Fricke gel dosimeters for the measurement of the anisotropy function of a HDR Ir-192 brachytherapy source Mauro Carrara 1, Stefano Tomatis 1, Giancarlo Zonca 1, Grazia Gambarini 2,3, Giacomo Bartesaghi 2,3, Chiara Tenconi 2, Annamaria Cerrotta 1, Carlo Fallai 1, 1 Fondazione IRCCS Istituto 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.

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 Fe 2+ into ferric ions Fe 3+ and the complex XO-Fe 3+ causes visible light absorption around 585nm, with yield proportional to the absorbed dose. Fe 2+ Fe 3+ RADIATION Fricke gel layer dosimetry (FGLD)

Optical imaging is performed by means of visible-light transmittance analysis. Transmitted light images are detected with a CCD camera. CCD Camera Gel-layer Plane uniform light source Computer FGLD image acquisition system

with Δ(OD) related to the absorbed dose D Δ(OD) = (OD) ir – (OD) ni = log 10 (I ni / I ir ) (OD) ni = -log 10 (I 0 / I ni ) I0I0 I ni Before irradiation (ni) I0I0 I ir After irradiation (ir) (OD) ir = -log 10 (I 0 / I ir ) 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- rates higher than 400 cGy/min FGLD characterisation

At doses lower than 400 cGy, the  (OD) response to the dose is not linear At doses lower than 400 cGy, the  (OD) response to the dose is not linear At doses higher than 2800 cGy and at dose-rates higher than 400 cGy/min, the  OD) response to the dose saturates At doses higher than 2800 cGy and at dose-rates higher than 400 cGy/min, the  OD) response to the dose saturates The dose response is independent on the energy over most of the adopted energy range The dose response is independent on the energy over most of the adopted energy range Each single dosimeter may require a specific calibration Each single dosimeter may require a specific calibration FGLD characterisation

FGLD calibration procedure I.II cGy k I. irradiation: elimination of the non-linear behaviour at low doses II. irradiation: determination of the sensitivity factor k A double 400cGy pre-irradiation of each FGLD resulted to be the optimal procedure for calibration. Applying this procedure, the obtained calibration curves were straight lines crossing the origin. D [cGy]

ANISOTROPY FUNCTION MEASUREMENT

Irradiation set-up: FGLD Tissue-equivalent phantom x y 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