapists (RTTs) and, using marks on the patient’s body and/or x-ray imaging devices, moved into a position that corresponds to the computerized treatment plan. When the patient is in the correct position, the RTTs can direct the linac to deliver the programmed treatment plan.

The number of treatment sessions (which can be referred to as “fractions”) and total radiation dose that a patient receives depends on the tumor type, the amount of dose tolerated by organs surrounding the tumor nearby, and the goal of treatment. The radiation for a single treatment session can often be delivered within a few minutes, but the total time for each treatment will vary depending on the time needed for patient set-up, the treatment method, and the total radiation dose delivered.

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