Commissioning an IMRT System for MLC Delivery Gary A. Ezzell., Ph.D. Mayo Clinic Scottsdale
Taking the broad view of commissioning
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
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
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...
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
Important for IMRT because there are so many junctions within a field
Measuring the offset Irradiate a series of strips that create matchlines Use different values of offset Select offset value that gives best uniformity
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.
Matchlines are very sensitive to positioning Offset 0.5 mm Offset 0.7 mm Offset 0.9 mm
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
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
Sweep all leaves at same rate Chamber reading at center should be proportional to MU Film should show uniformity
Sweep leaves at different rates e.g. Travel 5, 7, 9, 11, 13 cm in same MU Check relative dose
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)
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 1.100 1.050 1.000 0.950 Ratio to standard 5mm gap 0.900-0.4-0.2 0 0.2 0.4 0.6 Gap error
Routine QA checks for positioning accuracy Matchline films 1 mm gap films Check at different gantry, collimator angles Decide QA frequency
Mayo monthly includes matchline films at 5 combinations of gantry, collimator
Check visually. Patterns include programmed deviations 0.5 mm closed 0.5 mm open orientation 0.5 mm open 0.5 mm closed
Mayo monthly includes one profile analyzed quantitatively
Mayo monthly includes ratio of swept 5 mm gap to 10x10 open Similar test used with daily QA device
Summary for the delivery system Commission the delivery system independently of the planning system Establish QA tests to monitor performance
on to the planning system
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
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
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
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
6MV 21EX Profiles 10x30 100 90 80 70 Relative dose 60 50 40 Meas D=5 Peregrine D=5 Corvus D=5 30 20 10 0-25 -20-15 -10-5 0 5 10 15 20 25 Off axis
6MV 21EX Profiles 10x30 alternating 100 90 80 70 Relative dose 60 50 40 Meas D=5 Peregrine D=5 Corvus D=5 30 20 10 0-20 -15-10 -5 0 5 10 15 20 Off axis
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
Leak100 Leak0 patterns Corvus/Meas Peregrine/Meas Leak100 0.979 0.987 Leak50 0.978 0.977 Leak20 0.976 0.970 Leak10 0.960 0.955 Leak0 0.000 0.984
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
Ridge40 Ridge10 180 160 160 140 140 120 120 100 Dose (cgy) 100 80 60 Corvus Peregrine Chamber Film Dose (cgy) 80 60 Corvus Peregrine Chamber Film 40 40 20 20 0-10 -5 0 5 10-20 cm off axis 0-10 -5 0 5 10-20 cm off axis Trench40 Trench10 100 100 90 80 80 70 Dose (cgy) 60 50 40 30 20 Corvus Peregrine Chamber Film Dose (cgy) 60 40 20 Corvus Peregrine Chamber Film 10 0-10 -5 0 5 10-10 cm off axis 0-10 -5 0 5 10-20 cm off axis
5mm strips In c lin e 140 120 100 Dose (cgy) 80 60 40 Corvus Peregrine Chamber Film 20 0-10 -5 0 5 10-20 cm off axis
10mm strips Incline 140 120 100 Dose (cgy) 80 60 40 Corvus Peregrine Chamber Film 20 0-10 -5 0 5 10-20 cm off axis
Band100 Band0 patterns Bands0 140 120 100 Dose (cgy) 80 60 40 Corvus Peregrine Chamber Film Film Repeat Chamber 10 60 0% 80 20 20 0-10 -5 0 5 10-20 cm off axis
Finally, highly modulated single fields Random array
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
And once you are over the simple stuff
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
Mock prostate
Colors: film, Gray: plan
Mock Head/Neck
Colors: film, Gray: plan
Mock spine
Critical structure plane
Target plane
Colors: film, Gray: plan
Another question What is the role of quantitative assessment of individual IMRT fields vs. composite doses?
Some problems show up more clearly on individual fields than composite distributions e.g. Dynamic delivery with Corvus/Varian Step/Shoot Dynamic
Dosimetric validation (4) Test calculations in the presence of inhomogeneities Measured/Corvus: 0.985
Dose/MU at 5 cm below 12 cm inhomogeneity Measured/Corvus Bone 1.002 Air 0.988 Lung 0.985 Water 0.978
LUNG 140 120 100 80 60 40 20 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=20.0 0-15 -10-5 0 5 10 15-20
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
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%).
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?
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?
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
Welcome to IMRT there is a way out