LINEAR ACCELARATOR

INTROUDUCTION:

• A linear accelerator is a machine that accelerate the charge particle to higher energy in to the linear path.

• LINAC therapy allows to deliver higher doses of radiation to the tumor with limited damage to the surrounding healthy tissue and/or organs

• Uses higher energies for the treatment of cancer patients.

Anatomy of LINAC:

Vertical mounting (LINAC 1 at AKUH)

Horizontal Mounting (LINAC 2 at AKUH)

Auxiliary system: It consist of four system that are not involved in electron acceleration.

• A vacuum pumping system producing a vacuum pressure in the accelerating guide and the RF generator.• A water cooling system to accelerating waveguide,RF

generator. It provides stable temperature.• Air pressure system.• Shielding against leakage radiation.

Components:

• Modulator cabinet• Console control• Drive stand • Gantry• Treatment couch• Other accessories

Modulator cabinet:

• It distribute primary electrical power and high voltage to magnetron and klystron • it located in treatment room • it major components

The fan control: need arises for cooling the power distribution Auxiliary system: contains the emergency off button Primary power dist: Primary power-distribution system

Console control:

• Monitoring and controlling the LINAC

• All interlocks checks to allow beam started

• Provide digital display of monitor units

• Mechanical beam parameters

Drive Stand:

• Klystron/Magnetron: power source used to generate electromagnetic waves for the accelerator guides

• Waveguide: hollow tube-like structure that guide the electromagnetic waves from the magnetron to the accelerating guide where electrons are accelerated

• Circulator: connects between the klystron/magnetron and waveguide that prevents any reflected microwaves.

• Water-cooling system: allows many components in the gantry and drive stand to operate at a constant temperature

Magnetron:

Device that provides

high-frequency microwave

Accelerate the

electrons into the

accelerating waveguide.

Preferred for lower energies

<=6

Source of microwave about 3000

MHZ

Klystron:

• Source of microwave radio frequency RF

• Also microwave amplifier

• To accelerate the electron into waveguide

• For megavoltage energies >6MV

Electron gun:

• source of electrons; it is essentially a simple electrostatic accelerator called an electron gun.• Two types of electron gun are in use as sources of electrons

in medical linacs.• — Diode type;• — Triode type.

Beam transport system:

To transport electrons from accelerating waveguide to the x-ray target.• Bending magnet

90 degree bending270 degree bending

• Steering coils• Focusing coils

Diagram:

Gantry: Responsible for directing the photon (x-ray) energy or electron beam at a patients tumor.

• Electron gun: Produce electrons and injects them into the accelerator structure

• Accelerator structure: A special type of wave guide in which electrons are accelerated.

• Bending magnet. Alter the path of the electrons from waveguide into the x-ray target

• Treatment head: Components designed to shape and monitor the treatment beam X-ray target Primary collimator Ion chambers Flattening filters Secondary collimators Field light

Treatment Head Diagram:

X-ray target:

• High z material tungsten

• It thickness maximize the x-ray output and minimize electron contamination

Primary collimators Secondary collimators:• Fixed just beyond the x-ray

target

• Design to limit maximum field size

• It absorbs scatter from the target

• Place Away from the x-ray target

• Secondary collimators are typically independent jaws

• It allow the field to be shaped into a variety of rectangular shapes.

• There are two sets of jaws higher jaws (y-axis) lower jaws (x-axis)

•  independent jaws to perform dynamic wedging.

Flattening filter :

• The flattening filter is a cone shaped 

• change the beam profile at depth

• Absorbs photons on the central axis

• Producing a more uniform beam profile at the treatment distance.

Scattering Foils:

• Typically consist of dual lead foils.

• To ensure minimize the bremsstrahlung x-rays

•  narrow beam is usually spread by two scattering foils

• This converts the beam from a pencil beam to a usable wide beam

Ionization chambers:

• Ionization chambers embedded in linac clinical x-rays and electrons for dose monitoring for safety of the patients

• Two separately ion chambers

• It position between the flattening filters or scattering foils and secondary collimators

Dose monitoring:

• For patient safety, the linac have two electrodes to monitor beam output and connected to the counter.

• If the primary chamber fails during patient treatment, the

secondary chamber will terminate the irradiation.

• only a few per cent above the prescribed dose has been delivered.

• The beam energy, flatness and symmetry.

Multi Leaf Collimator (MLC):• Used to define any field shape for radiation

beams

• The number of leaves in commercial models with 120 leaves (60 pairs) covering fields up to 40 × 40 cm2

• MLCs are becoming valuable in supplying intensity modulated fields in conformal radiotherapy, either in the step and shoot mode or in a continuous dynamic mode.

Field light and lasers:

• It is a Field localizing device.

•  Used to display the position of the radiation field on the patient skin.

• The accuracy of the laser guides in determining isocentre position

•  Tolerance of laser position is 2 mm

Other accessories:

• Custom Blocks: In the event that MLCs are unable to produce the desired field shape. They are

limited by their cost, weight, and time issues • Electron Cone and Electron Cutout:

The electron cone attenuates electrons that are scattered outside of the required beam shape before they reach the cutout

• Electronic portal imaging:Electronic portal imaging refers to an electronic detector replacing the

radiographic film Provides feedback on single-event setup accuracy or observation of treatment

in near real-timethe patient can be repositioned before treatment takes place.

Patients supports:

• Treatment couch: mounted on a rotational axis around the isocenter Also called patient support assembly (PSA)

• Immobilization devices

THANK YOU ...