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MLC modeling and optimizing the DLG: Predicting and preventing a major mode of IMRT QA FailureMike GramsMayo ClinicRochester, MN
What makes a plan fail IMRT QA?• “It has too many MUs.”
• “It’s too modulated.”
• “We’re pushing the planning system too hard.”
• “I don’t know.”
Eclipse
Measurement
Spine SBRT Re-plan attempt
Physics: “Choose isocenter so the MLCs can park over the cord!”
Spinal Cord
Target
Eclipse – Across Cord
80.7% 2% (local)/2mm
95.0% 2% (global)/2mm
Eclipse
Measurement
Eclipse – Across CordEclipse – Along Cord
Eclipse
Measurement
World’s Worst Spine SBRT Plan Ver 1.0:
Collimator 0°, MLCs stationary but protecting “cord”, Simple case, should be easy for Eclipse to model
World’s Worst Spine SBRT Plan Ver 2.0:
Collimator 90°, MLCs stationary but protecting “cord”, Simple case, should be easy for Eclipse to model
Dose Profile Across “Cord” Region
Parallel to MLC –rounded end
Perpendicular to MLC –tongue and groove
Keep it simple: 2 x 2 cm2 open field
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Film Measurement
Eclipse
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Dose (cGy
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Distance (mm)
Dose Profiles Perpendicular to MLC
Film Measurement
Eclipse
What knobs can we turn in Eclipse?• Dosimetric Leaf Gap (DLG)
• Transmission
• Source spot size
Dosimetric Leaf Gap• On Varian machines, the ends of the MLC
leaves are rounded
• Because of this, some radiation passes through completely closed leaf pairs
• Eclipse models the leaf end as flat and compensates for the round end by shifting the leaf tip positions back by half the DLG
• The DLG is configurable in Eclipse
This is what the MLC looks like in real life
This is what Eclipse thinks the MLC looks like
This distance is half the DLG
Leaf Transmission• A small amount of radiation passes directly
through the leaves
• Transmission is configurable within Eclipse
Source spot size• “The spot size parameter affects effectively the
penumbra shape…”
• “…have a significant effect on the calculated absolute dose level for very small field sizes (<=1x1 cm2) and for the shape of the calculated penumbra for all field sizes.”
• Should “be manually adjusted for each treatment unit” (default value is x = y = 0 mm)
• Is configurable in Eclipse, but requires re-calculating beam data
Turn the knobs• Knob #1: DLG
Essentially widens the profile
everywhere
Not much help out here
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Film Measurement
Eclipse 2.5 mm DLG
Turn the knobs• Knob #2: Transmission
Mainly increases dose in the tails
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Eclipse 3.0% Transmission
Eclipse 1.5% Transmission
Film Measurement
Turn the knobs• Knob #3: Source spot size
Tends to slant the entire penumbra
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Film Measurement
Eclipse x = 1 mm
Turn the knobs• Turning the knobs individually, we still couldn’t
match the measured profiles in the problematic region
• Turning knobs in combination, we still couldn’t match the profiles and this became very complicated (lots of possible iterations)
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Film Measurement
Eclipse
We just need some extra dose in this region.
Can we “trick” Eclipse into putting more dose here???
We can artificially broaden the penumbra by summing two appropriately weighted fields: One with the original MLC opening and a second with the MLCs opened by 5
mm on each side
=100%
95%
+5%
=100% 95% + 5%
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Film Measurement
Eclipse
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0 10 20 30 40
Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Film Measurement
Modified Eclipse
World’s Worst Spine SBRT Plan Ver 1.0:
Collimator 0°, MLCs stationary but protecting “cord”, Simple case,should be easy for Eclipse to model
Original Eclipse Modified Eclipse
Modified Eclipse
• Deliver 95% of the MUs with the original MLC pattern, and the remaining 5% with the MLCs opened an additional 5 mm on each side
• MATLAB code edits the MLC positions and MUs and creates the additional fields
• Eclipse then calculates dose for all fields
• Works for simple cases, but what does it do for actual treatment plans?
