Fricke gel dosimeters for the measurement of the anisotropy function of a HDR Ir-192

brachytherapy source

Mauro Carrara1, Stefano Tomatis1, Giancarlo Zonca1, Grazia Gambarini2,3, Giacomo Bartesaghi2,3, Chiara Tenconi2, Annamaria Cerrotta1, Carlo

Fallai1,

1 Fondazione IRCCSIstituto Nazionale dei Tumori di Milano

2 Dipartimento di Fisica,Università degli Studi di Milano

3 Istituto Nazionale di Fisica Nucleare, Milano

Brachytherapy (from the Greek brachios, meaning “short”) is a form of radiotherapy where one or more sealed radioactive sources are placed inside or next to the area requiring treatment.

Brachytherapy

HDR-Brachytherapy

Ir-192 source (initial activity: 10Ci)

In high dose rate (HDR) brachytherapy a single sealed source (usually Ir-192) is adopted to deliver radiation to the target with a dose-rate of at least 12 Gy/h.

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rFrgrG

rGSrD K

The anisotropy function

Treatment planning systems (TPS) are adopted in clinical practice to optimize source dwell times and positions inside the catheters, with the aim of conforming the prescribed dose to the target volume.

TPS dose calculation algorithm:

The anisotropy function

The anisotropy function F(r,θ) has been recommended to account for the dose distribution anisotropy that results adopting sealed sources. This function is implemented in the AAPM dose calculation formalism and is widely adopted by the currently available treatment planning systems.

Radiation propagation is anisotropic due to:

• source self-absorbance

• absorbance by the capsule

F(r,θ)

To develop and verify a method based on Fricke gel layer dosimetry for the characterization of the anisotropy function F(r,θ) of an Ir-192 source.

Purpose of the work

Fricke gels are radiochromic tissue-equivalent dosimeters that can be established in form of layers. Fricke gel layer dosimeters FGLDs are prepared in our laboratory by infusing a ferrous sulphate solution and the metal-ion indicator Xylenol Orange (XO) in a tissue-equivalent gel matrix.

Exposure to ionizing radiation of a FGLD produces a conversion of ferrous ions Fe2+ into ferric ions Fe3+ and the complex XO-Fe3+ causes visible light absorption around 585nm, with yield proportional to the absorbed dose.

FeFe2+2+ FeFe3+3+ RADIATION

Fricke gel layer dosimetry (FGLD)

Optical imaging is Optical imaging is performed by means of performed by means of visible-light visible-light transmittance analysis.transmittance analysis.

Transmitted light Transmitted light images are detected images are detected with a CCD camera.with a CCD camera.

CCD CCD Camera Camera

Gel-Gel-layerlayer

Plane Plane uniform light uniform light sourcesource

ComputerComputer

FGLD image acquisition system

with Δ(OD) related to the absorbed dose D

Δ(OD) = (OD)ir – (OD)ni = log10 (Ini / Iir)

(OD)ni = -log10 (I0 / Ini)

I0 Ini

Before irradiation (ni)

I0 Iir

After irradiation (ir)

(OD)ir = -log10 (I0 / Iir)

FGLD analysis

FGLD CHARACTERISATION

tissue-equivalent phantom

Catheter

FGLD

Irradiation set-up:

FGLD characterisation

Source

Lack of linearity at doses lower than 400 cGy

Saturation at doses higher

than 2800 cGy

FGLD characterisation

Response saturation for dose-Response saturation for dose-

rates higher than rates higher than 400 cGy/min400 cGy/min

FGLD characterisation

• At doses lower than 400 cGy, the At doses lower than 400 cGy, the (OD) response to the dose is not linear(OD) response to the dose is not linear

• At doses higher than 2800 cGy and at dose-rates higher than 400 At doses higher than 2800 cGy and at dose-rates higher than 400

cGy/min, the cGy/min, the OD) response to the dose saturatesOD) response to the dose saturates

• The dose response is independent on the energy over most of the The dose response is independent on the energy over most of the

adopted energy rangeadopted energy range

• Each single dosimeter may require a specific calibrationEach single dosimeter may require a specific calibration

FGLD characterisation

FGLD calibration procedure

I. II.

400 800

cGyk

I.I. irradiation: elimination of irradiation: elimination of the non-linear behaviour at the non-linear behaviour at low doses low doses II. II. irradiation: determination irradiation: determination of the sensitivity factor k of the sensitivity factor k

A double 400cGy pre-irradiation of each FGLD A double 400cGy pre-irradiation of each FGLD resulted to be the optimal procedure for resulted to be the optimal procedure for calibration. Applying this procedure, the calibration. Applying this procedure, the obtained calibration curves were straight lines obtained calibration curves were straight lines crossing the origin. crossing the origin.

D [cGy]

ANISOTROPY FUNCTION MEASUREMENT

Irradiation set-up:

FGLDFGLD

Tissue-equivalent phantom

xy

z

Ir-192 source

A series of measurements of the same irradiation set-up composed of a

tissue-equivalent phantom and a FGLD with a built-in plastic catheter were

performed. The plastic catheter permitted us to convey the source directly

inside the dosimeter.

Anisotropy function measurement

The delivered dose was of 1500cGy at 10mm distance from the source. Images of the irradiated dosimeters were acquired and elaborated with a dedicated software developed in Matlab®.

Anisotropy function measurement

The measured anisotropy function at radial distances of 15mm and 20mm

at angles between 15° and 165°:

Anisotropy function measurement

The measured anisotropy function at radial distances of 25mm and 30mm

at angles between 15° and 165°:

Anisotropy function measurement

The measured anisotropy function at radial distances of 15mm and 20mm

at angles between 15° and 165°:

Anisotropy function measurement

The measured anisotropy function at radial distances of 25mm and 30mm

at angles between 15° and 165°:

Anisotropy function measurement

The measured anisotropy function at radial distances of 35mm, 40mm,

45mm and 50mm at angles between 15° and 165°:

Anisotropy function measurement

At short distances from the source, percentage differences between tabulated and measured data are almost always smaller than 3%.

At increasing distances, data become more scattered due to the low doses delivered to the dosimeter.

Measurements with a higher irradiation time will be performed to achiever better accuracy at higher distances from the source

A FGLD results to be an accurate tool for the Ir-192 anisotropy function measurement. This instrument could be easily adopted in QA protocols to verify the anisotropy function for each newly installed Ir-192 source.

Discussion and Conclusions

Thank you for your attention

Vancouver Island 2009