Lotte Verbunt Investigation of leaf positioning accuracy of two types of Siemens MLCs making use of an EPID

Lotte Verbunt

Investigation of leaf positioning accuracy

of two types of Siemens MLCsmaking use of an EPID

Overview

• background

• goals

• calibration

• image acquisition

• method I

• method II

• measurements

• comparison

• results

• conclusions

• recommendations

Dr. Bernard Verbeeten Institute Tilburg

• independent institute for radiotherapy and nuclear medicine:

- 11 radiotherapy oncologists

- 3 nuclear physicians

- 6 medical physicists

- 1 general doctor

• 2360 new radiation treatment patients a year (2004)

backgroundgoals

calibrationacquisitionmethod Imethod II

measurementscomparison

resultsconclusions

recommendations

Linear accelerator and EPIDbackground

goalscalibrationacquisitionmethod Imethod II


resultsconclusions

recommendations

accelerationbending

through patient

conversion into light

reflection by mirror

detection by camera

Multileaf collimator

two types of Siemens MLCs:

MLC 1: 29 leaf pairs; 2 mm accuracy

MLC 2: 41 leaf pairs; 1 mm accuracy

backgroundgoals



resultsconclusions

recommendations

Y jaw

leaf bank

central axis

Project goals

• development of a leaf verification method applicable to a Siemens linac using a CCD-camera based EPID

• verification of the leaf positions of two different types of Siemens MLCs and to check whether the MLCs and the current leaf calibration method are accurate enough for IMRT

backgroundgoals



resultsconclusions

recommendations

Why is this of interest?

• Intensity Modulated Radiation Therapy (IMRT):

- several beams from different directions

- each beam consist of several (abutting) segments

backgroundgoals



resultsconclusions

recommendations

Leaf calibration method: light field

• performed with EPACtool

• using light field and grid lines from graph paper

• 4-points calibration: 20, 10, 0, and -10 cm

backgroundgoals



resultsconclusions

recommendations left leaf bank right leaf bank

-20 -10 0 10 20

Leaf calibration method: light field

• performed with EPACtool

• using light field and grid lines from graph paper

• 4-points calibration: 20, 10, 0, and -10 cm

• uncertainties because:

– a difference between light and radiation field

– a blurred light field edge

– a possible rotation of graph paper

backgroundgoals



resultsconclusions

recommendations

Leaf verification methods: radiation field

• method I: 50 % dose value

• method II: from dose change to leaf positions

both methods use same input from the EPID

backgroundgoals



resultsconclusions

recommendations

Image acquisition

• 6 segments, 5 cm wide, 2 mm overlap of the leaves

black = blocked by the leaveswhite = unblocked irradiated part

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resultsconclusions

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Image acquisition


• barrel distortion correction

32

21 dddu RaRaRR

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resultsconclusions

recommendations

Image acquisition


• barrel distortion correction

• radiation center and pixel dimensions

backgroundgoals



resultsconclusions

recommendations

Method I

• take a mean profile for each segment and for each leaf pair

• fit a polynomial function between 20 and 80% of the dose value

• determine leaf position (50% dose value)

position [pixel]

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[%]

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resultsconclusions

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Accuracy method I

• images:

– nine 3 cm wide segments

– flat side of the leaves

– 90 rotation

• accuracy leaf position determination: 0.08 ± 0.18 mm (1 SD)

• shift of ± 0.45 mm at abutment leaf ends for MLC 2

backgroundgoals



resultsconclusions

recommendations

Method II

• sum all segments

• take a mean profile for each leaf pair

• calculate the dose change at each abutment

• convert the dose change into gap width

position [pixel]

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ixe

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extreme dose

average dose

ji

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D

EDd

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,, 1

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resultsconclusions

recommendations

Calibration function method II

• calibration function f :

• six tests with different gap widths (-1 to 4 mm)

• plot: intended gap width versus dose change

• dose change is 15-20 % per mm gap width

gap width dose changeif

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resultsconclusions

recommendations

-2,00

-1,00

0,00

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-0,15 0 0,15 0,3 0,45 0,6

dose change [-]

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Accuracy calibration function method II

• difference between measured (line) and expected gap (dots) width.

