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