LINEAR ACCELERATOR

Radiation Protection in Radiotherapy

5.3 Linear Accelerator

Part 5, lecture 2: Equipment - superficial, telecurie 1

INTRODUCTION5.3.1 History5.3.2 Components5.3.3 New Technologies

Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 3

INTRODUCTION

•Photon Beam (X-Ray):• 4 MV To 22 MV.• Single Beam.• Duel Beams

•Electron Beam:• Multi-Beams with energy grope between: 4- 22 MeV.

INTRODUCTION

WHAT IS LINACA linear accelerator is a device that us

es high Radio‐Frequency (RF)electromagnet

waves to accelerate charged particles (i.e. elect

rons) to high energies in a linear path, inside a

tube like structure called the accelerator waveg

uide. The resonance cavity frequency of the

medical linacs is about 3 billion Hertz (cycles/

INTRODUCTION

Used to treat all parts and organs of the body Uses microwave technology to accelerate electrons in the part of the accelerator called the wave guide, then allows these electrons to collide with a heavy metal targetThe high energy x-rays are shaped as they exit the machine to conform to the shape of the patient's tumor

How it worksINTRODUCTION

X-ray treatments are designed in a way that destroy the cancer cells while sparing the surrounding normal tissue

High energy photons enter the patient's body and aim to break the DNA in all the cells within the treatment area

The good cells are able to mend themselves The cancerous cells are unable to do this and therefore die

How It Works Continued

Patient lies on a moveable treatment couch which can move in any direction

The beam comes out of a part of the accelerator called a gantry, which can be rotated around the patient

Radiation is delivered to the tumor from any angle by rotating the gantry and moving the treatment couch

How it works continued

Advantages: particles are able to reach very high energies without the need for extremely high voltages

Linear accelerators attack the affected area with higher doses of radiation than other machines

Advantages

Disadvantages: A linear accelerator can cost anywhere between one million and three million dollars. Operating the machine costs about $900,000 annually.

The particles travel in a straight line, each accelerating segment is used only once. The segments run in short pulses, limiting the average current output and forcing the experimental detectors to handle data coming in short bursts, thus increasing the maintenance expense

Disadvantages

5.3.1 History

Early Accelerators (1953-1961):

Extremely large and bulkyLimited gantry motion

5.3.1 History

Second Generations (1962-1982):

360 degree rotational Allow treatment to a patent from any gantry angleImprovement in accuracy and dose delivery

5.3.1 History

• Third generation accelerators:

• Improved accelerator guide• Magnet systems• Beam-modifying systems to

provide wide ranges of beam energy, dose rate, field size

• Operating modes with improved beam characteristics

• Highly reliable• Compact design• May include: dual photon energies,

multileaf collimation, several electron energies & electronic portal verification systems

5.3.1 History

5.3.2 Components

1.Modulator cabinet 2.Console3.Drive Stand

I. KlystronII. WaveguideIII.CirculatorIV.Water-cooling system

4.Gantry I. Electron gunII. Accelerator structureIII.Treatment head

i. Bending magnetii. Flattening filteriii. Scattering foiliv. X-ray Targetv. Collimatorvi. Beam Modificatio

Treatment Head

(Straight Beam)

Power Supply

Modulator

Electron Gun

MagnetronOr

Klystron

Wave Guide system

Treatment Head

(Bent Beam)

Bending Magnet

Accelerator Tube

A block diagram of typical medical Linear Accelerator

5.3.2 Components

A power supply Provides DC power to the modulator

Modulator

the magnetron or klystron

Deliver the pulses to the electron gun

the accelerator tube or structure via a waveguide systems.

Assignment: Briefly describe the process within a linear accelerator starting from the power supply to the production of a 3 mm pencil beam.

5.3.2.1 Modulator Cabinet

• Modulator cabinet: contains components that distribute and monitor primary electrical power and high-voltage pulses to the magnetron or klystron

• Located in the treatment room• Three major components:

• The fan control: automatically turns the fans off and on as the need arises for cooling the power distribution

• Auxiliary power-distribution system: contains the emergency off button that shuts off the power to the treatment unit.

