Commissioning an IMRT System for MLC Delivery. Gary A. Ezzell., Ph.D. Mayo Clinic Scottsdale

Transcription

1 Commissioning an IMRT System for MLC Delivery Gary A. Ezzell., Ph.D. Mayo Clinic Scottsdale

2 Taking the broad view of commissioning

3 Commissioning elements Validating the dosimetry system Commissioning the delivery system Commissioning the planning/delivery systems for dosimetric accuracy Learning how to use the planning system for particular clinical problems Caveat: our experience is with Corvus + Varian

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

5 Delivery system tests Create test patterns independent of planning system - We use a text editor to create Varian files Need to check - Effect of rounded leaf ends (if applicable) - MLC positioning accuracy - MLC dynamic motion accuracy (if applicable) - MU linearity over range of interest...

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 Important for IMRT because there are so many junctions within a field

8 Measuring the offset Irradiate a series of strips that create matchlines Use different values of offset Select offset value that gives best uniformity

9 Measuring the offset No offset 0.6 mm offset 1.0 mm offset Here, 0.6 appears best, i.e. subtract 0.6 mm from MLC settings What is really needed?? most uniform at junction? Integrated dose? Check planning system requirements.

10 Matchlines are very sensitive to positioning Offset 0.5 mm Offset 0.7 mm Offset 0.9 mm

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

12 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

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

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

15 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 (Leaf gap error parameter)

16 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 Ratio to standard 5mm gap Gap error

17 Routine QA checks for positioning accuracy Matchline films 1 mm gap films Check at different gantry, collimator angles Decide QA frequency

18 Mayo monthly includes matchline films at 5 combinations of gantry, collimator

19 Check visually. Patterns include programmed deviations 0.5 mm closed 0.5 mm open orientation 0.5 mm open 0.5 mm closed

20 Mayo monthly includes one profile analyzed quantitatively

21 Mayo monthly includes ratio of swept 5 mm gap to 10x10 open Similar test used with daily QA device

22 Summary for the delivery system Commission the delivery system independently of the planning system Establish QA tests to monitor performance

23 on to the planning system

24 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

25 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

26 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

27 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

28 6MV 21EX Profiles 10x Relative dose Meas D=5 Peregrine D=5 Corvus D= Off axis

29 6MV 21EX Profiles 10x30 alternating Relative dose Meas D=5 Peregrine D=5 Corvus D= Off axis

30 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

31 Leak100 Leak0 patterns Corvus/Meas Peregrine/Meas Leak Leak Leak Leak Leak

32 Ridge and Trench patterns Useful for deciding minimum segment width 100% centers, 10% wings 10% centers, 100% wings Incline patterns 50% centers, 100% -10% wings

33 Ridge40 Ridge Dose (cgy) Corvus Peregrine Chamber Film Dose (cgy) Corvus Peregrine Chamber Film cm off axis cm off axis Trench40 Trench Dose (cgy) Corvus Peregrine Chamber Film Dose (cgy) Corvus Peregrine Chamber Film cm off axis cm off axis

34 5mm strips In c lin e Dose (cgy) Corvus Peregrine Chamber Film cm off axis

35 10mm strips Incline Dose (cgy) Corvus Peregrine Chamber Film cm off axis

36 Band100 Band0 patterns Bands Dose (cgy) Corvus Peregrine Chamber Film Film Repeat Chamber % cm off axis

37 Finally, highly modulated single fields Random array

38 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

39 And once you are over the simple stuff

40 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

41 Mock prostate

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43 Colors: film, Gray: plan

44 Mock Head/Neck

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46 Colors: film, Gray: plan

47 Mock spine

48 Critical structure plane

49 Target plane

50 Colors: film, Gray: plan

51 Another question What is the role of quantitative assessment of individual IMRT fields vs. composite doses?

52 Some problems show up more clearly on individual fields than composite distributions e.g. Dynamic delivery with Corvus/Varian Step/Shoot Dynamic

53 Dosimetric validation (4) Test calculations in the presence of inhomogeneities Measured/Corvus: 0.985

54 Dose/MU at 5 cm below 12 cm inhomogeneity Measured/Corvus Bone Air Lung Water 0.978

55 LUNG Meas d=1.25 Meas d=5.0 Meas d=10.0 Meas d=20.0 Corvus d=1.25 Corvus d=5.0 Corvus d=10.0 Corvus d=20.0 Peregrine d=1.25 Peregrine d=5.0 Peregrine d=10.0 Peregrine d=

56 Dosimetric evaluation: Acceptance criteria 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

57 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 acceptable degree of agreement with reality (3%) with the two-sigma uncertainty in the measurement technique (4%).

58 Questions What is the region of interest? What is the acceptable degree of agreement with reality? What is the uncertainty in the measurement technique? What percentage of the points should agree to the specified tolerance?

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

60 Dosimetric validation - Summary Know your measurement uncertainties Start with simple situations that can be more easily analyzed Design single beam tests that focus on specific parameters, e.g. - Transmission - Small or narrow fields - Penumbra and dose outside field Finish with multiple fields and mock clinical situations

61 Welcome to IMRT there is a way out