Mauro Valente

Dose Imaging in radiotherapy photon fields with Fricke and Normoxic-polymer Gels

Gel dosimeters are integrating dosimeters, that enable dose verification in three dimensions. Optical analysis of gel dosimeters has demonstrated to be an available technique for imaging the absorbed in-phantom dose exposed to radiotherapy beams. The goal is to demonstrate the ability of gel dosimeters to achieve accurately and spatial resolution in dose mapping, also when high dose regions are produced by a complex three dimensional treatment planning. Two types of dosimeters are investigated, a Fricke gel (Fricke-Xylenol-orange- infused gel) and a normoxic-polymer gel (polyacrylamide gel). Dosimeter gel samples of different shapes were exposed to photon fields, at various energies. Transmittance images were taken by means of a CCD camera and a spectrophotometer. In phantom 3-D images were realised with both dosimeter gels. An irradiation, using a linear accelerator, was realised in order to validate the method and techniques. Central axis depth dose profiles of the phantom were extracted and compared with ionization chamber measurements. Tissue-equivalence and other properties for both gels were studied using Monte Carlo techniques. Off-axis profiles and three dimensional dose distribution were obtained by simulations and compared with experimental dose distributions.

Gel dosimeters as useful dose and thermal-fluence detectors in boron neutron capture therapy

The dosimetry method based on Fricke–Xylenol-Orange-infused gels in form of layers has shown noticeable potentiality for in-phantom or in-free-beam dose and thermal flux profiling and imaging in the high fluxes of thermal or epithermal neutrons utilised for boron neutron capture therapy (BNCT). Gel-dosimeters in form of layers give the possibility not only of obtaining spatial dose distributions but also of achieving measurements of each dose contribution in neutron fields. The discrimination of the various dose components is achieved by means of pixel-to-pixel manipulations of pairs of images obtained with gel-dosimeters having different isotopic composition. It is possible to place large dosimeters, detecting in such a way large dose images, because the layer geometry of dosimeters avoids sensitive variation of neutron transport due to the gel isotopic composition. Some results obtained after the last improvements of the method are reported.

Characterization of ferric ions diffusion in Fricke gel dosimeters by using inverse problem techniques

Diffusion of ferric ions in ferrous sulfate (Fricke) gels represents one of the main drawbacks of some radiation detectors, such as Fricke gel dosimeters. In practice, this disadvantage can be overcome by prompt dosimeter analysis, and constraining strongly the time between irradiation and analysis, implementing special dedicated protocols aimed at minimizing signal blurring due to diffusion effects. This work presents a novel analytic modeling and numerical calculation approach of diffusion coefficients in Fricke gel radiation sensitive materials. Samples are optically analyzed by means of visible light transmission measurements by capturing images with a charge-coupled device camera provided with a monochromatic filter corresponding to the XO-infused Fricke solution absorbance peak. Dose distributions in Fricke gels are suitably delivered by assessing specific initial conditions further studied by periodical sample image acquisitions. Diffusion coefficient calculations were performed using a set of computational algorithms based on inverse problem formulation. Although 1D approaches to the diffusion equation might provide estimations of the diffusion coefficient, it should be calculated in the 2D framework due to the intrinsic bi-dimensional characteristics of Fricke gel layers here considered as radiation dosimeters. Thus a suitable 2D diffusion model capable of determining diffusion coefficients was developed by fitting the obtained algorithm numerical solutions with the corresponding experimental data. Comparisons were performed by introducing an appropriate functional in order to analyze both experimental and numerical values. Solutions to the second-order diffusion equation are calculated in the framework of a dedicated method that incorporates finite element method. Moreover, optimized solutions can be attained by gradient-type minimization algorithms. Knowledge about diffusion coefficient for a Fricke gel radiation detector is helpful in accounting for effects regarding elapsed time between dosimeter irradiation and further analysis. Hence, corrections might be included in standard dependence of optical density differences and actual, non-diffused, absorbed dose distributions. The obtained values for ferric ion diffusion coefficient are around 0.65 mm2 h−1, being in good agreement with previous works corresponding to similar Fricke gel dosimeter compositions. Therefore, more accurate 2D and 3D dose mapping might be attained, thus constituting valuable improvements in Fricke gel dosimetry, and parallely a high precision method of diffusion modeling and calculation has been developed.

