PDF Mustafa_Thesis presentation

WG Medical Radiation Physics, Pius-Hospital and Carl von Ossietzky University, Oldenburg
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Master Thesis :
The clinical performance of the DAVID-system for the
in vivo verification of VMAT irradiation
Presented by Mustafa Saibu Danpullo
1st supervisor
Prof. Dr.B.Poppe
2nd supervisor
Dr. HK. Looe,

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Layout
I Introduction
II Theory
 VMAT and IMRT
 MLC Design and Agility
 MWPC and DAVID system
III Materials & Methods
 Equipment, alignment, patient data, stability of DAVID chamber
 Beam property, Error detection
 Deconvolution
 DAVID QA software
IV Results / Discussion
V Conclusion

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I Introduction
IMRT (Intensity-modulated radiation therapy)
VMAT (Volumetric Modulated Arc Therapy):
 Why In vivo verification ?
 ICRU report 24 (1976) recommended that certain types of tumors
requires improve accuracy from 5% to 3.5%.
 To detect Equipment-related errors and deviations from the initial plan
 Complexity of planning and delivery techniques increases risk for
treatment-related error incidents.
 In 1992 to 2007, more than 4,000 near misses without adverse
outcome to patient’s case were reported, more than 50% were related
to the planning or treatment delivery stage.

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In vivo dosimetry methods:
 in vivo intracavitary dosimetry with TLD
 Diodes
DAVID (Device for Advanced Verification of IMRT deliveries
 In-vivo verification during treatment
 Online measurement of differences in dose to reference
 Error detection of the Multi Leaf Collimator (MLC)
I Introduction

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II Theory
Mostly Siemens, Elekta and Varian have introduced new LINAC
control systems that will be able to change the MLC leaf positions
IMRT uses many small fields to generated by beam-shaping
devices (MLC) to deliver a single dose of radiation
IMRT: Intensity-modulated Radiation Therapie

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II Theory
VMAT : Volumetric Modulated Arc Therapy
VMAT is a rotational IMRT that can be delivered using
conventional LINAC with MLC
Elekta and Varian have introduced new LINAC control systems
that will be able to change the MLC leaf positions and dose rate
while the gantry is rotating.
Precise Beam infinity and Rapid Arc

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A schematic drawing of the Siemens type A, Elekta type B and Varian type C MLC [18]
Stepped leafs for different manufacturers [34]
II Theory

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II Theory: Agility MLC design
• 160 tungsten leafs,
• rounded arc edge,
• 5 mm width,
• High speed(2x normal MLC) of up to
3cm/sec,
• large field MLC enable clinicians to
shape radiation,
• extremely low transmission of about
<0.5%
• 45 cm isocenter clearance from
accessory holder.
MLC Motor

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Inventor:
Prof. Georges Charpak,
France,1968
Nobel Prize in Physics (1992)
II Theory: Multi wire proportional chamber (MWPC)
The DAVID chamber is a multi-wire
ionization chamber designed by PTW
Freiburg based on Charpark's multi wire
proportional chamber.

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Compton scattering  Electric field causes electrons move to the
anode(wire) and ionizied atoms/molecules to the
cathode(plate)
 Each detection wire accumulates charge which
loads a C.
 After the voltage at the capacitor is read out, it is set
to zero and charged again
 The voltage achieved is read out by the associated
amplifier at a rate of 1 Hz.
 Performed by multi-channel electrometer
(MULTIDOS) + additional Software
.
II Theory: DAVID system functioning principle

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 Signal interpretation:
Ri: reading of a single channel (ion charge collected)
C: cross section of the lengthy collection volume along the wire
Ii: ionization density (x1 start of wire, x2 end of wire)
li1-li2: aperture of the associated leave pair
II Theory
II Theory

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Front Plate
Back Plate
Air Volume
II Theory: DAVID system signal recording
3 groups of secondary electrons contributing to the signal:
a)“primary signal”
b)scattered signal
c)leakage radiation
(background signal)

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VMAT Planning:
Treatment Planning System: ONCENTRA Masterplan Version 4.3
ELEKTA Synergy accelerator with an Agility 80 leaf-pair MLC
• Desktop Pro TM 7.011 is Elekta's third generation fully integrated
digital control system. MOSAIQ, DAVID software version 2.0
DAVID T34065
III Materials and Methods

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Patient data configuration chart
Reference
1st session

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DAVID Analysis
• PTW: DAVID 2.0 software
Warning level: 3%
Alarm level: 5%

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III Materials & Methods:
•VMAT: 4 (1 H&N, 3 Prostates)
•180° to -180° Clockwise and anti clockwise
Stability of the DAVID system
•IMRT : 1 (Prostate)
•0°
•90°
•270°
(Prostate and Head and Neck) 14 days

