Beam Therapy Beam Therapy EquipmentEquipment

2 Linear Accelerator Head2 Linear Accelerator Head

Photon / Electron Photon / Electron differencesdifferences

TargetTarget Flattening filterFlattening filter Any jaw sizeAny jaw size MLCsMLCs Shaping blocksShaping blocks Intensity Intensity

modulation e.g. modulation e.g. wedgeswedges

No targetNo target Scattering foilsScattering foils Jaw size Jaw size

dependent on dependent on energy and energy and applicatorapplicator

ee-- applicator applicator End-frameEnd-frame No intensity No intensity

modulationmodulation

Photon Head

Electron Head

Changes to accelerator Changes to accelerator when using electrons - when using electrons - automaticautomatic Photon target is moved out of the way.Photon target is moved out of the way. Carousel is moved to position scattering foil Carousel is moved to position scattering foil

for particular electron energy in place of for particular electron energy in place of photon flattening filter.photon flattening filter.

Gun volts on electron gun are reduced to Gun volts on electron gun are reduced to decrease the number of electrons passing decrease the number of electrons passing through wave guide.through wave guide.

Frequency of rf adjusted to give optimum Frequency of rf adjusted to give optimum acceleration of electronsacceleration of electrons

Bending magnet voltages adjusted to finely Bending magnet voltages adjusted to finely tune electron energies leaving bending tune electron energies leaving bending magnetmagnet

Changes to accelerator Changes to accelerator when using electrons – when using electrons – manualmanual Jaws moved to pre-defined position Jaws moved to pre-defined position

– dependent on applicator size and – dependent on applicator size and electron energy. Optimised for flat electron energy. Optimised for flat field (wide scattering)field (wide scattering)

Electron applicator attached to Electron applicator attached to headhead

Electron insert fitted into end of Electron insert fitted into end of applicator – defines final field sizeapplicator – defines final field size

Scattering foilsScattering foils

Scattering foils chosen for wide Scattering foils chosen for wide scattering angle (flat field), energy scattering angle (flat field), energy loss (none) and bremsstrahlung loss (none) and bremsstrahlung production (none)production (none)

Usually made of copperUsually made of copper Not necessarily of uniform Not necessarily of uniform

thicknessthickness Higher electron energies may use Higher electron energies may use

two foilstwo foils

Electron field shapingElectron field shaping Electrons are scattered by air Electrons are scattered by air

between scattering foils and between scattering foils and patient, therefore final shaping patient, therefore final shaping needs to be done as close to needs to be done as close to patient as possiblepatient as possible

Shaping done by:Shaping done by:– Secondary collimators – coarseSecondary collimators – coarse– Electron trimmers – scrape scattered Electron trimmers – scrape scattered

electronselectrons– Electron insert – fine field shapeElectron insert – fine field shape

ApplicatorsApplicators Applicator consists of Applicator consists of

progressively smaller progressively smaller scrapersscrapers

Electrons scattered by Electrons scattered by foil have a Gaussian foil have a Gaussian distribution, electrons distribution, electrons scattered by scrapers scattered by scrapers are used to flatten the are used to flatten the beambeam

Scrapers should be Scrapers should be made of low Z material made of low Z material (Aluminium) to limit X-(Aluminium) to limit X-ray productionray production

Sec.coll

Sec.coll

End Frame

Scrapers

Safety DevicesSafety Devices

The scattering foil, secondary collimator The scattering foil, secondary collimator settings, bending magnet voltages are checked settings, bending magnet voltages are checked for consistency with energy / applicator selected.for consistency with energy / applicator selected.

Presence of the applicator and correct insert Presence of the applicator and correct insert may be interlocked. (Applicator interlock very may be interlocked. (Applicator interlock very common, insert interlock rare)common, insert interlock rare)

Collision of applicator with patient, bed etc is Collision of applicator with patient, bed etc is also checked using sensors built into applicator.also checked using sensors built into applicator.

Radiation symmetry, output and energy are Radiation symmetry, output and energy are monitored by the monitor chamber during monitored by the monitor chamber during treatmenttreatment

Beam shapingBeam shaping

Beam can be modified in 3D – shape Beam can be modified in 3D – shape and intensityand intensity

CollimatorsCollimators ApplicatorsApplicators MLCsMLCs Cast blocksCast blocks WedgesWedges PenumbraPenumbra

MLCMLC

Used to shape radiation Used to shape radiation fieldfield

Can interdigitateCan interdigitate Fields set automaticallyFields set automatically 52 – 160 leaves52 – 160 leaves Typically 60mm tallTypically 60mm tall 2.5% leaf transmission; 2.5% leaf transmission;

4% interleaf leakage4% interleaf leakage Tongue and groove Tongue and groove

offsetoffset Focused in 1 directionFocused in 1 direction 7mm penumbra7mm penumbra

Blocks vs. MLCsBlocks vs. MLCsBLOCKsBLOCKs

Exact shapeExact shape Move small amountsMove small amounts Easy to check on Easy to check on

patientpatientX HeavyHeavyX Facing right wayFacing right wayX Individual – possibility Individual – possibility

of using wrong blockof using wrong block

MLCsMLCs

X SteppedSteppedX Movement need re-Movement need re-

planplanX Difficult to check on Difficult to check on

patientpatient Auto set-upAuto set-up Individual – Individual –

attached to fieldattached to field

Wedges – to shape the Wedges – to shape the radiation beamradiation beam

Hard – Usually lead, manually placed in Hard – Usually lead, manually placed in accessory mount, range of angles accessory mount, range of angles available. Increase energy of primary and available. Increase energy of primary and dose from scattereddose from scattered

Universal – Single 60Universal – Single 60° Wedge in ° Wedge in treatment head, mixed with open field to treatment head, mixed with open field to get intermediate angles. get intermediate angles. Increase energy Increase energy of primary and dose from scatteredof primary and dose from scattered

Dynamic – formed by sweeping one of Dynamic – formed by sweeping one of the jaws across the radiation field, range the jaws across the radiation field, range of angles available. Primary same energyof angles available. Primary same energy

PenumbraPenumbra

EnergyEnergy RadiationRadiation Source sizeSource size