mustafa_thesis presentation

WG Medical Radiation Physics, Pius-Hospital and Carl von Ossietzky University, Oldenburg1

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,


Layout

I IntroductionII 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 / DiscussionV Conclusion


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.


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


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


II TheoryVMAT : 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


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


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


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.

http://store.aip.org/OA_HTML/ecl.jsp?mode=detail&item=6903


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


Signal interpretation:

Ri: reading of a single channel (ion charge collected)C: cross section of the lengthy collection volume along the wireIi: ionization density (x1 start of wire, x2 end of wire)

li1-li2: aperture of the associated leave pair

II Theory


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)


VMAT Planning:Treatment Planning System: ONCENTRA Masterplan Version 4.3ELEKTA 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


Patient data configuration chart

III Materials & Methods

Patient data

Reference

1st session


DAVID Analysis• PTW: DAVID 2.0 software

III Materials & Methods

Warning level: 3%

Alarm level: 5%


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


III Materials & Methods: 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


1. Successive opening of 1 leaf on 1 side

III Materials & Methods: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)


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)

III Materials & Methods:


DeconvolutionIII Materials & Methods:

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


opened MLC at every 10th interval from 1st to 80th pairs.

Nine MLC slit through the entire DAVID chamber

IV Results and Discussion: Alignment


IV Results and Discussion: Stability of DAVID System

• IMRT: prostate• Deviation of ±2% (+2%)

• VMAT: prostate• Deviation of ±1% (-1%)


IV Results/Discussion: Stability of DAVID System

• IMRT: prostate• Deviation of ±2% (+2%)

• VMAT: H&N• Deviation of <0.5%


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


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


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


Surface dose Increase with increase in field sizeIncrease with increase in energyIncrease with decrease in SSD



Pronounce withdecrease in SSD and, increase in photon energy andincrease 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



Deconvolution test by iteration method

IV Results/Discussion: Deconvolution


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



IV Results/Discussion:

Deconvoluted slit signal at 40th and 65th wire

Deconvolution test


10x10cm fields before and after deconvolution

Single arc prostate plan before and after deconvolution (3 mm )

IV Results/Discussion: Deconvolution with DAVID software.


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.

IV Results/Discussion: Enhance sensitivity after deconvolution


before deconvolution after deconvolution H&N 6mm error..


False alarm/warning effect before deconvolution.

The effect is eliminated after deconvolution


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


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



Max Dose 75.99GyMax Dose 57.27Gc

IV Results/Discussion: Artificial MLC bang shift Error 2 cm


0 8 10 12 14 16 18 200

1

2

3

4

5

6

7

8dose in-crease / Gy

maximal deviation / %

shift / mm

Undetectable error! Design of the chamber

DAVID: Gap-Shift (prostate with OAR: rectum back wall) IV Results/Discussion


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• DD-Deconvoluted data�• ED- Electrical data



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



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.



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'



VMAT and IMRT plans sessions



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


V Conclusion

Single MLC bank shift error Maximum and total deviation to be analysedDeconvolution 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


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


Thank you for your attention


Additional Slides

WG Medical Radiation Physics, Pius-Hospital and Carl von Ossietzky University, Oldenburg

Description of the Elekta synergy DAVID160 system

50

David58

David160

http://www.ptw.de/index.php?eID=tx_cms_showpic&file=uploads/pics/david_mit.jpg&width=800m&height=600m&bodyTag=%3Cbody%20style=%22margin:0;%20background:


• 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

mustafa_thesis presentation

Documents