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The beam property of the DAVID chamber
•Percentage depth dose (PDD)
•Roos chamber 34001
•MP3 water phantom
•Transmission factor for 6 and 15 MV
•Semifex T31010 (Diff Field sizes)
Setup conditions
• With and without DAVID
• SSD 100 and 80 cm
• Photon energy 6 and 15
MV
• Different field sizes

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1. Successive opening of 1 leaf on 1
side
VMAT Plan Editing for error detection
• MATLAB script to change the MLC-positions
3. Field shift of a leaf gap (size of leave
gap remains)
2. Successive shift of a leaf gap (size of
leave gap remains)

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1. Successive opening of 1 leaf on 1
side
VMAT Plan Editing for error detection
• MATLAB script to change the MLC-positions
3. Field shift of a leaf gap (size of leave
gap remains)
2. Successive shift of a leaf gap (size of
leave gap remains)

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Deconvolution
S(x) measured signal as convolution of
P(x) True „dose“ profile with
LRF fɛ(x).
S(x) = P(x) * f(x)
„van Cittert“ iterative deconvolution
algorithm

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opened MLC at every 10th interval from 1st to 80th
pairs.
Nine MLC slit through the entire DAVID chamber
IV Results and Discussion: Alignment

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IV Results and Discussion: Stability of DAVID System
• IMRT: prostate
• Deviation of ±2% (+2%)
• VMAT: prostate
• Deviation of ±1% (-1%)

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IV Results/Discussion: Stability of DAVID System
• IMRT: prostate
• Deviation of ±2% (+2%)
• VMAT: H&N
• Deviation of <0.5%

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IV Results/Discussion: Transmission factor
The average KDAVID
• 0.939 ±0.003 for 6 MV
and
• 0.953 ± 0.004 for 15 MV
 Reduction of dose at isocenter
due to 8mm of PMMA
 By measuring the attenuation
factor the output value can be
corrected.
Attenuation of the beam by the DAVID chamber

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100 cm SSD,
Increased about
0.32% with DAVID
No change of
Dmax
1.4 cm with
and without DAVID
80 cm SSD,
An increased about
4.26% with DAVID
slight change of
Dmax
1.3 cm with DAVID
1.4 cm without DAVID
IV Results/Discussion: Changes in PDD

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100 cm SSD,
An increased about
0.67% with DAVID
Dmax
2.6 cm No change
80 cm SSD,
An increased about
18% with DAVID
slight change of
Dmax
1.8 cm with DAVID
2.4 cm without DAVID
IV Results/Discussion

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Surface dose
 Increase with increase in field size
Increase with increase in energy
Increase with decrease in SSD

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Pronounce with
decrease in SSD and,
increase in photon energy and
increase in field size.
Increase in surface dose is
due to scattered secondary
electrons from the DAVID
chamber reaching the
water phantom surface
(electron contamination).
6 cm
45 cm
100 cm

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Deconvolution test
by iteration method
IV Results/Discussion: Deconvolution

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Deconvolution
•does not depend on the length of the slit.
•40 cm slit results showed a small decrease in the tail signals.
10 x 10 cm
10 cm slit
40 cm slit
20 cm slit
30 cm slit

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IV Results/Discussion:
Deconvoluted slit signal at 40th and 65th wire
Deconvolution test

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10x10cm fields before and after deconvolution
Single arc prostate plan before and after deconvolution (3 mm )
IV Results/Discussion: Deconvolution with DAVID software.

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before deconvolution 6.1 mm
Gradients of the linear fit
before deconvolution : 0.47 and
after deconvolution 2.9 mm
Gradients of the linear fit
after deconvolution : 0.94
.
Enhance sensitivity after deconvolution

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before deconvolution after deconvolution
H&N 6mm error.
.
False alarm/warning effect before deconvolution.
The effect is eliminated after deconvolution

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Undeconvoluted
deconvoluted
Measuring the deconvolution matrix with
the DAVID software as manual. LSF of single middle slit is measured and
used to generate the 80x80 LRF matrix with
MAT LAB and installed in the DAVID software
for deconvolution.
IV Results/Discussion: Limitations of DAVID-160 system

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2mm MLC error, single bank shift
2mm MLC error, single leaf
Analyzing both the maximum deviation
and total deviation in two different plots
at the same time

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Max Dose 75.99GyMax Dose 57.27Gc
IV Results/Discussion: Artificial MLC bang shift Error 2
cm

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0
1
2
3
4
5
6
7
8
0 8 10 12 14 16 18 20
shift / mm
doseincrease/Gy
maximaldeviation/%
Undetectable error! Design of the chamber
DAVID: Gap-Shift (prostate with OAR: rectum back wall)

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DAVID Quality Assurance software (DQA)
MAT LAB 2011b and 2012b
analyzes the daily session for all patient's data with 2 clicks online.
• NDD- Non deconvoluted data
• ˆDD-Deconvoluted data
• ED- Electrical data

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Analysis only specific data on
specific date, session and only print
out the deconvoluted data (DD)

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Patient text file from DQA software
Sample patient 2014-01-12
Display only data's with MLC error
indicating the data type (DD),Beam
number, Session, segment and
particular MLC with error.