• Primary power-distribution system

5.3.2 Components

5.3.2.2 Console

• Console electronic cabinet: provides a central location for monitoring and controlling the linac• Take the form of a digital display, push button panel or video display terminal (VDT)

• All interlocks must be satisfied for the machine to allow the beam to be started

• Provides a digital display for prescribe dose (monitor units), mechanical beam parameters such as collimator setting or gantry angle

5.3.2 Components

5.3.2.3 Drive Stand • Drive Stand: a stand containing the apparatus that

drives the linear accelerator• Open on both sides with swinging doors for easy access

to gauges, valves, tanks, and buttons• 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: directs the RF energy into the waveguide and prevents any reflected microwaves from returning to the klystron

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

5.3.2 Components

I. KlystronII. Magnetron III.WaveguideIV.Water-cooling system

I- Klystron

5.3.2 Components

I. KlystronII. MagnetronIII.Water-cooling system

II- Magnetron

• Magnetron: device that provides high-frequency microwave power that is used to accelerate the electrons in the accelerating waveguide.

• Electrons are emitted from the cathode and spiral in the perpendicular magnetic field. The interaction of the spiraling electrons with the cavities in the anode creates the high-frequency EM waves.

• oscillator and amplifier used in low-energy

II- Magnetron

• Waveguides are evacuated or gas filled metallic structures of rectangular or circular cross-section used in the transmission of microwaves.

• Two types of waveguide are used in linacs: RF power transmission waveguides and accelerating waveguides.

• The power transmission waveguides transmit the RF power from the power source to the accelerating waveguide in which the electrons are accelerated.

• The electrons are accelerated in the accelerating waveguide by means of an energy transfer from the high power RF fields, which are set up in the accelerating waveguide and are produced by the RF power generators.

II- RF power transmission Waveguide

5.3.2 Components

I. KlystronII. WaveguideIII.Water-cooling system

III- Water-cooling system

5.3.2 Components

I. KlystronII. WaveguideIII.Water-cooling system

5.3.2.4 Gantry

• Gantry: responsible for directing the photon (x-ray) energy or electron beam at a patients tumor. it rotates 360 degrees around a line/point, called the Isocenter.• Electron gun: produce electrons and injects them

into the accelerator structure• Accelerator structure: a special type of wave

guide in which electrons are accelerated. • Treatment head: components designed to shape

and monitor the treatment beam

5.3.2 Components

I. KlystronII. Magnetron III.Water-cooling system

I – Electron Gun

Electron gun: produce electrons and injects them into the accelerator structure

• The injection system is the 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.

• Both electron gun types contain a heated filament cathode and a perforated grounded anode; in addition, the triode electron gun also incorporates a grid.

• Electrons are thermionically emitted from the heated cathode, focusedinto a pencil beam by a curved focusing electrode and acceleratedtowards the perforated anode through which they drift to enter the acceleratingwaveguide.

Radiation Protection in Radiotherapy 61

ELEKTA Electron beam source

ELECTRON GUN62

5.3.2 Components

II- Accelerator Structure/ waveguide

• Microwave power (produced in the klyston) is transported to the accelerator structure in which corrugations are used to slow up the waves synchronous with the flowing electrons. After the flowing electrons leave the accelerator structure, they are directed toward the target (for photon production) or scattering foil (for electron production) located in the treatment head.

5.3.2 Components

Treatment Head Diagram:

III- Treatment head

• Treatment head: components designed to shape and monitor the treatment beam For photon therapy, they consist of the bending magnet, target, primary collimator, beam flattening filter, ion chambers, secondary collimators and one or more slots for trays, wedges, blocks and compensators.

1. Bending magnet: direct the electrons vertically toward the patient2. X-ray target: 3. Primary collimator: designed to limit the maximum field size4. Beam flattening filter: shaped the x-ray beam in its cross sectional dimension

III- Treatment head

5. Ion chamber: monitors the beam for its symmetry in the right-left and inferior-superior direction6.Secondary collimators: upper and lower collimator jaws7. Field light: outlines the dimensions of the radiation field as it appears on the patient, allows accurate positioning of the radiation field in relationship to skim marks or other reference points

5.3.2 Components

1- Bending Magnet

5.3.2 Components

i. Bending magnetii. Flattening filteriii. Scattering foiliv. X-ray Targetv. Collimatorvi. Beam Modification

2- Flattening Filter

• Flattening filter: (lead, steel, copper etc.)• Modifies the narrow, non-uniform photon beam at the isocenter into a clinically useful beam through a combination of attenuation of the center of the beam and scatter into the periphery of the beam

• Measured in percent at a particular depth in a phantom (10 cm)

• Must be carefully positioned in the beam or the beam hitting the patient will be non-uniform, resulting in hot and cold spots

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.

