linear accelerator
Meaning of linear accelerator: A machine that drives electrons down a straight tube to near the speed of light and turns them into a treatment beam.
Definition of linear accelerator
A linear accelerator is a machine that speeds charged particles, usually electrons, along a straight tube using radio waves. In a hospital those electrons either strike a metal target to generate high-energy X-rays or are aimed at the body directly as an electron beam. The beam is then shaped and directed at a tumor during external beam radiation therapy.
How does it accelerate the particles?
Electrons are released from a heated filament at one end of a copper tube known as a waveguide. Powerful radio waves are fed into the tube, and their crests travel along it. Each electron is caught on a crest and pushed forward, in the way a surfer is carried by a wave. Over a couple of meters that repeated push brings the electrons close to the speed of light. The whole assembly is held under vacuum so nothing gets in the way.
X-rays or electrons?
| Photon beam | Electron beam | |
|---|---|---|
| How it is made | Electrons hit a metal target | Electrons leave the head directly |
| How deep it reaches | Through the body | Stops within a few centimeters |
| Typical use | Tumors deep inside | Skin and surface tumors, scars |
How is the beam shaped and aimed?
Leaving the head, the beam is a broad cone, which would irradiate far more tissue than intended. Inside the head sits a bank of tungsten leaves, each driven by its own motor, that slide in and out to cut the beam into the outline of the target. Moving those leaves during the exposure varies the radiation dose across the field, so more is delivered to the tumor and less to a nearby organ. The whole head swings on a gantry around the treatment couch, so beams enter from many angles and overlap only where the tumor is. Imaging built into the machine checks the position before each session.
What does the machine leave behind?
Nothing radioactive remains in the body, and a person treated on a linac is not a hazard to anyone afterward. The beam exists only while the machine is switched on. Staff stand outside the shielded room because they would otherwise accumulate exposure across many patients each day.
Also known as
Used in a sentence
Treatment was delivered on a linear accelerator over a course of daily fractions.
Good to know
External beam treatment leaves nothing radioactive in the body, which distinguishes it from implanted or injected radioactive sources.
Word origin
Where “linear accelerator” comes from
Linear derives from the Latin linea, meaning a line or thread, describing the straight path along which the particles are driven.