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The DQA software displays the warning and alarm errors
Warning and error dialogs at future date entry and invalid date entry respectively
year-month-day `yyyy-mm-dd'

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VMAT and IMRT plans sessions

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DAVID chamber:
• Linear dependency on leaf opening
• Sensitivity dependent on leaf gap opening
• How much radiation pass through the opening
• Deconvolution double the sensitivity
DAVID is design for specifics LINACS
Independent from the LINAC
In-vivo verification of MLC malfunction during VMAT
• Undetectability field shifts due to chamber design
(Suggestion: perpendicular wires or gradient)
V Conclusions

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V Conclusion
Single MLC bank shift error
Maximum and total deviation to be analysed
Deconvolution matrix
To be generated for each linac
To be generated by single single slit
• In comparison to other techniques measurement of undisturbed
signals
-> no dependence on patient position(EPID)
-> measurement of the complete delivered dose(TLD, diode or MOSFET
detectors)
• Suggestions for future development: Standard design for software
• Deconvolution program to be integrated,
• DQA to be integrated
• Design two chambers perpendicular to each other
Co-operate directly with LINAC vendors for specifics designs

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Sources
1. [1] Ezzel GA, Galvin JM, Low D, Palta JR, Rosen I , Sharpe MB, Xia P, Xiao Y, Xing L and Yu CX. Guidance on delivery,
treatment planning, and clinical implementation of IMRT: report of the IMRT subcommittee of the AAPM radiation
therapy committee Med Phys 2003; 30:2089-115.
2. [2] ESTRO Booklet No. 9, 2008. Guidelines for the verification of IMRT, edited by Ben Mijnheer and Dietmar Georg.
ISBN 90-804532-9.
3. [3] Schneider F, Polednik M, Wol D, Steil V, Delana A, Wenz F, Menegotti L. Optimization of the gafchromic EBT
protocol for IMRT QA. Z Med Phys 2009; 19(1):29-37.
4. [4] Poppe B, Blechschmidt A, Djouguela A, Kollho R, Rubach A and Harder D. Two-dimensional ionisation-chamber
arrays for IMRT plan verication. Med Phys 2006; 33:1005-15
5. [5] Poppe B, Thieke C, Beyer D, Kollho R, Djouguela A, Rühmann A, Willborn KC and Harder D. DAVID-a translucent
multi-wire transmission ionization chamber for in vivo verication of IMRT and conformal irradiation techniques. Phys
Med Biol 2006; 51:1237-48.
6. [9] Poppe B, Looe H K, Chofor N, Rühmann A, Harder D and Willborn K. Clinical Performance of a Transmission
Detector Array for the Permanent Supervision of IMRT Deliveries. Radiother. Oncol. 2010; 95:158-65
7. [10] Looe H K, Harder D, Rühmann A, Willborn K and Poppe B. Enhanced accuracy of the permanent surveillance of
IMRT deliveries by iterative deconvolution of DAVID chamber signal proles Phys. Med. Biol. 2010; 55:3981-92
8. [11] Heukelom, S., el al.Wedge factor constituents of high-energy photon beams: Head and phantom scatter dose
components Radiother. Oncol. 32: (1994) 66-3
9. 12] Jursinic, P. A. Changes in incident photon uence of 6 and 18 MV x rays caused by blocks and block trays Med
Phys 26 (1999) 2092-8
10.[13] v. Klevens, H. Dependence of the tray transmission factor on collimator setting and sourcesurface distance Med.
Phys. 27 (2000) 2117-3
11.[14] Sharma, S.C., el al., Recommendations for measurement of tray and wedge factors for high energy photons Med
Phys 21 (1994) 573-5

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Thank you for your
attention

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Additional Slides

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Description of the Elekta synergy DAVID160 system
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David58
David160

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•Φ constant with depth
(small # interactions)
•Same # electrons set in
motion in each square
•i.e., interactions per
volume constant through
target
•dose reaches a
maximum at R (kerma
constant with depth,
equals absorbed dose
beyond )
Number of electron tracks set in motion by
photon interaction

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