mlc modeling and optimizing the dlg: predicting and...

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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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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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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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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Distance (mm)

Dose Profiles Parallel to MLC

Film Measurement

Eclipse

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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

te

Measurement

Eclipse 1.5 mm DLG

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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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Measurement

Eclipse 1.5 mm DLG

Modified Eclipse

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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 Profiles Parallel to MLC

6X Film

Eclipse

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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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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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Film Measurement

Eclipse 1 mm grid

Eclipse 2 mm grid

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Distance (mm)

Dose Profiles Parallel to MLC

Eclipse 1 mm grid

Eclipse 2 mm grid