Kalin I. Penev

The effect of mixed dopants on the stability of Fricke gel dosimeters

Auto-oxidation and fast diffusion in Fricke gels are major drawbacks to wide-spread application of these gels in 3D dosimetry. Aiming to limit both processes, we used mixed dopants: the ferric-specific ligand xylenol orange with a ferrous-specific ligand (1,10-phenanthroline) and/or a bi-functional cross-linking agent (glyoxal). Markedly improved auto-oxidation stability was observed in the xylenol orange and phenanthroline doped gel at the expense of increased background absorbance and faster diffusion. Addition of glyoxal limited the diffusion rate and led to a partial bleaching of the gel. It is conceivable that these two new compositions may find useful practical application.

Multifactorial study and kinetics of signal development in ferrous–methylthymol blue–gelatin gel dosimeters

Purpose To develop and characterize a ferrous–methylthymol blue–gelatin gel dosimeter with low optical background and appropriate additives for reduced rate of auto‐oxidation and diffusion. Methods A mixed‐level multifactorial design of experiments was used to test the effects of the concentrations of sulfuric acid, 5‐nitro‐1,10‐phenanthroline (Nn), and glyoxal (Gx) on the background absorbance, dose sensitivity, and auto‐oxidation of the tested gel dosimeter. The dosimetric properties of the proposed ferrous–methylthymol blue–gelatin dosimeter, doped with Nn and Gx, were compared with the undoped formulation and with ferrous–xylenol orange–gelatin gel dosimeters. Irradiations were performed in both small‐scale cuvette samples and large 400‐mL bulk samples. In addition to that, a new kinetic model for the signal development postirradiation was derived based on chemical principles and used for comparison of the different formulations. Results The new formulation showed a reduced auto‐oxidation rate, while maintaining low background absorbance relative to the common ferrous–xylenol orange–gelatin gel dosimeter. Compared with undoped ferrous–xylenol orange or ferrous–methylthymol blue gels, the dose sensitivity of the new formulation is approximately 2 to 3 times lower, but remains clinically adequate. A previously unreported dose rate dependence of the dose sensitivity was observed, and a new kinetic model for the signal development postirradiation was used to investigate this effect. Similar dose rate dependences in gels containing either methylthymol blue or xylenol orange, with or without doping with Nn and Gx, were observed, suggesting that the low ferrous ammonium sulfate concentrations used in studied formulations were responsible for this effect. Conclusions A multifactorial design of experiments and a new kinetic model for the signal development postirradiation were successfully employed to optimize the composition and characterize the properties of a new ferrous–methylthymol blue–gelatin gel dosimeter doped with 5‐nitro‐1,10‐phenanthroline and glyoxal. Concrete recommendations were provided for precise dosimetry using the new formulation.

Characterization of a commercially-produced chemically stable Fricke gel dosimeter

The successful manufacturing of a modified Fricke gel with increased shelf life in a pilot scale (20-liter) commercial production facility is reported. The gels remained chemically stable and usable for over 30 days. Linear dose dependence was observed for cumulative doses up to 30 Gy. Dose-depth studies showed a good correlation between the experimental data and theoretical expectations, but the dose response appeared to be energy-dependent.

Gellan gum-based gels with tunable relaxation properties for MRI phantoms

OBJECT The research follows the analysis of gellan gum-based gels as novel MRI phantom material with the implementation of a design of experiments model to obtain tunable relaxation properties. MATERIALS AND METHODS Gellan gum gels doped with newly synthesized superparamagnetic iron oxide nanoparticles (SPIONs) and either MnCl2 or GdCl3 were prepared and scanned from 230 μT to 3 T. Nineteen gel samples were formulated with varying concentrations of contrast agents to determine the linear, quadratic, and interactive effects of the contrast agents by a central composite design of experiment. To inhibit microbial growth in the gels and to enable long-term use, methyl 4‑hydroxybenzoate (methylparaben) was utilized. RESULTS The model containing SPIONs and metal salts relaxivity was analyzed with ANOVA, and the resulting significant coefficients were tabulated. The mathematical model was able to accurately predict the intended relaxation property from the concentration of the contrast agent with adjusted R2 values > 0.97 for longitudinal (R1) relaxation rates and 0.87 for transverse (R2) relaxation rates. CONCLUSION The gel material maintained physical, chemical, and biological stability for at least four months and contained controllable relaxation properties while maintaining optical clarity.

Tetrazolium salt monomers for gel dosimetry I: Principles

Tetrazolium salts (TS) have been previously used for radiochromic dosimetry in solutions, films and three dimensional (3D) gelatine-based gels. However, widespread application for 3D dosimetry has not been achieved due to the required high concentrations and associated high costs of the TS dimer used in prior research. Through careful selection of TS monomer, sensitivity-enhancing additives and inert gel forming material, we report the preparation of a non-diffusing, chemically stable, 3D dosimeter with linear sensitivity between 0 and 80 Gy with submillimolar requirements for the active TS.

Tetrazolium salt monomers for gel dosimetry II: Dosimetric characterization of the ClearView™ 3D dosimeter

ClearView is a colourless radiochromic 3D dosimeter that is chemically and mechanically stable before and after irradiation. Upon irradiation, colour is generated from the radiochemical reduction of a tetrazolium salt suspended within a gellan gum gel. Here we present the dosimetric characterization of small and large volume samples (4 mL and 1 L, respectively) from three different batches of ClearView. The dose sensitivity was linear up to 80 Gy and constant within a batch but varied among batches between 3.4×10-3 and 4.1×10-3 Gy-1cm-1. Once generated, the radiation signal within the large samples remained unchanged for at least 58 days. Both electrons and photons produced the same signal at different energy levels; however, at dose rates below 400 cGy/min, a drop in sensitivity was observed. Overall, ClearView is best suited for relative dosimetry up to 80 Gy at high dose rates.