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IMRT Commissioning

IMRT Commissioning. Gary A. Ezzell., Ph.D. Mayo Clinic Scottsdale. Welcome to Arizona. Commissioning elements. Validating the dosimetry system Commissioning the delivery system Commissioning the planning/delivery systems for dosimetric accuracy Learning how to use IMRT.

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IMRT Commissioning

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  1. IMRT Commissioning Gary A. Ezzell., Ph.D. Mayo Clinic Scottsdale

  2. Welcome to Arizona

  3. Commissioning elements • Validating the dosimetry system • Commissioning the delivery system • Commissioning the planning/delivery systems for dosimetric accuracy • Learning how to use IMRT

  4. Step 0: the dosimetry system • Decide on the system that will be used to measure dose distributions • Determine the measurement uncertainty • Compare measurements of unmodulated fields to ion chamber scans • Needed to establish acceptance criteria

  5. Delivery system tests • Need to create test patterns independent of planning system • Need to check • MLC positioning accuracy • MLC dynamic motion accuracy • Effect of rounded leaf ends • MU linearity ...

  6. Radiation field “offset” for rounded leaf ends • For rounded MLC leaf ends, there is an offset between the light and radiation field edges: ~0.6 mm • Important in IMRT Light Radiation

  7. Why important for IMRT?

  8. Measuring the offset • Irradiate a series of strips that abut • Use different values of offset • Select offset value that gives best uniformity

  9. No offset 1.0 mm offset 0.6 mm offset Measuring the offset Here, 0.6 is best, i.e. subtract 0.6 mm from MLC settings Needed?? most uniform at junction? Integrated dose?

  10. Measuring the offset

  11. MLC accuracy • Tenths of millimeters are important for IMRT • Use patterns of abutting strips for a sensitive test • Test at different gantry and collimator angles • Select subset for routine QA

  12. Abutments are very sensitive to positioning Offset 0.9 mm Offset 0.7 mm Offset 0.5 mm

  13. DMLC tests • For dynamic MLC motion, leaf speed and dose rate also need to be controlled • Need to test for range of leaf speed and dose rates, as well as gantry and collimator angles • Create patterns that move a 1 cm gap using all leaves at same or variable rates

  14. Sweep all leaves at same rate Chamber reading at center should be proportional to MU Film should show uniformity

  15. Sweep leaves at different rates e.g. Travel 5, 7, 9, 11, 13 cm in same MU Check relative dose

  16. Special problem of matching accelerators • In ’99 Mayo Clinic Scottsdale had two “matched” Varian 2100C linacs, but IMRT doses differed by ~2.5% • Needed to adjust MLC calibration on one machine by 0.13 mm

  17. Delivery system QA • Sweep a 5mm gap across a 10 cm span with Farmer chamber at center • Take ratio to 10x10 open field • 2.5% for 0.2 mm change

  18. Routine QA checks for positioning accuracy • Abutment films • 1 mm gap films • Check at different gantry, collimator angles • Check frequency(?)

  19. …and now for a change

  20. Planning system: Dosimetric accuracy • For IMRT, the MLC leaves move through the area of interest • Final distribution is created by summing many beamlets • New things become important • Leakage through MLC leaves • Penumbra defined by MLC leaves • Small fields

  21. MLC leakage - measure average value • Leakage through leaf (~2%) • Between neighboring leaves (~5%) • Measure using a pattern that fully closes all leaves - careful not to be under carriage or jaw

  22. Penumbra • Measure with film, diode, or microchamber, conventional scanning chamber too wide • Subtle effects make a difference in IMRT Beam model based on penumbra measured with chamber Beam model based on penumbra measured with film

  23. Dosimetric validation (1) Start with the basics • Define simple open fields irradiating a simple phantom • check calculated output, PDD, profiles against standard measurements • (just like commissioning any planning system) • And shaped static fields also

  24. Dosimetric validation (2) • Define simply-modulated fields with wide regions that can be measured with chamber and film (or equivalent) • Check: • Leakage/transmission • Basic modulation • Penumbra • Out of field dose

  25. “Leak100” – “Leak0” patterns

  26. 100% centers, 10% wings 10% centers, 100% wings “Incline” patterns 50% centers, 100% -10% wings “Ridge” and “Trench” patterns Useful for deciding minimum segment width

  27. 5mm strips

  28. 10mm strips

  29. “Band100” – “Band0” patterns 0% 60 20 10 80

  30. Finally, highly modulated single fields

  31. What to do about differences? • May need to adjust the beam model • May need to live with it • That is, take known deficiencies into account when evaluating plans • Very important to know about it, especially for critical structures

  32. And once you are over the simple stuff …

  33. Dosimetric validation (3) • Define targets and structures in a simple phantom: mock clinical situations • Create inverse plan • Measure doses with chamber, film, … • Evaluate dose/MU in low gradient region • Evaluate isodoses on many planes

  34. Mock prostate

  35. Colors: film, Gray: plan

  36. Mock Head/Neck

  37. Colors: film, Gray: plan

  38. Mock spine

  39. Critical structure plane

  40. Target plane

  41. Colors: film, Gray: plan

  42. Dosimetric evaluation of mock clinical plans • What kind of agreement to expect in high dose, low gradient regions? • ~2 - 4% is reasonable (chamber) • What kind of agreement to expect for isodoses? • Much harder to specify • ~2 - 4 mm for 50% - 90% lines

  43. How to be more quantitative? • Acceptance criteria should be stated statistically, such as: “within the region of interest, 95% of the calculation points should agree with the measurement to within 5%,which combines the desired degree of agreement with reality (3%)with the two-sigma uncertainty in the measurement technique (4%).”

  44. Questions • What is the region of interest? • What is the desired degree of agreement with reality? • What is the uncertainty in the measurement technique? • What percentage of the points should agree to the specified tolerance?

  45. Questions • For the points that do not agree to that tolerance, is there an outer limit of acceptability? • Should the agreement be expressed as a percent of the local dose, prescription dose, or some other dose? • Should distance to agreement be incorporated?

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