FRICKE GEL DOSIMETER TISSUE-EQUIVALENCE: A MONTE CARLO STUDY

Gel dosimetry has proved to be a valuable technique for absorbed dose distribution measurements in radiotherapy. FriXy-gel dosimeters consist of Fricke (ferrous sulphate) solution infused with xylenol orange. The solution is incorporated to a gel matrix in order to fix it to a solid structure allowing good spatial resolution and is imaged with a transportable optical system, measuring visible light transmittance before and after irradiation. This paper presents an evaluation of total photon mass attenuation coefficients at energies in the range of 50 keV-10MeV for the radiochromic FriXy gel dosimeter sensitive material. Mass attenuation coefficient estimations have been performed by means of Monte Carlo (PENELOPE) simulations. These calculations have been carried out for the FriXy gel sensitive material as well as for soft tissue (ICRU) and pure liquid water; a comparison of the obtained data shows good agreement between the different materials.

Development and characterization of a microCT facility

This work reports the design, construction, characterization and application of a novel X-ray imaging beamline at Laboratorio de Investigación e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X - LIIFAMIRx - University of Cordoba and Institute of Physics E. Gaviola - CONICET, Argentina. This development is the first phase in the construction of the integral facility for combining different imaging modalities, like absorption contrast images by primary and scattering contributions, high-resolution micro-tomography, elastic scattering and X-ray fluorescence scanning for chemical composition and surface characterizations. The progress and results here reported concern mainly to the micro-tomography beam-line. This technique is already operative and it was used for several academic researches, application studies and services. The obtained results for the characterization of organic and inorganic samples demonstrated the feasibility and reliability of the developed facility. Accordingly, this work reports specific characteristics about its design, construction, and operation supporting its employment in a wide range of especial applications that might not be accomplished by other available techniques. Moreover, some application studies, mainly focused on biological samples, are presented.

Dosimetry for Beta-Emitter Radionuclides by Means of Monte Carlo Simulations

Nowadays, there are interests as well as active investigations devoted to the study and application of radiolabeled molecules able to selectively target and irradiate tumoral cells during nuclear medicine procedures. With this kind of pharmaceuticals, spatial activity distribution with extremely non-uniform characteristics may be assessed in patients. Actually, this feature constitutes precisely themain advantage in view ofmaximizing the discrimination between affected and healthy tissue. The mentioned situation constitutes the main motivation for the present work. In this sense, the chapter is focused on nuclear medicine dosimetry pointing out the main features about how to implement Monte Carlo (MC) approaches to this aim. Nowadays, from a general point of view, therapies with radiopharmaceuticals using beta-emitter radionuclides are growing significantly and very fast. Beta-emitters can be emitters of β− or β+ radiation. Commonly, β+ emitters, like 18F are used for imaging techniques, whereas β− are mainly used with therapeutic purposes, to deliver high dose rate on tumors. Therefore, β− emitters are usually those of more interest for dosimetry. During nuclear medicine procedures, radiopharmaceutical activity distribution may be determined by means of different modalities. Nowadays it is mainly assessed using imaging techniques but otherwise it is also possible to infer it [Stabin (2008)]. This information is then incorporated in the treatment planning system in order to obtain an estimation of the dose distribution. More specifically, patient-specific dose distribution owing to alpha, beta and/or gamma emitters can be calculated starting from activity distribution by means of either direct MC simulation or analytical methods. On the other hand, patient-specific dosimetry requires anatomical information, which shall be further considered as input for establishing mass distribution during MC computations. Patient anatomical information can be suitably extracted from typical non-invasive imaging techniques, like computed tomography (CT) or magnetic resonance imaging (MRI). Many studies have been performed by means of MC applications in Nuclear Medicine up today, both in the imaging field, and regarding dosimetry calculations [F. Botta & Valente (2011), Zubal & Harrel (1992), H. Yoriyaz & dos Santos (2001), M. Ljungberg & Strand (2002)]. I troduction 11

Optimization of the sensitivity/doses relationship for a bench-top EDXRF system used for in vivo quantification of gold nanoparticles

