Mark K. Murphy

Evaluation of the new cesium‐131 seed for use in low‐energy x‐ray brachytherapy

Characterization measurements and calculations were performed on a new medical seed developed by IsoRay Inc. in Richland, Washington, that utilizes the short-lived isotope 131Cs. This model has recently received FDA 510(k) clearance. The objective of this work was to characterize the dosimetric properties of the new seed according to the AAPM Task Group 43 recommendations. Cesium-131 is a low-energy x-ray emitter, with the most prominent peaks in the 29 keV to 34 keV region. The intended application is brachytherapy for treating cancers in prostate, breast, head and neck, lung, and pancreas. The evaluations performed included air-kerma strength, radial dose function, anisotropy in phantom, half-life, energy spectra, and internal activity. The results indicate the CS-1 seeds have a dose-rate constant of 0.915 cGy hr(-1) U(-1) in water, dose penetration characteristics similar to 125I and 103Pd, anisotropy function values on the order of 0.71 at short distances and small angles, and an average anisotropy factor of 0.964. The overall dosimetric characteristics are similar to 125I and 103Pd seeds with the exception of half-life, which is 9.7 days, as compared to 17 days for 103Pd and 60 days for 125I. The shorter half-life may offer significant advantages in biological effectiveness.

A variable-energy electron microbeam: a unique modality for targeted low-LET radiation.

Abstract Sowa, M. B., Murphy, M. K., Miller, J. H., McDonald, J. C., Strom, D. J. and Kimmel, G. A. A Variable-Energy Electron Microbeam: A Unique Modality for Targeted Low-LET Radiation. Radiat. Res. 164, 695–700 (2005). We have designed and constructed a low-cost, variable-energy low-LET electron microbeam that uses energetic electrons to mimic radiation damage produced by γ and X rays. The microbeam can access lower regions of the LET spectrum, similar to conventional X-ray or 60Co γ-ray sources. The device has two operating modes, as a conventional microbeam targeting single cells or subpopulations of cells or as a pseudo broad-beam source allowing for direct comparison with conventional sources. By varying the incident electron energy, the target cells can be selectively exposed to different parts of the energetic electron tracks, including the track ends.

Application of High-Resolution 1H MAS NMR Spectroscopy to the Analysis of Intact Bones from Mice Exposed to Gamma Radiation

Abstract Herein we demonstrate that high-resolution magic angle spinning (MAS) 1H NMR can be used to profile the pathology of bone marrow rapidly and with minimal sample preparation. The spectral resolution obtained allows several metabolites to be analyzed quantitatively. The level of NMR-detectable metabolites in the epiphysis + metaphysis sections of mouse femur were significantly higher than that observed in the diaphysis of the same femur. The major metabolite damage to bone marrow resulting from either 3.0 Gy or 7.8 Gy of whole-body γ radiation 4 days after exposure were (1) decreased total choline content, (2) increased fatty acids in bone marrow, and (3) decreased creatine content. These results suggest that the membrane choline phospholipid metabolism (MCPM) pathway and the fatty acid biosynthesis pathway were altered as a result of radiation exposure. We also found that the metabolic damage induced by radiation in the epiphysis + metaphysis sections of mouse femur was higher than that of the diaphysis of the same femur. Traditional histopathology analysis was also carried out to correlate radiation damage with changes in metabolites. Importantly, the molecular information gleaned from high-resolution MAS 1H NMR complements the pathology data.

Detecting Radiation-Induced Injury Using Rapid 3D Variogram Analysis of CT Images of Rat Lungs

RATIONALE AND OBJECTIVES To investigate the ability of variogram analysis of octree-decomposed computed tomography (CT) images and volume change maps to detect radiation-induced damage in rat lungs. MATERIALS AND METHODS The lungs of female Sprague-Dawley rats were exposed to one of five absorbed doses (0, 6, 9, 12, or 15 Gy) of gamma radiation from a Co-60 source. At 6 months postexposure, pulmonary function tests were performed and four-dimensional (4D) CT images were acquired using a respiratory-gated microCT scanner. Volume change maps were then calculated from the 4DCT images. Octree decomposition was performed on CT images and volume change maps, and variogram analysis was applied to the decomposed images. Correlations of measured parameters with dose were evaluated. RESULTS The effects of irradiation were not detectable from measured parameters, indicating only mild lung damage. Additionally, there were no significant correlations of pulmonary function results or CT densitometry with radiation dose. However, the variogram analysis did detect a significant correlation with dose in both the CT images (r = -0.57, P = .003) and the volume change maps (r = -0.53, P = .008). CONCLUSION This is the first study to use variogram analysis of lung images to assess pulmonary damage in a model of radiation injury. Results show that this approach is more sensitive to detecting radiation damage than conventional measures such as pulmonary function tests or CT densitometry.

Beyond Californium—A Neutron Generator Alternative for Dosimetry and Instrument Calibration in the U.S.

Abstract Evaluations of neutron survey instruments, area monitors, and personal dosimeters rely on reference neutron radiations, which have evolved from the heavy reliance on (&agr;,n) sources to a shared reliance on (&agr;,n) and the spontaneous fission neutrons of californium‐252 (252Cf). Capable of producing high dose equivalent rates from an almost point source geometry, the characteristics of 252Cf are generally more favorable when compared to the use of (&agr;,n) and (&ggr;,n) sources or reactor-produced reference neutron radiations. Californium‐252 is typically used in two standardized configurations: unmoderated, to yield a fission energy spectrum; or with the capsule placed within a heavy-water moderating sphere to produce a softened spectrum that is generally considered more appropriate for evaluating devices used in nuclear power plant work environments. The U.S. Department of Energy 252Cf Loan/Lease Program, a longtime origin of affordable 252Cf sources for research, testing and calibration, was terminated in 2009. Since then, high-activity sources have become increasingly cost-prohibitive for laboratories that formerly benefited from that program. Neutron generators, based on the D-T and D-D fusion reactions, have become economically competitive with 252Cf and are recognized internationally as important calibration and test standards. Researchers from the National Institute of Standards and Technology and the Pacific Northwest National Laboratory are jointly considering the practicality and technical challenges of implementing neutron generators as calibration standards in the U.S. This article reviews the characteristics of isotope-based neutron sources, possible isotope alternatives to 252Cf, and the rationale behind the increasing favor of electronically generated neutron options. The evaluation of a D-T system at PNNL has revealed characteristics that must be considered in adapting generators to the task of calibration and testing where accurate determination of a dosimetric quantity is necessary. Finally, concepts are presented for modifying the generated neutron spectra to achieve particular targeted spectra, simulating 252Cf or workplace environments.

Physical characterization of a 137Cs field used for proficiency-test irradiations

This paper describes a series of measurements and calculations that were undertaken at the Pacific Northwest Laboratory to determine the physical and dosimetric characteristics of a 137Cs source, which is housed within a commercially manufactured irradiator. These measurements and calculations helped to demonstrate that: (1) the exposure rate for this source was consistent with (traceable to) radiation standards maintained by the National Institute of Standards and Technology; (2) the radiation field at the surface of the irradiation phantom, in a 10 cm X 10 cm central area, was uniform to within +/- 2%; (3) the contribution of scattered photons was minimal; and (4) the ratio of the shallow to the deep dose equivalent was nearly unity. Because there are a number of similar irradiators in use, it is hoped that the methodology and results described in this paper will be of use to others.