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