The present work is devoted to optimizing the sensitivity-doses relationship of a bench-top EDXRF system, with the aim of achieving a detection limit of 0.010mg/ml of gold nanoparticles in tumor tissue (clinical values expected), for doses below 10mGy (value fixed for in vivo application). Tumor phantoms of 0.3cm3 made of a suspension of gold nanoparticles (15nm AurovistTM, Nanoprobes Inc.) were studied at depths of 0-4mm in a tissue equivalent cylindrical phantom. The optimization process was implemented configuring several tube voltages and aluminum filters, to obtain non-symmetrical narrow spectra with fixed FWHM of 5keV and centered among the 11.2-20.3keV. The used statistical figure of merit was the obtained sensitivity (with each spectrum at each depth) weighted by the delivered surface doses. The detection limit of the system was determined measuring several gold nanoparticles concentrations ranging from 0.0010 to 5.0mg/ml and a blank sample into tumor phantoms, considering a statistical fluctuation within 95% of confidence. The results show the possibility of obtaining a detection limit for gold nanoparticles concentrations around 0.010mg/ml for surface tumor phantoms requiring doses around 2mGy.

Internal dosimetry for alpha emitters radiopharmaceuticals in biological tissue studied with the FLUKA code

Nuclear medicine clinical practices for neoplasic disease diagnose and treatment are based on the incorporation of α , β and γ radiotracers and radiopharmaceuticals, which might be associated with potential damage. Thus, being necessary accurate dosimetry strategies. In vivo absorbed dose appears as an ideal solution. However, its implementation in clinics does not attain enough reliability. In this sense, different approaches were proposed for internal dosimetry calculations. This work presents a novel analytical-numerical approach for internal dosimetry purposes. Dedicated Monte Carlo simulations were performed by subroutines adapted from the FLUKA code. In-water EDK were evaluated at different photon energies and some typical γ-emitters radiopharmaceuticals; whereas DPK were obtained for both αand β emitters. Additionally, EDK and DPK were calculated for several biological tissues.

Feasibility of dose enhancement assessment: Preliminary results by means of Gd-infused polymer gel dosimeter and Monte Carlo study

This work reports the experimental development of an integral Gd-infused dosimeter suitable for Gd dose enhancement assessment along with Monte Carlo simulations applied to determine the dose enhancement by radioactive and X-ray sources of interest in conventional and electronic brachytherapy. In this context, capability to elaborate a stable and reliable Gd-infused dosimeter was the first goal aimed at direct and accurate measurements of dose enhancement due to Gd presence. Dose-response was characterized for standard and Gd-infused PAGAT polymer gel dosimeters by means of optical transmission/absorbance. The developed Gd-infused PAGAT dosimeters demonstrated to be stable presenting similar dose-response as standard PAGAT within a linear trend up to 13 Gy along with good post-irradiation readout stability verified at 24 and 48 h. Additionally, dose enhancement was evaluated for Gd-infused PAGAT dosimeters by means of Monte Carlo (PENELOPE) simulations considering scenarios for isotopic and X-ray generator sources. The obtained results demonstrated the feasibility of obtaining a maximum enhancement around of (14 ± 1)% for 192Ir source and an average enhancement of (70 ± 13)% for 241Am. However, dose enhancement up to (267 ± 18)% may be achieved if suitable filtering is added to the 241Am source. On the other hand, optimized X-ray spectra may attain dose enhancements up to (253 ± 22) %, which constitutes a promising future alternative for replacing radioactive sources by implementing electronic brachytherapy achieving high dose levels.

Integration of Fricke gel dosimetry with Ag nanoparticles for experimental dose enhancement determination in theranostics

The use and implementation of nanoparticles in medicine has grown exponentially in the last twenty years. Their main applications include drug delivery, theranostics, tissue engineering and magneto function. Dosimetry techniques can take advantage of inorganic nanoparticles properties and their combination with gel dosimetry techniques could be used as a first step for their later inclusion in radio-diagnostics or radiotherapy treatments. The present study presents preliminary results of properly synthesized and purified silver nanoparticles integration with Fricke gel dosimeters. Used nanoparticles presented mean sizes ranging from 2 to 20 nm, with a lognormal distribution. Xylenol orange concentration in Fricke gel dosimeter was adjust in order to allow samples optical readout, accounting nanoparticles plasmon. Dose enhancement was assessed irradiating dosimeters setting X-ray beams energies below and above silver K-edge. Monte Carlo simulations were used to estimate the dose enhancement in the experiments and compare with the trend obtained in the experimental results.