Washington Chapter 7 Linear Accelerators Definitions

Modulator cabinet

contains components that distribute and monitor primary electrical power and high-voltages pulses to the magnetron or klystron – usually located in the treatment room – contains fan control switch, primary  and auxiliary power distribution

Drive Stand

large rectangular cabinet containing the apparatus that drives the linear accelerator, open to both sides for easy access to gauges, valves, tanks and buttons, houses the klystron, waveguide, circulator and cooling system 

Klystron

converts kinetic energy to microwave energy in the linac. Also amplifies microwaves

Magnetron

similar to klystron, except does not require a separate oscillator.

Waveguide (accelerator wave guide, Accelerator structure)

hollow, tube-like structure used to accelerate injected electrons to near the speed of light prior to striking a target to produce photons

Circulator

prevents backflow of microwave power back to the klystron

Electron Gun

responsible for producing electrons and injecting them into the accelerator structure

Bending Magnet

used in high energy linear accelerators to bend the electron stream within the head of the gantry, at 270 or 70 degrees.

Treatment Head

houses the components designed to shape and monitor the beam

Flattening Filter

located on the carousel with the electron scattering foils, that is cone shaped and placed in the photon beam to shape the beam in its cross-sectional dimension.  “Flattens” the beam for more uniform treatment. 

Field Defining Light

light from an iodine-quartz bulb, reflected from a mirror in the treatment head, that outlines the dimensions of the radiation field as it appears on a patient.

Range Finder\Optical Distance Indicator

projects a scale onto the patient’s skin which corresponds to the SSD used during the simulation or treatment process

Electron Applicator

also electron cone, accessory used to direct and focus electrons into the field, brings collimation of beam closer to patient’s surface, improving radiation-dose distribution by sharpening the dose gradient at the beam edges, holds electron mask for custom beam shaping

Target

flowing electrons from the accelerator structure hit the target for photon production, target is moved out of the way if electron beam is desired.

Scattering Foil

most common method of producing an electron beam that is wide enough for clinical use; thin sheet of material with a high Z number is placed in the path of the pencil beam of electrons.  There may be two scattering foils, the first then widens the beam, the second improves the beam flatness.

Primary collimator

within the treatment head, after emerging from the target, the photon beam then is shaped by the primary collimator; it is a stationary collimator that limits the beams maximum field size.

Secondary Collimator

used to set field size, these are adjustable collimators that are designed to reduce transmission of the beam to less than 2% of the central axis dose – in other words, they define the field size. 

Record and Verify Program

verification system that verifies treatment parameters set on the machine; allows incorrect setup parameters to be corrected before the machine is turned on, and also provides other data such as computer assisted setups, recording of patient data, allows for data transfer from the simulator or treatment planning computer, and assists with quality control.

Beam Stopper

opposes the treatment head on the gantry, and is designed to intercept the primary beam after it interacts with the patient to reduce room shielding requirements

PSA

patient support assembly, treatment table or couch, on which patients are positioned to receive treatment.

Pendant

set of handheld local controls suspended from the ceiling or attached to the treatment couch that mimics the movements of the treatment unit.  Controls most machine movements.

HVL

Half value layer – the thickness of absorbing material necessary to reduce the x-ray intensity to one half its original value

Treatment table

patient support assembly, or couch, on which patients are positioned to receive treatment

Lipowitz Metal

used in radiotherapy to create shielding blocks and compensators for x and gamma rays.  Aka cerrobend, it is 50% bismuth, 26.7% lead, 13.3% tin, 10% cadmium, has lower melting point than lead at 70° C (327° C for lead).  It is also harder and lighter than lead at room temp.

Accelerator Guide

Accelerator structure is mounted in the gantry.  Microwave power is transported to the accelerator guide, at the same time where electrons have been shot into it.  The microwaves travel in ‘waves’, the electrons ride the ‘wave’ and are accelerated to nearly the speed of light before emerging to either hit a target (produces photon beam) or continue on as an electron beam

Conformal Therapy

ttherapy that with the use of 3D treatment planning, allows the delivery of higher tumor doses to select target volumes without increasing treatment morbidity

Multileaf collimator

part of the secondary collimator system, allow treatment field shaping and blocking through the use of 20-120 motorized leaves located in the treatment head of the machine.  Used in conformal therapy in place of heavy straight edge or custom made divergent blocks

Cyclotron

a charged particle accelerator used mainly for nuclear research and form generating proton and neutron beams.  It accelerates the participles in a spiral pattern

Betatron

a megavoltage unit that can provide x-ray and electron therapy beams from less than 6 MeV to more that 40 MeV.  Largely obsolete for clinical treatments.