Medical Linear Accelerator

Overview

A medical linear accelerator, or linac, is the standard machine for external beam radiotherapy. It accelerates electrons to several million electron-volts and either uses them directly or smashes them into a metal target to make high-energy X-rays. The beam is then shaped to the three-dimensional outline of a tumour and aimed from many angles so that a high dose builds up in the target while each individual entry path through healthy tissue receives much less. A typical course delivers 1.8–2 Gy per daily session over several weeks.

The machine is built around the Accelerating Waveguide and its RF Power Source source, mounted on a rotating Rotating Gantry above a robotic Treatment Couch, with beam shaping in the Treatment Head and Multileaf Collimator.

Accelerating the electrons

Electrons start at the Electron Gun, where a heated Dispenser Cathode emits them and a Control Grid chops the emission into bunches timed to the microwave field. They enter the Accelerating Waveguide, an evacuated copper structure whose Accelerating Cavities resonate at about 2,856 MHz. Microwave power fed through the RF Window sets up an electric field that the electron bunch "surfs", gaining roughly 15 MeV per metre, so a metre-long structure reaches the megavolt range. The structure is held below 1e-7 torr by a Ion Pump because any gas would scatter the beam and spoil the vacuum the cathode needs.

The microwave power comes from the RF Power Source section. A Klystron amplifies a low-power signal to several megawatts of pulsed RF, driven by the Pulse Modulator and its Thyratron switch, which fire microsecond high-voltage pulses. A RF Circulator protects the klystron from reflected power. Smaller machines use a self-oscillating magnetron instead of a klystron-plus-driver chain. After acceleration the Bending Magnet turns the beam through 270° and refocuses it onto the target, correcting energy spread in the process.

Producing and shaping the beam

In photon mode the electron beam strikes the X-ray Target, a tungsten slug that converts the electrons' energy to bremsstrahlung X-rays. These emerge sharply peaked forward, so a Flattening Filter evens them into a uniform field, and the Primary Collimator defines the maximum cone. A sealed Transmission Ion Chamber continuously measures the delivered dose and beam symmetry and terminates the beam when the prescribed monitor units are reached. The Collimator Jaws set a rectangular field up to 40×40 cm, and the Multileaf Collimator sculpts that field: up to 120 tungsten MLC Leafs, each driven by its own Leaf Drive Motor, slide in and out under the MLC Controller to match the tumour outline. By moving the leaves while the beam is on, the linac modulates intensity across the field (IMRT) so the dose conforms tightly to concave targets.

Geometry and positioning

The whole beam line rides on the Rotating Gantry, whose Gantry Ring rotates ±185° about a fixed point in space called the isocentre, kept within about 1 mm by the Gantry Drive and balanced by a Counterweight. Power and signals cross the rotating joint through the Slip Ring. The patient lies on the Treatment Couch, whose carbon-fibre Couch Top and six-axis Couch Translation Drive place the target volume precisely at the isocentre. Rotating the gantry and couch lets the beam enter from many directions that all converge on the tumour.

Imaging, cooling, and safety

Before each fraction the On-board Imaging System confirms setup: an orthogonal kV X-ray Source and Flat-Panel Detector acquire cone-beam CT or planar images that are matched to the plan, and the couch is shifted to correct any offset. The Cooling System holds the Klystron, waveguide, target, and magnets at constant temperature, since thermal drift changes the resonant frequency and beam energy; a Coolant Flow Switch inhibits the beam if coolant flow falls. All of this is sequenced by the Control System, whose Safety Interlock System is a hard-wired chain that prevents beam-on unless the door is shut, dose monitors agree, collimators match the plan, and cooling is healthy, the layered protection that defines a Class I radiotherapy device.