Polyvinyl alcohol-Fricke hydrogel and cryogel: two new gel dosimetry systems with low Fe3+ diffusion.

Two new Fricke dosimeter gel systems with low diffusion rates have been developed for 3D radiation dosimetry purposes. Both systems consist of a solution of 20% (by weight) polyvinyl alcohol (PVA) in a 50 mM H2SO4 solution with 0.4 mM ferrous ammonium sulphate and xylenol orange (FX). The difference in the two gels is the way that the gelation process was initiated: either by bringing the temperature to (a) +5 degrees C or (b) -20 degrees C before returning them to room temperature. These gels are termed 'hydrogel' and 'cryogel', respectively. The hydrogel is optically transparent, and can be used with either optical or MRI detection methods for dosimetric imaging. The cryogel is rubbery in texture but opaque, so its internal Fe3+ concentration can only be measured with MRI. The hydrogel's optical attenuation coefficient is linear (r2 = 0.99) with dose from 0 to 20 Gy with a sensitivity of 0.106 cm(-1) Gy(-1) (at 543 nm). In terms of MR relaxation rate, the dose response for both the hydrogel and cryogel was linear (r2 = 0.99) with a sensitivity of 0.020 s(-1) Gy(-1) (at 1.5 T). The Fe3+ diffusion coefficient (at 20 degrees C) was measured to be 0.14 mm2 h(-1), which is significantly lower than similar preparations reported for porcine gelatin or agarose. The PVA-FX gels can be stored for long periods of time before exposure to radiation, since the auto-oxidation rate was 10 times less than that of gelatin-Fricke recipes. The new gels developed in this work are a significant improvement on previous Fricke gel systems.

Optical CT reconstruction of 3D dose distributions using the ferrous-benzoic-xylenol (FBX) gel dosimeter.

In recent years, magnetic-resonance imaging of gelatin doped with the Fricke solution has been applied to the direct measurement of three-dimensional (3D) radiation dose distributions. However, the 3D dose distribution can also be imaged more economically and efficiently using the method of optical absorption computed tomography. This is accomplished by first preparing a gelatin matrix containing a radiochromic dye and mapping the radiation-induced local change in the optical absorption coefficient. Ferrous-Benzoic-Xylenol (FBX) was the dye of choice for this investigation. The complex formed by Fe3+ and xylenol orange exhibits a linear change in optical attenuation (cm-1) with radiation dose in the range between 0 and 1000 cGy, and the local concentration of this complex can be probed using a green laser light (lambda = 543.5 nm). An optical computed tomography (CT) scanner was constructed analogous to a first-generation x-ray CT scanner, using a He-Ne laser, photodiodes, and rotation-translation stages controlled by a personal computer. The optical CT scanner itself can reconstruct attenuation coefficients to a baseline accuracy of < 2% while yielding dose images accurate to within 5% when other uncertainties are taken into account. Optical tomography is complicated by the reflection and refraction of light rays in the phantom materials, producing a blind spot in the transmission profiles which, results in a significant dose artifact in the reconstructed images. In this report we develop corrections used to reduce this artifact and yield accurate dosimetric maps. We also report the chemical reaction kinetics, the dose sensitivity and spatial resolution (< 1 mm3) obtained by optical absorption computed tomography. The article concludes with sample dose distributions produced by "cross-field" 6 MV x-ray beams, including a radiosurgery example.

Determination of the cleavage specificity of Streptomyces griseus protease B in the presence of guanidinium chloride.

Streptomyces griseus protease B (SGPB), a Pronase enzyme, has been shown to be stable and active in the presence of 6.0 M guanidinium chloride (Siegel, S. et al. (1972) J. Biol. Chem. 247, 4155-4159). In order to determine the cleavage specificity of this unusual enzyme under denaturing conditions, 12 peptides of known amino acid sequence were hydrolyzed by SGPB in the absence and presence of 6.0 M guanidinium chloride. The new N-terminal amino acids produced by the action of SGPB were dansylated and quantitatively identified by reverse phase HPLC. The results indicate that SGPB retained its cleavage specificity for phenylalanyl, tyrosyl, tryptophanyl, and leucyl peptide bonds in the presence of 6.0 M guanidinium chloride. Of these peptide bonds, SGPB exhibited a greater cleavage preference for phenylalanyl and tryptophanyl bonds, which was relatively unaffected by the presence of the denaturant. The SGPB-catalyzed cleavages of the leucyl peptide bonds examined (Leu-Met, Leu-Arg, Leu-Val, Leu-Thr, and Leu-Ile) were substantially decreased under denaturing conditions, while Leu-Gly bond cleavage by SGPB was virtually unaffected by denaturant. The demonstrated predictability of the catalytic preference of this unusual protease for phenylalanyl, tyrosyl, tryptophanyl, and leucyl-glycine peptide bonds under denaturing conditions enhances its utility in the site-specific proteolysis of insoluble or otherwise proteolysis-resistant polypeptide substrates.