2- Flattening Filter

• Flatness: a wide beam that is nearly uniform in intensity from one side to the other (+/- 6%)

• Symmetry: the measure of intensity difference between its opposite sides (+/- 4%)• Causes include the use of a wedge, misalignment of the flattening filter, and misdirection of the electron beam before hitting the target.

5.3.2 Components

3- Scattering foil

• Scattering foil: thin metal sheets provide electrons with which they can scatter, expanding the useful size of the beam

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

5.3.2 Components

4- X-ray Target

5-Collimator

5.3.2 Components

6- Beam Modification

Field blocking and shaping device:

• SHEILDING BLOCKS

• Aims of Shielding:• Protect critical organs• Avoid unnecessary radiation to surrounding normal tissue• Matching adjacent fields

• An ideal shielding material should have the following characteristics:• High atomic number.• High-density.• Easily available.• Inexpensive.

The most commonly used shielding material for photons is Lead (Pb).

Radiation Protection in RadiotherapySeminar on Beam Modification Devices. Moderator : Dr. S.C. Sharma. Department of Radiotherapy.

Compensators• A beam modifying device which evens

out the skin surface contours, while retaining the skin-sparing advantage.

• It allows normal depth dose data to be used for such irregular surfaces.

• Compensators can also be used for • To compensate for tissue

heterogeneity. • To compensate for dose irregularities

arising due to reduced scatter near the field edges (example mantle fields), and horns in the beam profile.

Notice the reduction in the hot spot

Compensators

h’/h

• Can be constructed using thin sheets of lead, lucite or aluminum.

• This results in production of a laminated filter

Compensators

BEAM SPOILER

• A tissue equivalent material used to reduce the depth of the maximum dose (Dmax).

• Better called a “build-up bolus”.• A bolus can be used in place of a

compensator for kilovoltage radiation to even out the skin surface contours.

• In megavoltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions.

Bolus• Commonly used materials are:

• Cotton soaked with water.• Paraffin wax.

• Other materials that have been used:• Mix- D (wax, polyethylene, mag oxide)• Temex rubber (rubber)• Lincolnshire bolus (sugar and mag carbonate in form of spheres)• Spiers Bolus (rice flour and soda bicarb)

• Commercial materials:• Superflab: Thick and doesn't undergo elastic deformation. Made of synthetic oil gel.• Superstuff: Add water to powder to get a pliable gelatin like material.• Bolx Sheets: Gel enclosed in plastic sheet.

5.3.2 Components

i. Bending magnetii. Flattening filteriii. Scattering foil

5.Treatment couch6.Other accessories

5.3.2.5 Treatment Couch

• Treatment couch: mounted on a rotational axis around the isocenter• Also called patient support assembly (PSA)• Move mechanically in a horizontal and lengthwise

direction- must be smooth and accurate allowing for precise and exact positioning of the isocenter during treatment positioning

• Support up to 450 lbs• Range in width from 45-50 cm• Racket-like frame should be periodically tightened

to provide more patient support and reduce the amount of sag during treatment positioning.

5.3.3 The Linac X-Ray Beam

• Production of x-rays• Electrons are incident on a target of a high-Z material (e.g. tungsten)

• Target – need water cooled & thick enough to absorb most of the incident electrons

• Bremsstrahlung interactions•Electrons energy is converted into a spectrum of x-rays energies

•Max energy of x-rays = energy of incident energy of electrons

•Average photon energy = 1/3 of max energy of x-rays

• Designation of energy of electron beam and x-rays• Electron beam - MeV (million electron volts,

monoenergetic)• X-ray beam – MV (megavolts, voltage across an x-ray

tube, heterogeneous in energy)

x-rays produced from high energy electrons impinging on a target tend to be scattered in the forward direction

x-rays produced by lower energy electrons tend to be scattered at right angle to the direction of the electron beam