Spine SBRT Re-plan attempt
Physics: “Choose isocenter so the MLCs can park over the cord!”
Spinal Cord
Target
Eclipse – Across Cord
80.7% 2% (local)/2mm
95.0% 2% (global)/2mm
Modified Eclipse – Across Cord
99.5% 2% (global)/2mm
93.1% 2% (local)/2mm
Modified Eclipse – Across CordModified Eclipse – Along Cord
What about other treatment plans?• Measured 25 IMRT QA plans with film
• Compared measurement to the original Eclipse dose plane and also to the Modified Eclipse dose plane
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2% (M
ax)/0 mm DTA
Pass Ra
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Measurement
Eclipse 1.5 mm DLG
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0 5 10 15 20 25
2% (M
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Pass Ra
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Measurement
Eclipse 1.5 mm DLG
Modified Eclipse
Modified Eclipse
• Has lots of benefits:• Gets square fields and IMRT/VMAT plans right• Can calculate a new dose distribution and DVH• Can predict pre-QA where a plan might fail
Modified Eclipse Plan – Original Eclipse Plan
Modified Eclipse
• Has lots of benefits:• Gets square fields and IMRT/VMAT plans right• Can calculate a new dose distribution and DVH• Can predict pre-QA where a plan might fail
• But adds more steps and time to an already busy workflow
• Is there something easier we can do that’s good enough?
Can we just change the DLG?• Modified Eclipse method adds dose by making
MLC pair openings 1 cm wider for 5% of the MU
• Will opening all MLCs by some ∆DLG do the same thing?
• How much did the Modified Eclipse method change the mean MLC opening?
Can we just change the DLG?
Original Eclipse mean:orig
Modified Eclipse mean:new 0.95 orig 0.05 orig 1cmnew orig 0.05cm
Same as increasing DLG by 0.05 cm
Can we just change the DLG?• Our previous DLG in Eclipse was 0.15 cm
• Based on the preceding analysis, the DLG was changed to 0.20 cm
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Film Measurement
Eclipse 2.0 mm DLG
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2% (M
ax)/0 mm DTA
Pass Ra
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Measurement
Eclipse 1.5 mm DLG
Modified Eclipse
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2% (M
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Pass Ra
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Measurement
Eclipse 1.5 mm DLG
Modified Eclipse
Eclipse 2.0 mm DLG
Spine SBRT Re-plan attempt
Physics: “Choose isocenter so the MLCs can park over the cord!”
Spinal Cord
Target
Eclipse 1.5 mm DLG Eclipse 2 mm DLGModified Eclipse
If the cord is relatively straight, you can rotate the collimator so that the MLCs completely cover it by fully extending the leaves
More desirable, entire length of MLC is parked over the cord for large portions of the arcs
Summary• Inaccurate modeling of the end of the MLC can
lead to IMRT QA failures
• By artificially broadening the penumbra, better agreement between measurement and Eclipse is obtained
• Based on the “Modified Eclipse” approach, we were able to find an optimal DLG
• Plans calculated with this optimized DLG normally agree just as well with measurement as the Modified Eclipse
• There are a few exceptions, most notably, spine SBRT cases
Thanks• Amanda Deisher
• John Antolak
• Luis Fong de los Santos
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
6X Film
Eclipse
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
6X Film
6X Pinpoint
Eclipse
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‐12 ‐10 ‐8 ‐6 ‐4 ‐2 0 2 4 6 8 10 12
Dose (cGy
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Distance (cm)
6X X direction ‐ profile parallel to MLC leaf direction
MatriXX
Eclipse
What about Acuros?• Acuros is reported to be a more accurate
calculation than AAA
• It has not yet been commissioned at Mayo
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Acuros
Film Measurement
A quick note on calculation grid size• Calculating on a 1 mm grid vs. a 2 mm grid
results in even worse agreement in the cord region
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Distance (mm)
Film Measurement
Eclipse 1 mm grid
Eclipse 2 mm grid
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Dose (cGy
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Distance (mm)
Dose Profiles Parallel to MLC
Eclipse 1 mm grid
Eclipse 2 mm grid
