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Trainees Intellectual Forum

Medical Linear Accelerators(Linacs)

Medical Physics, Aga Khan University-Karachi 1

Ghufran Ali

Trainee Medical Physicist

ACADEMIC SESSION

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Contents

1. Particle Accelerators

2. Medical Linacs

Schematic diagram

Linac configurations Linac components

Beam interlocks

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PARTICLE ACCELERATORS

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PARTICLE ACCELERATORS

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PARTICLE ACCELERATORS

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Betatron

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Cyclotron

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Microtron

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Medical Linacs

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Medical Linacs

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Schematic Diagram (simple)

1-80keV electrons

pulse- injected into

accelerator tube

Evacuated Cu tube;

Transfer microwave

energy (from magnetron

or klystron) to electrons

X-ray target, flattening

filter, electron scattering

foil, collimators, etc

Emits high voltage pulses (PRF: 30-300 Hz) to magnetron & e - gun; to

synchronize electron injection with

microwave power.

Source of microwave power

(~3000MHz);

Magnetrons up to ~20 MeV;

Klystrons at higher energies

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Linac Configurations:

vertical mounting (LA 1 at AKUH)

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For 4-6 MV linacs

e-gun & x-ray target

part of waveguide

aligned with isocentreno bending magnet

& transport system

X-ray target embedded into waveguide; therefore, this design

cannot produce electron beam

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Linac Configurations:

horizontal mounting(LA 2 at AKUH)

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RF Power Generation

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RF power amplifier

Source of microwave power (~3000MHz);

Magnetrons up to ~20 MeV; Klystrons at

higher energies

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RF Power Generation

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to synchronize electron injection with microwave power.

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Waveguides

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RF Power transport

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Travelling Waveguide

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Above 8 MeV, accelerator structures becomes too long (> 1 m)

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Standing Waveguide

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More efficient; half as long;

but more expensive

Most modern linacs

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Auxiliary System

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Electron Beam Transport

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Coils & Bending Magnet

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Electron Beam Transport

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Linac Treatment Head

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d

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Linac Treatment HeadElectron

Beam

IonChamber

Secondary

Collimators

Electron

applicator

Patient

Primary

Collimator

Flattening Filter

Patient

Target

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Target and Flattening Filter

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Carousel

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Photon Beam Collimation

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Production of Electron Beams

6 to 30 MeV energy beams are available

Electron exits through a thin beryllium window

Two techniques for producing uniform beam

Pencil beam scattering (Pb or Cu foils)

Pencil beam scanning

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Electron Beam Collimation

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Electron Mode

INOR

Electron

Beam

Scattering Foil

IonChamber

SecondaryCollimator

Electron

applicator

Patient

Primary

Collimator

Scattering Foil

Patient

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Dose Monitoring

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Dose Monitoring

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Dose Monitoring

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Dose Monitoring

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Dose Monitoring

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References

Chapter 4:The Physics of radiation Therapy, Faiz M.KHAN

Sections 4.3 - 4.8

Chapter 5:

Radiation Oncology Physics: A Handbook

for Teachers and Students, IAEA, Vienna,2007

Section 5.5