linac
Meaning of linac: The everyday shorthand used in radiotherapy departments for the machine that produces the treatment beam.
Definition of linac
Linac is the standard abbreviation for linear accelerator, the machine used to deliver external beam radiation therapy. It drives electrons along a straight tube and either sends them into the body directly or converts them into high-energy X-rays. Its head rotates on a gantry around a treatment couch, so a beam can be directed from many angles in a single session.
What is in the treatment room?
The machine dominates it: a large rotating arm on one side, a flat couch that moves in several directions in the middle, and imaging panels that fold out. The walls are thick concrete and the door is heavy, because the room has to contain the beam. Lasers mounted on the walls project crosshairs used to line the body up in the same position at every visit. Staff step outside during the exposure and watch on camera and intercom.
Why is it called megavoltage?
| Kilovoltage X-ray | Megavoltage beam | |
|---|---|---|
| Energy | Thousands of volts | Millions of volts |
| Where the dose peaks | At the skin | Below the skin surface |
| Main purpose | Diagnostic imaging | Treatment of deep targets |
The higher energy is what allows a beam to reach a target deep in the body without burning the skin on the way in, a property known as skin sparing. It is the central advantage a linear accelerator has over the older X-ray machines used before it.
How does a course of treatment run?
Planning comes first. A scan is taken in the exact position that will be used each day, often with a mold or mask made to hold the body still, and small permanent skin marks are placed as reference points. Physicists and clinicians then design the arrangement of beams and calculate the dose.
Treatment is usually given in small daily amounts, called fractions, spread over weeks. Dividing it this way gives healthy tissue time to repair between sessions while the tumor recovers less well. Each visit takes minutes, most of it spent on positioning rather than on the radiation itself, which lasts moments. Nothing is felt during the exposure, and no radioactivity remains afterward.
Also known as
Used in a sentence
The department scheduled the remaining fractions on a second linac while the first was serviced.
Good to know
Linac and linear accelerator name the same machine, and the term megavoltage describes the beam energy rather than a different type of equipment.