Temperature behavior of radiochromic poly(vinyl-alcohol)–glutaraldehyde Fricke gel dosimeters in practice

The use of synthetic gel matrices prepared with poly(vinyl-alcohol) (PVA) cross-linked by glutaraldehyde (GTA) contributed to enhance the interest toward radiochromic Fricke gel (FG) dosimeters. As it occurs in several chemical dosimeters, the response of PVA-GTA Fricke gels could be affected by temperature. Aim of this work is to study the dependence of the dosimetric properties of PVA-GTA Fricke gel dosimeters both on the irradiation temperature and on temperature changes possibly occurring between the irradiation and readout phases. Such effects were investigated by means of magnetic resonance imaging (MRI) and optical absorbance (OA) measurements. The results did not reveal any significant dependence of the sensitivity of the dosimeters on the irradiation temperature in the investigated interval 20\ub0C-35\ub0C. By contrast, auto-oxidation phenomena confirmed to be a critical aspect for FG dosimeters, also in case of use of PVA matrix. The extent such phenomena, that might impair the accuracy of dose estimations, proved to critically depend on the temperature at which FG dosimeters are subjected before and after irradiation, as well as on the duration of possible thermal-stress

CHARACTERIZATION OF THE BNCT EPITHERMAL COLUMN OF THE FAST REACTOR TAPIRO (ENEA) AND DOSE MEASUREMENTS IN PHANTOM UTILISING NOT-CONVENTIONAL DETECTION

The epithermal column of TAPIRO reactor has been characterized and in-phantom dose has been imaged, with the purpose of determining parameters and data whose knowledge will be of main importance for future experimentation regarding boron neutron capture therapy (BNCT). In-phantom measurements have been carried out mainly utilizing a recently developed method for absorbed dose imaging, based on gel-dosimeters. Gel-dosimeters have revealed to give significant support to thermal or epithermal neutron dosimetry: in fact, the modality of energy release in gel-dosimeters is very similar to that in tissue, and with proper adjustment of the isotopic composition of gel matrix, the various dose components are separated

dosimetry methods in boron neutron capture therapy

Dosimetry studies have been carried out at thermal and epithermal columns of LVR- 15 research reactor for investigating the spatial distribution of gamma dose, fast neutron dose and thermal neutron fluence. Two different dosimetry methods, both based on solid state detectors, have been studied and applied and the accuracy and consistency of the results have been inspected. One method is based on Fricke gel dosimeters that are dilute water solutions and have good tissue equivalence for neutrons and also for all the secondary radiations produced by neutron interactions in tissue or water phantoms. Fricke gel dosimeters give the possibility of separating the various dose contributions, i.e. the gamma dose, the fast neutron dose and the dose due to charged particles generated during thermal neutron reactions by isotopes having high cross section, like 10B. From this last dose, thermal neutron fluence can be obtained by means of the kerma factor. The second method is based on thermoluminescence dosimeters. In particular, the developed method draw advantage from the different heights of the peaks of the glow curve of such phosphors when irradiated with photons or with thermal neutrons. The results show that satisfactory results can be obtained with simple methods, in spite of the complexity of the subject. However, the more suitable dosimeters and principally their utilization and analysis modalities are different for the various neutron beams, mainly depending on the relative intensities of the three components of the neutron field, in particular are different for thermal and epithermal columns

Effects of the pre-irradiation storage procedure on the dose response of a Fricke xylenol orange gel dosimeter

The Fricke xylenol orange (FX) gel system is a chemical dosimeter characterized by good sensitivity, linear dose response, tissue equivalence, no toxicity, easy preparation, reproducibility and low cost. Thanks to the presence of the gelatinous matrix, the system is particularly suitable to perform reliable 3D mapping of the absorbed dose spatial distribution via magnetic resonance imaging (MRI) or optical techniques. The aim of this work is to study in a systematic way the influence of the pre-irradiation storage procedure upon sensitivity, dose response stability and lifetime of use of a FX gel system made with gelatin from porcine skin subjected to homogeneous irradiation. For this purpose, different pre-irradiation storage procedures, in terms of temperature and duration of each storage step, were investigated. In order to evaluate the dose response stability, the optical analyses of the samples were performed up to 6 hours after irradiation. Moreover, the samples were irradiated at time intervals of 24 hours for up to 7 days after preparation in order to evaluate the system lifetime of use. Regardless of their thermal and temporal life, the samples show linear dose responses in the investigated dose range (3-24 Gy) and an increase of sensitivity with the time elapsed between preparation and irradiation. Among the three pre-irradiation storage procedures considered here, a procedure that provides the best dose response stability and lifetime of use was identified and recommended for further use. The analyzed dosimetric system possesses good properties that make it promising for medical application, particularly concerning the evaluation of pre-treatment plan quality assurance within the conformational external beam radiotherap