X-Ray Emission

The electron beam• Electron beam exits the window of accelerator tube is

narrow pencil beam• In electron mode, instead of striking the target, is made

strike an electron scattering foil in order to spread the beam as well as get a uniform electron fluence across the treatment field

Uniform electron fluence across the treatment field

e.g. lead

Narrow pencil about 3 mm in diameter

Electron scatter readily in air

Beam collimator must be achieved close to the skin surface

Q: With an aid of a diagram, explain the production of X-ray beam and

electron beam in a linear accelerator machine.

Answer:

1. POWER DISTRIBUTION Modulator cabinet: contains components that distribute and monitor primary electrical power and high-voltage pulses to the magnetron or klystron and electron gun

2.PRODUCTION OF ELECTRONSElectrons are produced in an electron gun. A hot cathode emits electrons, which are accelerated towards an anode, passing through an aperture to reach the accelerating waveguide. 3. ACCELERATION OF ELECTRONSUpon entering the accelerating waveguide, the electrons are accelerated by a magnetic field generated by microwaves. These microwaves are produced in either a klystron or a magnetron and are supplied to the waveguide.

4. BEAM TRANSPORTThe length of waveguides capable of generating high megavoltage photon beams (> 6 MV) makes a straight beam treatment head unfeasible. High energy linacs use a beam transport system to deliver the electrons to the treatment head. This is accomplished using strong electromagnets which bend the beam through 270o. The three turns cause the electrons to initially diverge and then converge upon the scattering foil as a pencil beam. 5. TREATMENT HEAD

PRODUCTION OF X-RAYS BEAM• Electrons are incident on a target of a high-Z material.• A typical spectrum of a clinical X-ray beam consists of line spectra that are characteristic of the

target material and that are superimposed on to the continuous bremsstrahlung spectrum. The bremsstrahlung spectrum originates in the X ray target, while the characteristic line spectra originate in the target and in any attenuators placed into the beam

PRODUCTION OF ELECTRON BEAM• Electron beam exits the window of accelerator tube is narrow pencil beam• In electron mode, instead of striking the target, is made strike an electron scattering foil in order to

spread the beam as well as get a uniform electron fluence across the treatment field• Electrons are not collimated by the secondary or tertiary collimators. This is due to the lateral scatter

of electrons would cause a significant geometric penumbra at the target surface.

A power supply Provides DC power to the modulator

Modulator

the magnetron or klystron

Deliver the pulses to the electron gun

the accelerator tube or structure via a waveguide systems.

Assignment: Briefly describe the process within a linear accelerator starting from the power supply to the production of a 3 mm pencil beam.

5.3.3 New Technologies

1. Three-dimensional conformal therapy (3D-CRT): the field shape and beam angle change as the gantry moves

around the patient

2. Intensity Modulated radiation therapy (IMRT): beneficial in escalating the dose to the tumor volume and

reducing the dose to normal tissue

• Independent collimators (dual asymmetrical jaws): provide increased flexibility in treatment planning

• MLCs allow an increased number of treatment fields without the use of heavy Cerrobend blocking

• Dynamic wedge: computerized shaping of the treatment field

• Electronic portal imaging: provides feedback on single-event setup accuracy or observation of treatment in near real-time

• Verification and record devices: • Allow incorrect setup parameters to be corrected before the machine is turned

on• Provide data in computer assisted setup• Recording of patient data• Allowing for data transfer from the simulator or treatment planning computer• Assisting with quality control

• Stereotactic radiation therapy: involves the aiming and delivery of a well defined narrow beam to extremely hard to reach places

Flattening Filter Free (FFF)FFF X-rays is to provide much higher dose rates available for treatments.

For example, FFF X-rays from Varian TrueBEAM can deliver 1400

MU/minute for 6 MV X-rays and 2400 MU/minutes for 10 MV X-rays.

The VERO system, a novel platform for image guided

stereotactic body radiotherapy, is a joint product of BrainLAB and MHI. A new type of 6MV linac

with attached MLC is mounted on an O-ring gantry

THANK YOU ...

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