Prepared bySALIM SHA N.M.MSc. MRP

LINEAR ACCELERATOR(LINAC)

LINAC ?

It is a device that uses high frequency electromagnetic waves to accelerate charged particles such as electrons to high energies through a linear tube.

The high energy electron beam itself can be used to treat superficial tumors, or it can be made to strike a target to produce x rays for treating deep seated tumors.

HISTORY

Late 1940’s idea of using LINAC in medical application become interested.

Medical LINAC have been in clinical use since the early 1950’s.

First one was installed at Hammersmith in 1952. 1956, the first patient was treated at Stanford

university in U.S. There had an 8 MeV x ray beam and limited

gantry motion. There were large and bulky

A 2-year-old boy the first patient to receive radiation in operation therapy from the medical linear accelerator at Stanford

COMPONENTS

SIX CLASSES

(i) Injection system; (ii) RF power generation system; (iii) Accelerating waveguide; (iv) Auxiliary system; (v) Beam transport system; (vi) Beam collimation and beam

monitoring system.

Injection system

The injection system is the source of electrons; it is essentially a simple electrostatic accelerator called electron gun.

Electron gun contain a heated filament cathode and a perforated grounded anode Electrons are thermionic ally emitted from the heated

cathode, focused into a pencil beam by a curved focusing electrode and accelerated towards the perforated anode through which they drift to enter the accelerating waveguide.

The electrostatic fields used to accelerate the electrons are supplied directly from the pulsed modulator in the form of a negative pulse delivered to the cathode of the gun.

Radiofrequency power generation system

The microwave radiation used in the accelerating waveguide to accelerate electrons to the desired kinetic energy is produced by the RF power generation system, which consists of two major components:

A pulsed modulator. An RF power source. Power supply provides dc power to the modulator, which

includes the pulse forming network and a switch tube known as hydrogen thyratron.

High voltage pulses from the modulator section are flat topped dc pulses of few microseconds in duration.

These pulses are delivered to magnetron or klystron and simultaneously to the electron gun.

The RF power source is either a magnetron or a klystron.

MAGNETRON

Device that produce microwaves.

It has a cylindrical construction, having a central cathode and outer cathode.

Space between cathode and anode is evacuated.

Cathode is heated by an inner filament and the electrons are generated by thermionic emission.

MAGNETRON

Static magnetic field is applied perpendicular to the plane of cross-section of cavities and a pulsed dc electric field is applied between cathode and anode.

Electrons emitted by cathode are accelerated towards anode by the action of pulsed dc electric field and by the influence of magnetic field electrons move in complex spirals towards resonant cavities, radiating energy in the form of microwaves.

KLYSTERON

It is a microwave amplifier.

The electrons produced by the cathode are accelerated by a negative pulse of voltage into the first cavity called buncher cavity.

The microwaves set up an alternating electric field across the cavity.

The velocity of the electron is altered by the action of this electric field to a varying degree-velocity modulation

KLYSTERON

Some electrons are speed up while others slow down and some are unaffected result and this results in bunching of electrons.

As the electron bunch reaches catcher cavity they induce charges on the ends of the cavity and there by generate a retarding electric field .

The electrons suffer deceleration and by the principle of conservation of energy the K.E of electrons is converted into high power microwaves.

GANTRY

Source of radiation can rotate 360 degree.

As the gantry rotates collimator axis moves in a vertical plane. The point of intersection of collimator axis and the axis of rotation of the gantry is known as iso center.

The gantry consist of: Electron gun, Accelerator Structure, Treatment Head

ACCELERATOR STRUCTURE

It consists of a copper tube with its interior divided by copper discs or diaphragms of varying aperture and spacing.

This section is evacuated to allow free propagation of electrons.

Electrons interact with the electromagnetic field of microwaves.

Electrons gain energy from the sinusoidal electric field by an acceleration process.

Auxiliary system

The linac auxiliary system comprises four systems:

A vacuum pumping system producing a vacuum pressure of ~10–6 torr in the accelerating guide and the RF generator;

A water cooling system used for cooling the accelerating guide, target, and RF generator;

An optional air pressure system for pneumatic movement of the target and other beam shaping components;

Shielding against leakage radiation.

Electron beam transport

Two systems for electron bendinghave been developed:

90º bending; 270º bending; The system use bending magnets.

Linac treatment head

The important components found in a typical head of a fourth or fifth generation linac include:

X ray targets; Flattening filters and electron scattering foils

(also called scattering filters); Primary and adjustable secondary collimators; Dual transmission ionization chambers; A field defining light wedges; Optional MLC.

Schematic diagram of linac head component (a) photon beam (b) electron beam

Scattering foil &Flattening filter Electron beam exist the window, is a

narrow pencil about 3mm in diameter. The narrow pencil beam is made to

strike an electron scattering foil to spread the beam and uniformity.

Made of thin metallic foil, usually lead. Flattening filter to make the beam

intensity uniform across the field Made of lead, aluminum, tungsten,

uranium, steel.

Beam collimation

In a typical modern medical linac, the photon beam collimation is achieved with two or three collimator devices:

A primary collimator; Secondary movable beam defining

collimators; An MLC (optional).

Techniques used with Medical Linacs

With the current trend using linear accelerator a variety of treatment techniques are possible. Some of the techniques as follow, 1. 3D conformal radiotherapy 2. Intensity Modulated Radiotherapy 3. Stereotactic Radiotherapy(SRT) 4. Stereotactic Radiosurgery(SRS) 5. Dynamic Adoptive Radiotherapy (DART) 6. Image guided Radiotherapy (IGRT)

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