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
Ratio to stan dard 5m m g ap
1.100
1.050
1.000
0.950
0.900
-0.4
-0.2
0
0.2
G ap error
0.4
0.6
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
Meas D=5
50
Peregrine D=5
Corvus D=5
40
30
20
10
0
-25
-20
-15
-10
-5
0
Off axis
5
10
15
20
25
6MV 21EX Profiles
10x30 alternating
100
90
80
70
Relative dose
60
Meas D=5
Peregrine D=5
50
Corvus D=5
40
30
20
10
0
-20
-15
-10
-5
0
Off axis
5
10
15
20
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
Ridge10
Ridge40
180
160
160
140
140
120
120
Dose (cGy)
Corvus
Peregrine
80
Chamber
Film
60
Dose (cGy)
100
100
C hamber
Film
40
20
20
0
0
-5
Peregrine
60
40
-10
C orvus
80
0
5
-10
10
-5
-20
0
5
10
-20
cm off axis
cm off axis
Trench40
Trench10
100
100
90
80
80
70
60
Corvus
Peregrine
Chamber
50
40
Film
C orvus
Dose (cGy)
Dose (cGy)
60
Peregrine
C hamber
40
Film
30
20
20
10
0
-10
0
-10
-5
-10
0
cm off axis
5
-5
0
10
-20
cm off axis
5
10
5mm strips
In c lin e
140
120
100
80
Dose (cGy)
C o rvus
P e re g rine
60
C ha m b e r
F ilm
40
20
0
-1 0
-5
0
-2 0
c m o ff a x is
5
10
10mm strips
In c lin e
140
120
100
Dose (cGy)
80
C orvus
P eregrine
C ham b er
F ilm
60
40
20
0
-10
-5
0
-20
cm o ff axis
5
10
“Band100” – “Band0” patterns
Bands0
140
120
100
C o rvu s
80
Dose (cGy)
P e re g ri n e
C ha m b e r
F i lm
60
F i lm R e p e a t
40
C ha m b e r
20
0
-1 0
-5
0
-2 0
c m o ff a x is
5
10
10 60
0%
80 20
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
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
80
60
40
20
0
-15
-10
-5
0
-20
5
10
15
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