1. Beam Therapy Equipment 3 Patient Treatment and Accessories

2. Dosimetry System Dual channel – main and back-up channels Dual channel – main and back-up channels Segmented Ionisation Chambers Segmented Ionisation Chambers Ionisation chambers produce a current Ionisation chambers produce a current Difference in opposite segments used to steer electron beam radial and transverse Difference in opposite segments used to steer electron beam radial and transverse Outer radial segments used to adjust number of electrons emitted by gun Outer radial segments used to adjust number of electrons emitted by gun Sum of segments (electric charge) gives dose Sum of segments (electric charge) gives dose Rate of segments (current) gives dose-rate Rate of segments (current) gives dose-rate Separate back-up Timer Separate back-up Timer

3. Dosimetry

4. Shielding Shielding is used for two reasons: Protect healthy tissue from damage by radiation Protect healthy tissue from damage by radiation Modify the intensity of the radiation beam Modify the intensity of the radiation beam There are three main ways of shaping the radiation beam for photon treatments: Jaws Jaws MLCs MLCs Blocks Blocks

5. Jaws Bulk or back-up shielding, rectangular Bulk or back-up shielding, rectangular Tungsten Tungsten Density 19.32 g cm -3 Density 19.32 g cm -3 Typically ~8cm thick Back-up jaws ~3cm thick Typically ~8cm thick Back-up jaws ~3cm thick 6MV transmission ~1.5% 6MV transmission ~1.5%

6. Multi Leaf Collimators (MLCs) Field shaping, occasionally bulk shielding Field shaping, occasionally bulk shielding Tungsten Tungsten Density 19.32 g cm -3 Density 19.32 g cm -3 Typically ~7.5cm thick Typically ~7.5cm thick 6MV transmission ~2.5% Inter-leaf leakage ~4% 6MV transmission ~2.5% Inter-leaf leakage ~4%

7. Blocks Field shaping, attached to tray on machine head Field shaping, attached to tray on machine head MCP 96 – Low melting point alloy (52% Bismuth, 32% Lead, 16 % Tin) MCP 96 – Low melting point alloy (52% Bismuth, 32% Lead, 16 % Tin) Density 9.7 g cm -3 Density 9.7 g cm -3 Typically ~8cm thick Typically ~8cm thick 6MV transmission ~6% 6MV transmission ~6%

8. Penumbra Geometric penumbra Geometric penumbra –Dependent upon source size –Larger effective source size –> larger penumbra Profile (and thus penumbra) not exactly as expected (geometric profile) because of scatter Profile (and thus penumbra) not exactly as expected (geometric profile) because of scatter Scatter depends upon Scatter depends upon –Radiation type (photons or electrons) Electrons scattered easier by air and solids –> larger penumbra Electrons scattered easier by air and solids –> larger penumbra –Energy Higher energies harder to scatter –> smaller penumbra Higher energies harder to scatter –> smaller penumbra Higher energies more penetrating –> larger penumbra Higher energies more penetrating –> larger penumbra

9. Patient Set-up There are a number of linac devices used for patient set-up: Lasers Lasers Light Fields Light Fields Distance Meters Distance Meters Pointers Pointers

10. Lasers Lasers used to identify isocentre Laser lines pass through isocentre perpendicular to each other A typical set of lasers comprises Cross lasers (coronal and lateral lines) placed on both walls lateral to isocentre Cross lasers (coronal and lateral lines) placed on both walls lateral to isocentre Line laser (sagittal) placed at foot of bed passing through isocentre Line laser (sagittal) placed at foot of bed passing through isocentre Cross laser (lateral and sagittal) placed on ceiling above isocentre Cross laser (lateral and sagittal) placed on ceiling above isocentre

11. Light Field Optical indication of radiation field Optical indication of radiation field Light source at same location as radiation source (using mirrors) Light source at same location as radiation source (using mirrors) Cross on field indicates central axis (and wedge direction) Cross on field indicates central axis (and wedge direction)

12. Distance Meter Uses the light field and a second light source to indicate distance from radiation source to patient surface Uses the light field and a second light source to indicate distance from radiation source to patient surface Second light source marked with lines at set distances Second light source marked with lines at set distances Intersection of the two light beams varies with distance to surface – indicating distance Intersection of the two light beams varies with distance to surface – indicating distance

13. Pointers Physical pointers attached to a holder on machine head, running along central axis of beam Physical pointers attached to a holder on machine head, running along central axis of beam Indicate distance from source to tip (usually patient / shell surface) Indicate distance from source to tip (usually patient / shell surface) Used when distance meter not suitable Used when distance meter not suitable

14. Cobalt Unit Accelerator, bending chamber and target replaced by solid source Accelerator, bending chamber and target replaced by solid source Co-60 disk, typically 1cm in diameter Co-60 disk, typically 1cm in diameter Activity of 100’s Ci Activity of 100’s Ci Dose-rate drops with time (T ½ = 5.29 yrs) Dose-rate drops with time (T ½ = 5.29 yrs) Photons of 1.17 and 1.33 MV Photons of 1.17 and 1.33 MV Penetration equivalent to an ~4MV linac Penetration equivalent to an ~4MV linac Isocentre 75-80 cm Isocentre 75-80 cm