Test of the proposed methodTest of the proposed method

IntroductionIntroduction

CCDController C

CD

Illuminator

gel

Filter 585nm

Assembling the phantom before its irradiation.

The phantom, ready to be irradiated at a 60Co unit.

Phantom for test measurementsPhantom for test measurementsThis phantom offers the opportunity to select the number of gel dosimeter layers to be inserted.Both phantom’s component (poystirene) and gel-dosimeter matrix have good tissue-equivalence to high energetic X-rays.

The developed softwareThe developed softwarefor dose distributions’ renderingfor dose distributions’ rendering

A suitable software has been developed, able to properly process the acquired dosimeters’ images to get the interactive rendering of dose profiles, surfaces and volumes, as well as isodose curves.

Examples of graphical user interfaces:

1. Depth dose profiles 1. Depth dose profiles Dose reliability has been tested by inter-comparing the obtained depth dose profiles with those measured with a cylindrical ionization chamber (Farmer, 0.6cc), in the same field configuration.The good agreement between results is evident.

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Dos

e (G

y)

Ionisation Chamber

Gel

60Co Unit

2. Dose distributions’ imaging2. Dose distributions’ imaging

In the last decade, technological improvements of radiotherapy (RT) hardware and software have been significant and consequently the use and importance of RT in cancer treatment have increased greatly.A fundamental advance has been the development of external beam techniques aimed at dose delivery that is highly localized on the tumour volume, sparing at the same time most of the surrounding healthy tissues. These techniques include conformal RT as well as intensity modulated RT (IMRT).

Experimental 3D rendering of in-phantom absorbed dose, with high reliability and good spatial resolution.

In a tissue-equivalent gel matrix, a ferrous sulphate solution and the metal-ion indicator Xylenol Orange (XO) are infused. Ionizing radiation causes a conversion of ferrous ions Fe2+ into ferric ions Fe3+.

Proposed methodProposed method

RADIATION Fe2+ Fe3+

The complex of XO with Fe3+ produces visible light absorption around 585nm, with yield proportional to the absorbed dose. Then, this system (gel-dosimeter) acts as a continuum dosimeter.

Grey-level (GL) images of light transmittance at 585nm are detected by a CCD camera, before and after the irradiation of the dosimeters. With the properly developed software, the stored images are propcessed with a semi-automatical procedure (recognition and registration through reference points; filtering and noise removal; artefacts removal; conversion to dose matrices…), until 3D dose distributions are obtained.The instrumentation for the optical imaging of gel dosimeters is transportable, and can be set up close to the radiation source, in order to perform image detection short time before and after gel dosimeter exposure.

Gel dosimeters are in form of layers of convenient shape and thickness (1-3 mm). For the analysis, they are placed on a plane light source near a grey-level calibration standard.

Planned dose distributions for a patient suffering prostate

cancer.

These methodological improvements require corresponding improvements in the dosimetry methods, in order to ensure that the values calculated with computer treatment planning systems (TPS), adopted in the clincal praxis, agree with the delivered dose distributions.Considering that traditional dosimeters (i.e. ion chambers, films, diodes or thermoluminescent detectors) are not well suited to this task, an alternative technique is proposed.

GoalGoal

Some gel-dosimeters of different shapes and exposures

Variation of optical density between irradiated samples and reference sample

The method’s reliability has been verified comparing the obtained dose distributions with those measured by means of ionization chambers or calculated with Monte Carlo (MC) simulations (Penelope), adopting field geometries in which the last are reliable.

In two different experimental configurations (both with Varian Clinac 2100C, Eγ=18MV), dose profiles, dose surfaces, isodose curves and 3D isodose distributions have been obtained. For inter-comparisons, experimental results are reported together with treatment planning system (TPS) (Prowess 3D) calculations and with MC simulations (Penelope).

phantom3x2 field

gel layers12 3 4 5 6

phantom3x2 field

gel layers12 3 4 5 6

90°270°

Configuration 1 (C1):

Configuration 2 (C2):

ConclusionsConclusions

Dose profiles along beam direction (a) and orthogonal to it (b) [C1]

3D distribution of relative isodoses for

C1 (95% blue, 80% yellow, 40% red)

3D distribution of relative isodoses for C2 (95% blue, 80% yellow, 40% red)

Relative isodose curves obtained with a single gel-layer (a), TPS (b) and MC (c) (95% red, 90% blue, 85% green, 80% yellow, 60% light blue, 40% orange) [C2]

Dose surface obtained with a single gel-layer (a) and with MC simulation (b) [C2]

(a)

(b)

(b)(a)

Dose profiles along beam direction (a) and orthogonal to it (b) [C2]

(a) (b)

(a)

(b)

Two different depth dose profiles measured with a single gel-layer and with a ionization chamber

The proposed method allows reliable 3D imaging of absorbed dose with good spatial resolution. (c)

Congresso del Dipartimento di Fisica

Highlights in Physics 200511–14 October 2005, Dipartimento di Fisica, Università di Milano

A method for 3D imaging of absorbed dosein conformal radiotherapy

M. Carrara*, G. Gambarini *,†, S. Gay*,†, L. Pirola* and M. Valente*,†

* Dipartimento di Fisica, Università di Milano† INFN – Sezione di Milano

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se (

Gy)

FriXy Gel

Ionization chamber

Siemens Mevatron MX2

Eγ=6MV