1. Gary A. Ezzell., Ph.D. Mayo Clinic Scottsdale
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. Measuring the offset No offset 0.6 mm offset 1.0 mm 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 0.5 mm
Offset 0.7 mm
Offset 0.9 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

26. 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

27. “Leak100” – “Leak0” patterns

28. “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

30. 5mm strips

31. 10mm strips

32. “Band100” – “Band0” patterns60 0% 80 20

33. Finally, highly modulated single fields

34. 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

35. And once you are over the simple stuff …

36. 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

37. Mock prostate

39. Colors: film, Gray: plan

40. Mock Head/Neck

42. Colors: film, Gray: plan

43. Mock spine

44. Critical structure plane

45. Target plane

46. Colors: film, Gray: plan

47. 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

48. 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%).”

49. 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?

50. 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?

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

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

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

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

56. Finding your way around …

57. General principles (1) Don’t ask for the impossible
If you ask for NO dose to the cord, 60 Gy may appear just as bad as 40 Gy to the optimizer
Look at a good 3D conformal alternative to get a starting point

58. General principles (2) Explain the problem sufficiently to the system
Define what needs to be treated
Define what needs to be avoided
The system is going to choose the beam shapes according to the structures defined

59. General principles (3)
Define the problem sufficiently to the human who is doing the planning
What is absolutely necessary
What is desirable
Where you are able to compromise

60. General principles (4)
Understand the difference between three kinds of “prescriptions”:
What you want and tell the human planner to get
What the planner tells the system to try for
What you eventually get and treat with

61. General principles (5) Learn what “knobs” there are
DVH criteria for targets and structures
Relative weights or tissue types
Number of intensity levels
Number and direction of beams …
Try each individually and systematically
on idealized and actual patients

62. General principles (6) Dose uniformity vs Conformality
Target dose uniformity can be expected to decrease with
increasing concavity
increasing dose gradient
decreasing number of beams

63. General principles (7) Don’t assume IMRT is the way to go
3D conformal, judged by the same criteria, may be better
parallel-opposed beam pairs are often best
IMRT system may have limitations
1x1 beamlets
insufficient weight to dose uniformity

64. Building experience with artificial problems
Designed to illustrate performance for certain types of situations
Observe how changing various planning parameters affects plan quality and delivery efficiency
For each, decide on relevant measures of plan quality

65. Simple cylindrical geometry
Single target with 1 cm PTV expansion (PTV is 8 cm diam, 8 cm long)
Goal: target dose uniformity
PTV max/min
PTV D2/D98
Compare to:
3 open fields
evenly weighted

66. Target dose uniformity
As you vary the requested degree of target dose uniformity, how do the results change?
Corvus V5 “Prescription Panel”

67. Goal: target uniformity (Corvus V5)
% vol below x10 10x12
Min
Max
Rx Max/Min
Max/Min
D2/D
# segments
# MU
Forward plans
Inverse plans

68. Simple cylinder with 4 structures PipesEasy
Goal: structure sparing vs target uniformity
PTV D2/D98
Structure mean/PTV mean
15 fields equispaced
Try different structure goals
50, 20, 10, 2% of target dose

69. PipesEasy: Effect of changing structure goals

70. PipesEasy: Effect of changing structure goals

71. PipesEasy: comments
Asking for too much sparing degrades target uniformity with little improvement
90% and 20% isodoses
Structure at 10%
Structure at 2%

72. What is achievable? Limiting dose gradient

73. C Shape Goal: structure sparing vs target uniformity PTV D2/D98
Structure D5/PTV 98
15 fields equispaced
Try different structure goals
60, 50, …, 10% of target dose

74. Corvus: C Shaped Target
Structure at 10%
Structure at 60%

75. Corvus

76. Limiting dose gradient
6%/mm

77. Buildup Expand PTV by 2-10 mm Evaluate dose uniformity CTV max/min
Volume max/CTV mean

78. 100%
110%
5 mm
below
0 mm
below

79. Comments on these artificial problems
Good for gaining some feel
for the “knobs”
for the limiting conditions (e.g. maximum dose gradients)
for the consequences of being unrealistic in the problem statement

80. Dealing with real clinical plans
Determine method/conventions for defining structures and targets
Determine margins (CTV and PTV)
These do not change whether IMRT or 3D conformal

81. Dealing with real clinical plans
Decide on criteria for an acceptable plan
e.g. PTV dose must be sufficiently uniform: PTV D2/D98  1.15
Decide on parameter to be optimized
e.g. minimize mean parotid dose
These will often be competing and cannot both be “optimized”

82. Dealing with real clinical plans
Start with a 3D conformal plan to get a sense of what is achievable
Use these results as a starting point for DVH goals for the inverse planner
Start with relaxed goals and gradually tighten them

83. Welcome to Arizona … and IMRT
… there is a way out