Beam Therapy Equipment 2 Linear Accelerator Head

Photon / Electron differences Target Target Flattening filter Flattening filter Any jaw size Any jaw size MLCs MLCs Shaping blocks Shaping blocks Intensity modulation e.g. wedges Intensity modulation e.g. wedges No target No target Scattering foils Scattering foils Jaw size dependent on energy and applicator Jaw size dependent on energy and applicator e - applicator e - applicator End-frame End-frame No intensity modulation No intensity modulation

Photon Head

Electron Head

Changes to accelerator when using electrons - automatic Photon target is moved out of the way. Photon target is moved out of the way. Carousel is moved to position scattering foil for particular electron energy in place of photon flattening filter. Carousel is moved to position scattering foil for particular electron energy in place of photon flattening filter. Gun volts on electron gun are reduced to decrease the number of electrons passing through wave guide. Gun volts on electron gun are reduced to decrease the number of electrons passing through wave guide. Frequency of rf adjusted to give optimum acceleration of electrons Frequency of rf adjusted to give optimum acceleration of electrons Bending magnet voltages adjusted to finely tune electron energies leaving bending magnet Bending magnet voltages adjusted to finely tune electron energies leaving bending magnet

Changes to accelerator when using electrons – manual Jaws moved to pre-defined position – dependent on applicator size and electron energy. Optimised for flat field (wide scattering) Jaws moved to pre-defined position – dependent on applicator size and electron energy. Optimised for flat field (wide scattering) Electron applicator attached to head Electron applicator attached to head Electron insert fitted into end of applicator – defines final field size Electron insert fitted into end of applicator – defines final field size

Scattering foils Scattering foils chosen for wide scattering angle (flat field), energy loss (none) and bremsstrahlung production (none) Scattering foils chosen for wide scattering angle (flat field), energy loss (none) and bremsstrahlung production (none) Usually made of copper Usually made of copper Not necessarily of uniform thickness Not necessarily of uniform thickness Higher electron energies may use two foils Higher electron energies may use two foils

Electron field shaping Electrons are scattered by air between scattering foils and patient, therefore final shaping needs to be done as close to patient as possible Electrons are scattered by air between scattering foils and patient, therefore final shaping needs to be done as close to patient as possible Shaping done by: Shaping done by: –Secondary collimators – coarse –Electron trimmers – scrape scattered electrons –Electron insert – fine field shape

Applicators Applicator consists of progressively smaller scrapers Applicator consists of progressively smaller scrapers Electrons scattered by foil have a Gaussian distribution, electrons scattered by scrapers are used to flatten the beam Electrons scattered by foil have a Gaussian distribution, electrons scattered by scrapers are used to flatten the beam Scrapers should be made of low Z material (Aluminium) to limit X- ray production Scrapers should be made of low Z material (Aluminium) to limit X- ray production

Safety Devices The scattering foil, secondary collimator settings, bending magnet voltages are checked for consistency with energy / applicator selected. The scattering foil, secondary collimator settings, bending magnet voltages are checked for consistency with energy / applicator selected. Presence of the applicator and correct insert may be interlocked. (Applicator interlock very common, insert interlock rare) Presence of the applicator and correct insert may be interlocked. (Applicator interlock very common, insert interlock rare) Collision of applicator with patient, bed etc is also checked using sensors built into applicator. Collision of applicator with patient, bed etc is also checked using sensors built into applicator. Radiation symmetry, output and energy are monitored by the monitor chamber during treatment Radiation symmetry, output and energy are monitored by the monitor chamber during treatment

Beam shaping Beam can be modified in 3D – shape and intensity Collimators Collimators Applicators Applicators MLCs MLCs Cast blocks Cast blocks Wedges Wedges Penumbra Penumbra

MLC Used to shape radiation field Used to shape radiation field Can interdigitate Can interdigitate Fields set automatically Fields set automatically 52 – 160 leaves 52 – 160 leaves Typically 60mm tall Typically 60mm tall 2.5% leaf transmission; 4% interleaf leakage 2.5% leaf transmission; 4% interleaf leakage Tongue and groove offset Tongue and groove offset Focused in 1 direction Focused in 1 direction 7mm penumbra 7mm penumbra

Blocks vs. MLCs BLOCKs Exact shape Exact shape Move small amounts Move small amounts Easy to check on patient Easy to check on patient X Heavy X Facing right way X Individual – possibility of using wrong block MLCs X Stepped X Movement need re- plan X Difficult to check on patient Auto set-up Auto set-up Individual – attached to field Individual – attached to field

Wedges – to shape the radiation beam Hard – Usually lead, manually placed in accessory mount, range of angles available. Increase energy of primary and dose from scattered Hard – Usually lead, manually placed in accessory mount, range of angles available. Increase energy of primary and dose from scattered Universal – Single 60° Wedge in treatment head, mixed with open field to get intermediate angles. Increase energy of primary and dose from scattered Universal – Single 60° Wedge in treatment head, mixed with open field to get intermediate angles. Increase energy of primary and dose from scattered Dynamic – formed by sweeping one of the jaws across the radiation field, range of angles available. Primary same energy Dynamic – formed by sweeping one of the jaws across the radiation field, range of angles available. Primary same energy

Penumbra Energy Energy Radiation Radiation Source size Source size