1. Use of the IC Profiler detector array for comprehensive machine QA ESTRO QA & Dosimetry Satellite Symposium 9.5.11 Steve Morgan, Medical Physics Dept, Sussex Cancer Centre

2. Outline 1.Device description 2.Streamlining beam dosimetry QA 3.MLC calibration 4.Summing up

3. Device description

4. Detectors 251 parallel plate ion chambers IC volume0.05cm 3 IC dimensions3mm x 7mm IC spacing5mm Field of view32cm x 32cm Build-up0.9g/cm 2 Weight5kg

5. Beam steering  4 plot axes captured simultaneously – highly efficient  Particularly advantageous for primary steering (1R, 1T) investigations in which adjustments simultaneously affect both inplane and crossplane symmetry

6. Fine adjustments Profile minimum re-centred  5mm pitch allows subtle optimisations to be made  eg Bending Fine adjustment following Bending Magnet replacement:

7. Integrated exposures  Excellent signal-to-noise ratio  Good performance down to small MUs 100MU 10MU 3MU

8. Photon energy  Flatness: sensitive indicator of energy BC 38.0V, d 10 67.0% BC 40.5V, d 10 67.5% BC 42.5V, d 10 68.0%

9. Streamlining beam dosimetry QA

10. Guiding principles  Reduce the number of different equipment deployments  Reduce the number of ‘excursions’ into the treatment room for set-up adjustments  Make each exposure ‘multi-purpose’ where possible  Improve efficiency and quality  Use chambers only for absolute dose output at gantry 0°  Use IC Profiler for all other measurements

11. Electron Wedge Gantry 90° Electron set-up  Gantry mount: accommodates 25x25cm applicator  Electron wedge: simultaneous output, energy & symmetry

12. Traditional schedule Annual test frequency TestSet-upG0°G90°G180°G270°ReadingsExcursions Photons (2 energies) OutputFarmer/Solid W 12444 4824 Energy (QI)Farmer/Solid W 12111 9045 Wedge FactorFarmer/Solid W 12444 7224 Output factorsICO4/Large SW 4 404 Basic flatnessDoublecheck 12 24 Full flatnessSchuster 4444 1284 TPS profilesICO4/Water tank 1 1 day set-up & measurement S Electrons (7 energies) OutputRoos/Solid W 1211 98 EnergyRoos/Solid W 1211 196 Output factorsRoos/Solid W 1 49 Basic flatnessDoublecheck 12 8412 Full flatnessSchuster 411 426 TPS profilesICO4/Water tank 1 1 day set-up & measurement Totals 6871486

13. Proposed schedule Annual test frequency TestSet-upG0°G90°G180°G270°ReadingsExcursions Photons (2 energies) OutputFarmer(G0°)/ ICP 12444 2412 Energy (QI)ICP 12444 4812 Wedge FactorICP 12444 48- Output factorsICP 4 404 Basic flatnessDaily QA3 (260) Covered in daily run-up Full flatnessICP 12444 48* TPS profilesICP/Large SW 1 1 hour measurement time S Electrons (7 energies) OutputRoos(G0°)/ ICP 1211 84 EnergyICP/Elec Wedge 1211 9812 Output factorsICP 1 49 Basic flatnessDaily QA3 (260) Covered in daily run-up Full flatnessICP 1211 -- TPS profilesICP/Large SW 1 2 hours measurement time Totals 6 3871 439486 173

14. Possible schedule Annual test frequency TestSet-upG0°G90°G180°G270°ReadingsExcursions Photons (2 energies) OutputFarmer(G0°)/ ICP 12 2412 Energy (QI)ICP 12 9612 Wedge FactorICP 12444 48- Output factorsICP 4 404 Basic flatnessDaily QA3 (260) Covered in daily run-up Full flatnessICP 12 96* TPS profilesICP/Large SW 1 1 hour measurement time S Electrons (7 energies) OutputRoos(G0°)/ ICP 12 84 EnergyICP/Elec Wedge 12 25212 Output factorsICP 1 49 Basic flatnessDaily QA3 260 Covered in daily run-up Full flatnessICP 12 -- TPS profilesICP/Large SW 1 2 hours measurement time Totals 6 3871 689486 173 Thus saving 30 miles of walking in one year (for a four linac department)

15. MLC calibration

16. Alternate chambers sit under centre of MLC leaf Elekta MLCi 7mm 3mm 10mm MLC leaf 50% 0% 100% 20%/mm 0 -1mm -2mm 1mm 2mm × Partial volume effect MLC relative position (‘minor offsets’)

17. Alternate chambers sit under centre of MLC leaf Elekta MLCi 7mm 3mm 10mm MLC leaf 50% 0% 100% 20%/mm 0 -1mm -2mm 1mm 2mm × Partial volume effect MLC relative position (‘minor offsets’)

18. Alternate chambers sit under centre of MLC leaf Elekta MLCi 7mm 3mm 10mm MLC leaf 50% 0% 100% 20%/mm 0 -1mm -2mm 1mm 2mm × Partial volume effect MLC relative position (‘minor offsets’)

19. Alternate chambers sit under centre of MLC leaf Elekta MLCi 7mm 3mm 10mm MLC leaf 50% 0% 100% 20%/mm 0 -1mm -2mm 1mm 2mm × Partial volume effect MLC relative position (‘minor offsets’)

20. Alternate chambers sit under centre of MLC leaf Elekta MLCi 7mm 3mm 10mm 50% 0% 100% 20%/mm 0 -1mm -2mm 1mm 2mm MLC leaf × Partial volume effect

21. MLC calibration is referenced to backup jaw MLC19 MLC21 MLC20 MLC22 Backup Jaw 100% 90% 110% Reference signal 100% 90% 110% MLC position signal 20%/mm

22. Perfect alignment not necessary! MLC19 Backup Jaw 100% 90% 110% Reference signal 100% 90% 110% MLC position signal MLC20 MLC21 MLC22

23. Major gains and offsets - penumbra interpolation D penumbra (s) = 1 – (t + s c ) 2 × 1 - s √(s 2 + n) + (t + s c ) where (t + s c ) = Transmission + Scatter (outside beam) and n = (20-80% penumbra width / 1.5) 2 75% 100% 50% 25% 0% 0 -5mm -10mm 5mm 10mm  5mm detector spacing - unlikely to have two detectors sitting within the linear (20-80%) portion of the penumbra  Square root interpolation model finds 50% edge  Model and parameters tested by shifting ICP in 1mm increments (and comparisons with film)

24. Calibration sequence – 25MU exposures Y2 Jaw Y2 MLC Y1 Jaw Y1 MLC 3 412 +5+15 -5 -15 5 6 -5+5 +15 -15 MLC minor offsets (central 30 leaves); Backup jaw major gains & offsets Major gains & offsets Further 4 exposures if calibrating all 40 MLC leaves

25. PiMLiCo software interface Out-of-tolerance results Export minor offsets to linac Major gain/offset calculator

26. Electronic update of minor offsets White cells indicate updated items Import

27. System performance  Can detect and correct minor offsets of ~0.2mm  Agreement with film/water tank within experimental error QA of major gains/offsets + central 30 leaf pairs 5 mins Re-calibration of all MLC and diaphragm settings 20 mins

28. Summing up

29. Advantages and opportunities Efficient and precise tool for beam optimisation. Fewer test set-ups. Multi-purpose exposures. Time-to-competence for new staff expected to reduce. Significant time and consumables savings associated with MLC calibration and X-ray/light coincidence tests. Alternative to water tank for TPS data baseline checks.

30. Thank you steve.morgan@bsuh.nhs.uk