Novel GTA-PVA Fricke gels for three-dimensional dose mapping in radiotherapy

One of the most recent and promising developments in radiotherapy dosimetry was the introduction of 3D radiation-sensitive gels. These gels present tissue equivalent composition and density, so they also serve as phantoms, and their response is largely independent of radiation quality and dose rate. Some gels are infused with ferrous sulfate and rely on the radiation-induced oxidation of ferrous ions to ferric ions (Fricke-gels). These formulations suffer from spontaneous-oxidation and diffusion of ferric ions after irradiation; chelating agents such as xylenol-orange significantly reduces the latter. Other gel types consist of dispersed monomers, and rely on radiation-induced cross-linking; they form stable polymer structures, but they are typically affected by significant toxicity. In a large multicenter study, we developed and investigated new formulations of Fricke-gels based on poly-vinyl alcohol chemically cross-linked with glutaraldehyde. The formulation is safe and easy to manufacture, with a sensitivity of 0.073 Gy-1and a diffusion coefficient of 0.17 mm2/h, it arguably offers the best all-around performance of current Fricke-infused gels. The main original outcomes of the study are described in this work, while reference is made to separate reports for specific procedures and results

Optical absorbance properties of PVA-GTA Fricke gel dosimeters irradiated with 6 MV and 15 MV X-rays

Purpose. Quality assurance procedures required in the modern radiotherapy would greatly benefit by the development of tissue equivalent dosimeters able of rendering 3D dose profiles with high spatial resolution. In this scenario, Fricke gel (FG), consisting in gel matrices infused with Fricke ferrous sulphate solution combined with xylenol orange, could be good candidates, but some limits have prevented their introduction in clinical practice. Recently, new FG formulations based on gel matrices of poly-vinyl alcohol (PVA) cross-linked with glutaraldehyde (GTA) have shown important decrease of the limitations affecting FGs obtained with gelatin or agarose. Purpose of this study is the characterization of the optical absorbance (OA) and dosimetric properties of PVA-GTA FG dosimeters. Methods. PVA-GTA FG dosimeters were prepared using 9.1 w/w of Mowiol-PVA and 26.4 mM of GTA. The dosimeters, placed in spectrophotometry cuvettes (10 mm optical path), were uniformly irradiated using 6 and 15 MV X-rays generated by a Varian Clinac-2100. OA spectra were measured by means of an Agilent Cary-100 spectrophotometer. Dosimetric properties of PVA-GTA-FG dosimeters, such as reproducibility, dose\u2013response, sensitivity, dose-rate dependence and energy dependence, were studied. Furthermore, FG in form of layers (3 mm optical path) were irradiated producing steep dose gradients and used to investigate ferric ions diffusion phenomena by means of sequential light transmittance images. Results. The analysis of the OA spectra of the PVA-GTA-FG dosimeters in the wavelength interval of interest for dosimetry purposes (around 550\u2013600 nm) revealed an intra-batch reproducibility of the order of 1.5%. The OA proved to increase linearly with increasing dose, in the investigated interval 0\u201315 Gy, if the wavelength used for the analysis is properly chosen. The optical response of the PVA-GTFG dosimeters was independent of both energy and dose-rate. Furthermore, PVA-GTA matrix enables to achieve ferric ions diffusion rates approximatively 3 times lower than those observed in natural gel matrices. Conclusions. This study confirmed the interesting properties of PVA-GTA FG dosimeters and indicated the best parameters for their optical analysis. Currently, studies are in progress in order to optimize Magnetic Resonance Imaging procedures for the assessment of 3D dose distributions in PVA-GTA FG phantoms

Characterization of radiochromic poly(vinyl-alcohol)–glutaraldehyde Fricke gels for dosimetry in external x-ray radiation therapy

Quality assurance procedures required in modem radiotherapy would greatly benefit from the development of tissue equivalent dosimeters able to render three-dimensional dose profiles with a high spatial resolution. In this scenario, Fricke gel (FG) dosimeters could be good candidates, but some limitations have restricted their use in clinical practice. Recently, formulations based on gel matrices of poly(vinyl-alcohol) (PVA) chemically cross-linked with glutaraldehyde (GTA) have shown improvements compared to FGs with natural matrices.The purpose of this study is the characterization of the dosimetric properties of radiochromic PVA-GTA FG dosimeters by means of absorption spectroscopy measurements. Xylenol orange (XO) FG dosimeters, prepared in spectrophotometry cuvettes using 9.1 w/w of Mowiol (R)-PVA and 26.5mM of GTA, were uniformly irradiated with a Cs-137 source and with 6 and 15 MV x-rays generated by a medical linear accelerator. Ultraviolet-visible absorbance spectra collected at consecutive times post-irradiation showed that a time of approximately 15 min is sufficient to reach a stable absorbance value, indicating the achievement of a chemical equilibrium in the complexation processes between Fe3+ and XO. The analysis of the change of the absorbance spectra shape with the cumulated dose demonstrated that a linear dose-response curve of PVA-GTA FG dosimeters is obtained in the entire investigated dose interval of 0.5-15 Gy by properly choosing the wavelength used for the absorbance measurements. Furthermore, PVA-GTA FG dosimeters proved to be nearly water- and tissue-equivalent and characterized by a response independent of the energies and dose rates in the investigated intervals. These findings suggested that PVA-GTA FGs are promising tools for dosimetry applications in external x-ray radiation therapy