Beam Therapy Beam Therapy EquipmentEquipment

1 Linear Accelerator Basics1 Linear Accelerator Basics

X-ray productionX-ray production

Treatment beams produced by linear Treatment beams produced by linear acceleratorsaccelerators

Consists of an electron gun, RF source, Consists of an electron gun, RF source, waveguide, bending magnet and waveguide, bending magnet and treatment headtreatment head

Electrons emitted from the gun, Electrons emitted from the gun, accelerated down the waveguide using accelerated down the waveguide using microwaves, re-directed with a magnet microwaves, re-directed with a magnet and hit a tungsten target to produce x-and hit a tungsten target to produce x-raysrays

Electron gunElectron gun

Made of thermionic material e.g. ThoriumMade of thermionic material e.g. Thorium Cathode – negative charge, electrons Cathode – negative charge, electrons

accelerated from itaccelerated from it Electrons emitted when heated – ‘boil’ offElectrons emitted when heated – ‘boil’ off Gun often gridded – has a positive grid in Gun often gridded – has a positive grid in

front to accelerate the electronsfront to accelerate the electrons Grid is pulsed so get pulses of electronsGrid is pulsed so get pulses of electrons Have pulses of electrons entering Have pulses of electrons entering

waveguidewaveguide

Microwave productionMicrowave production

Two devices invented in 1939 for Two devices invented in 1939 for WWII radar – Magnetron (GEC) WWII radar – Magnetron (GEC) and Klystron (Varian brothers)and Klystron (Varian brothers)

Operated at frequency of 3 GHz – Operated at frequency of 3 GHz – wavelength of 10cmwavelength of 10cm

Required for high power radar Required for high power radar systemssystems

Magnetron – self Magnetron – self oscillatoroscillator Cathode in centre Cathode in centre

of evacuated, of evacuated, lobed chamberlobed chamber

Magnetic field Magnetic field perpendicularperpendicular

Electrons spiral out Electrons spiral out inducing resonant inducing resonant radio-fieldradio-field

Sampled and fed Sampled and fed to waveguideto waveguide

Klystron - amplifierKlystron - amplifier

Electron gun Electron gun produces stream of produces stream of electronelectron

Pulsed by low Pulsed by low energy microwavesenergy microwaves

Pulsed electrons Pulsed electrons induce high energy induce high energy microwaves in microwaves in tuned cavitytuned cavity

WaveguideWaveguide Hollow, pressurised, rectangular tube – Hollow, pressurised, rectangular tube –

carries RF to linac waveguidecarries RF to linac waveguide Circular, linac waveguide under vacuumCircular, linac waveguide under vacuum Made of copper – to conduct chargeMade of copper – to conduct charge Electric fields from microwaves produce Electric fields from microwaves produce

electric currents produce new magnetic electric currents produce new magnetic and electric fields that travel along pipeand electric fields that travel along pipe

Electrons enter waveguide accelerated by Electrons enter waveguide accelerated by microwave radiation into bunchesmicrowave radiation into bunches

Varian – Standing waveVarian – Standing wave Elekta – Travelling waveElekta – Travelling wave

Standing waveStanding wave

Microwaves Microwaves reflected from reflected from distal surface to distal surface to produce a produce a standing wavestanding wave

Travelling WaveTravelling Wave

Microwaves Microwaves absorbed at absorbed at distal surface – distal surface – travelling wavetravelling wave

Beam controlBeam control

Linacs have focusing magnets and Linacs have focusing magnets and steering magnetssteering magnets

Electric currents through wires produce Electric currents through wires produce magnetic fieldsmagnetic fields

These magnetic fields are used to move These magnetic fields are used to move the electron beam (-ve charge)the electron beam (-ve charge)

They focus the electrons into thin beamThey focus the electrons into thin beam Also used to steer the beam vertically Also used to steer the beam vertically

and horizontally in the waveguide for and horizontally in the waveguide for flatnessflatness

When electron When electron beam exits beam exits accelerator it is accelerator it is bent (90bent (90°/270°)°/270°) and strikes and strikes target target producing x-producing x-raysrays

These get These get flattened, flattened, measured and measured and shaped and exit shaped and exit treatment headtreatment head

Bending magnetBending magnet

Only required if Only required if need to redirect need to redirect electron beamelectron beam

Doubly achromaticDoubly achromatic Needs to be Needs to be

cooledcooled Change field Change field

strength and thus strength and thus beam energy by beam energy by adjusting currentadjusting current

Problems with 90Problems with 90° bending ° bending magnetsmagnets