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Use of the IC Profiler detector array for comprehensive machine QA

ESTRO QA & Dosimetry Satellite Symposium 9.5.11. Use of the IC Profiler detector array for comprehensive machine QA. Steve Morgan, Medical Physics Dept, Sussex Cancer Centre. Outline. Device description Streamlining beam dosimetry QA MLC calibration Summing up. Device description.

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Use of the IC Profiler detector array for comprehensive machine QA

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

  2. Outline • Device description • Streamlining beam dosimetry QA • MLC calibration • Summing up

  3. Device description

  4. Device description

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

  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, d10 67.0% BC 40.5V, d10 67.5% BC 42.5V, d10 68.0%

  9. Streamlining beam dosimetry QA

  10. Use chambers only for absolute dose output at gantry 0° • Use IC Profiler for all other measurements 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

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

  12. Traditional schedule

  13. Proposed schedule 50% fewer readings 65% fewer trips down the maze

  14. Thus saving 30 miles of walking in one year (for a four linac department) Possible schedule 20% fewer readings 65% fewer trips down the maze

  15. MLC calibration

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

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

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

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

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

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

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

  23. Major gains and offsets - penumbra interpolation • 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) 100% 75% 50% 25% 0% -10mm 0 5mm -5mm 10mm 1 – (t + sc) s + (t + sc) × 1 - Dpenumbra(s) = √(s2 + n) 2 where (t + sc) = Transmission + Scatter (outside beam) and n = (20-80% penumbra width / 1.5)2

  24. Calibration sequence – 25MU exposures MLC minor offsets (central 30 leaves); Backup jaw major gains & offsets 3 1 2 4 +5 +15 -5 -15 Y2 Jaw Y2 MLC Y1 Jaw Y1 MLC Major gains & offsets 5 Further 4 exposures if calibrating all 40 MLC leaves 6 -15 +15 -5 +5

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

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

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

  28. Summing up

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

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

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