p. pérez, v. galván, g. castellano & m. valente. to develop a simple and versatile dosimetric...

Developing and improving a scanning system for dosimetric

applications

P. Pérez, V. Galván, G. Castellano & M. Valente

Facultad de Matemática, Astronomía y Física Universidad Nacional de Córdoba

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To develop a simple and versatile dosimetric method capable of determining changes in matter (Xylenol Orange added Fricke gel) due to ionizing radiation.

To design and construct a preliminar 2D-scanning device (hardware and software) in order to quantify absorbed dose by means of visible light absorption.

To establish applicability limits of extremely simple devices.

Outlines & main goals

P. Pérez, V. Galván, G. Castellano & M. Valente - Developing and improving a scanning system for dosimetric applications

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Nowadays, radiotherapy is one of the most used techniques for the treatment of different pathologies.

The accuracy regarding the application of these treatments depends mainly on the dosimetric measurements of absorbed dose within irradiated tissues.

The development of novel dosimetric techniques has become great interests.

A dosimetric device capable of performing bidimensional dose mapping suggests a high pontenciality for clinical applications

Introduction


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Optical analysis: Standard Fricke solution presents maximum absorption at 294nm (ultraviolet).

X. O. Fricke gel has maximum absorption at 585nm (visible).

Spatial resolution can be obtained fixing the X.O. Fricke solution to a gel matrix.

X. O. Fricke gel dosimeters, which constitute tissue-equivalent media for radiotherapy purposes, are capable of providing real 3D dose mapping.

Introduction


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Boltzmann Radiation Transport Equation

Coherent and parallel beam

ma>>ms

Theoretical Background

drIrIrIntc

ssa

,4

ˆ1


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Contiuous (stable) light source

Then,

Wich solution is the Lambert-Beer eq.

zI

dsenzIzIdrIrI

a

sa

ssa

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,4

z

zI

kz

zI

t

zI

crIn

tc

,0,0

ˆ1ˆ

1

zIdz

zdIa

zIaeIzI 0



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From L-B

Light absorption law

z≈Cd, then

And the Optical Density Differences (ΔOD) become:

zI

zI

0

ln

dCA

zA

d

a

I

IOD 10log



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Dosimeter preparation: Chemical composition (150ml final volume)

Protocol

Materials and methods

Comp. H2O G. H2O F. G.P.S. X.O. Sulf. Fe+2 H2SO 4

100ml 50ml 47ml 3,0389g 0,0125g 0,0196g 1,39ml


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Experimental Set Up: Device description for 1D acquisition



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Experimental Set Up: Device description for 2D acquisition

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Developed software for 1D acquisition:

Get image Get red channel

Calculate XCM Calculate 2° moment

Calculate s(Rm)

Get ROI

Get definitive image

Calculate VM on new ROI and new s



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Set up for dose mapping:

Conventional X-Ray tube

W anode. Mo cathode. V=40kV. 40mA

In-home X-Y shifter + dedicated control software

LINAC Varian 6/100



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Measurement, data processing and some results. Experimental Set Up characterization. Preliminary tests.

Reproducibility and stability of the source-dosimeter-camera system for 1D acquisition

Results:Stability: I=25,6+0,6 Reproducibility : I=26,7+0,4Discrepancies between different modalities lees than

4%Software validation by means of testing with

Gaussian function Performance for Gaussian function center recognition Examples of test distributions


(ic,jc) y s escrito (ic,jc) calculado

(50,25) y 10 (50,25)

(25,50) y 10 (25,50)

(50,50) y 5 (50,50)

(75,75) y 5 (75,75)

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Software validation 2 Mean Value (VM) a priori well-knownfunctions

Constant I píxel to píxel (VM) Variable I pixel to pixel (ROI)Results:

Fixed I: VM=1, s=0 Variable I:


Measurement, data processing and some results. Software validation

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Characterization of the developed device for optical analysis

Results: Serie 1: DOD=0,16+0,01. Error: 6,7%. Serie 2: characteristic curve “exposition time-response”


Measurement, data processing and some results. On X-Ray Tube

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Two elaboration series to be irradiated by means of CLINAC: characteristic curve of “dose-response” and “depth dose” (to compare with standard dosimetric system -ionization chamber).

One elaboration serie to be irradiated by means of X-Ray tube and repeat the experience to obtain characteristic curve “exposition-response”


Measurement, data processing and some results. Preliminary tests on CLINAC

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Irradiated samples

In-depth dose and comparison with ionization

chamber

Preliminary measurementsOn CLINAC – in-depth dose


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Preliminary validations for 2D acquisition system


Adjusting camera focus

System sensor calibration by means of well-

known reference sample

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Preliminary measurementsOn 2D acquisition system


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Preliminary measurementsOn 2D acquisition system


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Discussion, conclusions and outlook

Software validation for 1D acquisition It has been established the consistency for the realization of the diferent steps

relative to communication with hardware, signal acquisition and data interpretation and subsequent processing.

Response of the integral system (source-dosimeter-software) It was possible to stablish the REPRODUCIBILITY (non-distinguishable), stability

(less than 1.6%) and repetitivity (less than 2.4%). The reults show clearly linearity region, as it was expected, even with supra

and sub linear zones. Response curves, which have been found to be indistinguishable,

demonstrated the reproducibility of the whole dosimeter elaboration procedure.

In addittion Device dimensions (20cm side box) allow its portability, therefore improving its

comfortability for CLINAC measurements. Preliminary PDD determination, as compared with ionization chamber, show

hopeful results.


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Discussion, conclusions and outlookSome considerations for 2D aquisitions

The development of the 2D dose acquisition system should be based on the same theoretical background.

Disadvantages: need of a quite parallel incident beam (hard to assess). The lack of it may produce a unstable system.

To decide Approach 1: To image the sample (mounted on the X-Y positioning system) at each

mechanical step and to reconstruct (integrate) whole sample image at the end of the process.

Approach 2: To image the sample at each mechanical step acquiring signal analogue to the 1D case, i.e. to insert a collimation system along with automatized image processing algorithms.

In addittion Device dimensions (40x30x20 cm3) allow its portability, therefore improving its

comfortability for clinic measurements. Preliminary measurements can determine de irradiated zone but have some problems with

the border. In view of obtained results, the overall performance of the system suggests its feasibility


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Main conclussions It was demonstrated that, even considering extremely simplcity of

the the device component, it is possible to develop suitable systems capable of determining absorbed dose by means of visible light transmittance measurements, particularly with radiochromic dosimeters.

Perspectives A lot of future improvements are been considered for the developed

device regarding its applicability limits and accuracy. Specially, a better detector quality may improve significantly the overall performance.

Several techniques devoted to 1D and 2D dose mapping are under investigation.

Discussion, conclusions and future lines


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Thanks

p. pérez, v. galván, g. castellano & m. valente. to develop a simple and versatile dosimetric...

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