• accuracy calibration functions: – 0.09 mm (1 SD) for linac 1 – 0.12 mm (1 SD) for linac 2

backgroundgoals



resultsconclusions

recommendations

1,50

2,00

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0,38 0,42 0,46 0,50 0,54 0,58

dose change [-]

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p w

idth

[m

m]

Measurements

gap width and gap position:

• long-term reproducibility: segments from left to right (20 times in 10 weeks)

• short-term reproducibility: segments from left to right (5 times in succession)

• hysteresis:

– segments from right to left (5 times in 10 weeks)

– segments in random order (5 times in 10 weeks)

backgroundgoals



resultsconclusions

recommendations

Comparison

• t-test for comparison of both methods:

– difference is 0.004 ± 0.14 mm (1SD)

• statistical: different because of many measurements

• clinical: difference not relevant

• only results obtained with method I are shown

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resultsconclusions

recommendations

Results

• results of MLC 2 are split because of calibrations

* Vieira et al. Radiother. Oncol. (2005)

• mean gap width error: 1.26 mm due to:

– a difference between light and radiation field

– a blurred light field edge

backgroundgoals



resultsconclusions

recommendations

gap width error [mm] gap position error [mm]

MLC 1 1.12 ± 0.43 -0.10 ± 0.32

MLC 2 (1-7) 1.26 ± 0.49 0.43 ± 0.30

MLC 2 (8-13) 1.30 ± 0.35 -0.07 ± 0.60

MLC 2 (14-20) 1.61 ± 0.51 -0.32 ± 0.58

Vieira et al. * 1.21 ± 0.22

Results

• criterion dose error: 3%

criterion position error: 3 mm

• mean gap width error: 1.26 mm

under dosage of 19-25% (15-20% per mm)

4-5% per plan (if plan consists of 5 beams)

• calibration method is not accurate enough for IMRT

backgroundgoals



resultsconclusions

recommendations

Results: dependency

• gap width and gap position not dependent on:

– abutment position

– leaf pair number

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resultsconclusions

recommendations

Results: gap width error in time

MLC 1

MLC 2 drift (0.35 mm)

-0,50

0,00

0,50

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days of measurement [-]

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resultsconclusions

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Results: gap position error in time

MLC 1

MLC 2

-1,50

-1,00

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backgroundgoals



resultsconclusions

recommendations

rotation: 0.6 *

* Bayouth et al. Med. Phys. 30 (2003)

Results: reproducibility gap width

• short-term reproducibility (1 SD):

0.17 mm and 0.12 mm for MLC 1 and MLC 2

• long-term reproducibility (1 SD):

0.22 mm and 0.18 mm for MLC 1 and MLC 2

3% dose error criterion 15% dose error in beam (using 5 beams) maximum allowed gap width error: 0.75 mm (3 SD) (using 20% per mm)

maximum gap width deviation for each leaf pair from its average: 0.75 mm and 0.70 mm for MLC 1 and MLC 2

MLCs are accurate enough for IMRT

backgroundgoals



resultsconclusions

recommendations

Results: hysteresis

• gap width:

– no hysteresis

• gap position:

– small hysteresis for MLC 1 ( 0.40 mm)

– no hysteresis for MLC 2

backgroundgoals



resultsconclusions

recommendations

Conclusions

• an accurate leaf verification method has been developed

• the current leaf calibration method using the light field is not accurate enough for IMRT

• the day-to-day variation in gap width is accurate enough for typical IMRT plans

backgroundgoals



resultsconclusions

recommendations

Recommendations

• not possible to calibrate the leaves using this leaf verifation method

• method can be used for:

– checking the accuracy of the leaf calibration

– verifying the leaf positions in order to change the encoder value using MLCCHK

reduce calibration frequency

• investigate hysteresis for different gantry angles

backgroundgoals



resultsconclusions

recommendations

Questions?

Documents

Lotte Verbunt Investigation of leaf positioning accuracy of two types of Siemens MLCs making use of an EPID