Beam Therapy Beam Therapy Equipment Equipment

3 Patient Treatment and 3 Patient Treatment and AccessoriesAccessories

Dosimetry SystemDosimetry System

Dual channel – main and back-up channelsDual channel – main and back-up channels Segmented Ionisation ChambersSegmented Ionisation Chambers Ionisation chambers produce a current Ionisation chambers produce a current Difference in opposite segments used to Difference in opposite segments used to

steer electron beam radial and transversesteer electron beam radial and transverse Outer radial segments used to adjust Outer radial segments used to adjust

number of electrons emitted by gunnumber of electrons emitted by gun Sum of segments (electric charge) gives Sum of segments (electric charge) gives

dosedose Rate of segments (current) gives dose-rateRate of segments (current) gives dose-rate Separate back-up TimerSeparate back-up Timer

DosimetryDosimetry

ShieldingShielding

Shielding is used for two reasons:Shielding is used for two reasons: Protect healthy tissue from damage by Protect healthy tissue from damage by

radiationradiation Modify the intensity of the radiation Modify the intensity of the radiation

beambeam There are three main ways of shaping the There are three main ways of shaping the

radiation beam for photon treatments:radiation beam for photon treatments: JawsJaws MLCsMLCs BlocksBlocks

JawsJaws

Bulk or back-up shielding, Bulk or back-up shielding, rectangularrectangular

TungstenTungsten Density 19.32 g cmDensity 19.32 g cm-3-3

Typically ~8cm thick Typically ~8cm thick Back-up jaws ~3cm thickBack-up jaws ~3cm thick

6MV transmission ~1.5%6MV transmission ~1.5%

Multi Leaf Collimators Multi Leaf Collimators (MLCs)(MLCs)

Field shaping, occasionally bulk Field shaping, occasionally bulk shieldingshielding

TungstenTungsten Density 19.32 g cmDensity 19.32 g cm-3-3

Typically ~7.5cm thickTypically ~7.5cm thick 6MV transmission ~2.5% 6MV transmission ~2.5%

Inter-leaf leakage ~4%Inter-leaf leakage ~4%

BlocksBlocks

Field shaping, attached to tray on Field shaping, attached to tray on machine headmachine head

MCP 96 – Low melting point alloy MCP 96 – Low melting point alloy (52% Bismuth, 32% Lead, 16 % (52% Bismuth, 32% Lead, 16 % Tin)Tin)

Density 9.7 g cmDensity 9.7 g cm-3-3

Typically ~8cm thickTypically ~8cm thick 6MV transmission ~6%6MV transmission ~6%

PenumbraPenumbra Geometric penumbraGeometric penumbra

– Dependent upon source sizeDependent upon source size– Larger effective source size –> larger penumbraLarger effective source size –> larger penumbra

Profile (and thus penumbra) not exactly as Profile (and thus penumbra) not exactly as expected (geometric profile) because of expected (geometric profile) because of scatterscatter

Scatter depends uponScatter depends upon– Radiation type (photons or electrons)Radiation type (photons or electrons)

Electrons scattered easier by air and solids –> larger Electrons scattered easier by air and solids –> larger penumbrapenumbra

– EnergyEnergy Higher energies harder to scatter –> smaller penumbraHigher energies harder to scatter –> smaller penumbra Higher energies more penetrating –> larger penumbraHigher energies more penetrating –> larger penumbra

Patient Set-upPatient Set-up

There are a number of linac devices There are a number of linac devices used for patient set-up:used for patient set-up:

LasersLasers Light FieldsLight Fields Distance MetersDistance Meters PointersPointers

LasersLasers

Lasers used to identify isocentreLasers used to identify isocentre

Laser lines pass through isocentre Laser lines pass through isocentre perpendicular to each otherperpendicular to each other

A typical set of lasers comprisesA typical set of lasers comprises Cross lasers (coronal and lateral lines) Cross lasers (coronal and lateral lines)

placed on both walls lateral to isocentreplaced on both walls lateral to isocentre Line laser (sagittal) placed at foot of bed Line laser (sagittal) placed at foot of bed

passing through isocentrepassing through isocentre Cross laser (lateral and sagittal) placed Cross laser (lateral and sagittal) placed

on ceiling above isocentreon ceiling above isocentre

Light FieldLight Field

Optical indication of Optical indication of radiation fieldradiation field

Light source at Light source at same location as same location as radiation source radiation source (using mirrors)(using mirrors)

Cross on field Cross on field indicates central indicates central axis (and wedge axis (and wedge direction)direction)

Distance MeterDistance Meter

Uses the light field and a second Uses the light field and a second light source to indicate distance light source to indicate distance from radiation source to patient from radiation source to patient surfacesurface

Second light source marked with Second light source marked with lines at set distanceslines at set distances

Intersection of the two light Intersection of the two light beams varies with distance to beams varies with distance to surface – indicating distance surface – indicating distance

PointersPointers Physical pointers Physical pointers

attached to a holder on attached to a holder on machine head, running machine head, running along central axis of along central axis of beambeam

Indicate distance from Indicate distance from source to tip (usually source to tip (usually patient / shell surface)patient / shell surface)

Used when distance Used when distance meter not suitablemeter not suitable

Cobalt UnitCobalt Unit

Accelerator, bending chamber and target Accelerator, bending chamber and target replaced by solid sourcereplaced by solid source

Co-60 disk, typically 1cm in diameterCo-60 disk, typically 1cm in diameter Activity of 100’s CiActivity of 100’s Ci Dose-rate drops with time (TDose-rate drops with time (T½½= 5.29 yrs)= 5.29 yrs) Photons of 1.17 and 1.33 MVPhotons of 1.17 and 1.33 MV Penetration equivalent to an ~4MV linacPenetration equivalent to an ~4MV linac Isocentre 75-80 cmIsocentre 75-80 cm