Mechanical and Architectural Changes in Animal Bone Following Fast Neutron Irradiation.

ABSTRACT: Damage to healthy bone following exposure to ionizing radiation has been well documented for at least seven decades. Among the reported effects are a transient increase in stiffness and a reduction in breaking strength. These changes have been linked to a decrease in osteoblast proliferation and differentiation, inducing cell cycle arrest, reducing collagen production, and increasing sensitivity to apoptotic agents. In this work, we analyzed some mechanical and structural changes in compact costal bovine bone (Hereford breed, n = 9) subjected to escalating doses of fast neutrons from a 7 Li(p,n) 7 Be reaction. The mean neutron energy was 233 keV with calculated absorbed doses ranging from 0 to 4.05 ± 10% Gy. Samples were subjected to Young's Modulus (YM) and breaking strength testing with a Universal Testing Machine (UTM). We found an increase in Young's Modulus and a decrease in breaking strength as functions of increasing dose equivalent. Optical coherence tomography (OCT) revealed trabecular displacement into compact bone in an irradiated sample (D = 4.05 ± 10% Gy), with breaching of the endosteal wall. OCT further revealed a "crack-like" structure across the irradiated sample, potentially consistent with damage from a proton track resulting from an elastic (n,p) reaction. No previous report has been found on mechanical changes in large mammalian bones following fast neutron doses, nor of the OCT imaging of such samples.

Some radiological, structural, and physical properties of a clinical X-ray unit oil.

Radiological, structural, and physical properties of an x-ray oil which absorbed a photon dose of approximately 2.0 ± 0.2 Gy during its operational life (∼10 years), were investigated. An attenuation coefficient of 0.198 ± 0.003 cm-1 (at 122 keV) was obtained. NMR, ESR, and FTIR spectroscopy suggest a preponderance of pentane, and hexane. Mass spectrometry detected tertrapentacontane, nonadecane, and docosane. No gamma-emitters were revealed with gamma spectroscopy. A boiling temperature of approximately 300 °C was measured. Microscopy revealed the presence of gas bubbles.

The development and experimental evaluation of a simple analytical model for the TPR in the build-up region of megavoltage photon beams.

PURPOSE: The purpose of this work was to develop a simple analytical model for the tissue phantom ratio (TPR) in the build-up region of megavoltage photon beams and to experimentally evaluate the model under a variety of clinically relevant field configurations. METHODS: Considering electron contamination and primary photons as the main components of the absorbed dose in the build-up region, an analytic expression for the TPR was derived. The electron contamination component was addressed with a biexponential function; the primary photon component was treated as nonlocal energy transport, i.e., assuming the energy deposited by secondary electrons can be described by a biexponential mode similar to that of contaminating electrons. The model contains five independent constants, which were fitted experimentally. The accuracy of the model was evaluated by comparing its results with in-phantom measurements taken on square, rectangular, irregular, and wedged fields, for 6 and 15 MV photon beams on a GE-Saturne 41 accelerator. More specifically, the accuracy of the model was quantified using the gamma index with 2% dose and 2 mm spatial tolerances as described by Low et al. [Med. Phys. 25, 656-661 (1998)]. RESULTS: For square cerrobende blocked fields, the maximum recorded gamma indices were 0.42 and 0.54 for 6 and 15 MV beams, respectively. For "I" shaped fields, the corresponding maxima were 0.64 and 0.52, respectively, while for "cross" shaped fields they were 0.42 and 0.76. For rectangular 10 × 30 cm fields, the corresponding maxima were 0.32 and 0.42, and for 7 × 20 cm fields, they were 0.70 and 0.35, respectively. For square 10 × 10 cm and 15 × 15 cm fields with an acrylic tray, the maxima were 0.57 and 0.45 for 6 MV and 0.32 and 0.77 for 15 MV beams, respectively. For a 10 × 10 cm 60° wedged field, the maxima were 0.53 and 0.33 for 6 and 15 MV beams, respectively. In all examined cases of irregular, rectangular, square (with and without tray), and wedged fields, the gamma index was less than unity. Thus, the model correctly predicted TPR in all cases, using the defined criteria. CONCLUSIONS: A simple analytical model for the TPR in the build-up region was developed and evaluated experimentally. The model's predicted TPR values were compared with physical measurements for irregular, square (with and without tray), rectangular, and wedged fields, for 6 and 15 MV photon beams. In every case examined, the results of the model agreed with the experimental measurements based on specific quantitative agreement criteria. The model appears useful for predicting the TPR in the build-up region of megavoltage beams for different types of fields, in different configurations.

Some physical and clinical factors influencing the measurement of precision error, least significant change, and bone mineral density in dual-energy x-ray absorptiometry.

Dual-energy X-ray absorptiometry (DXA) is the standard method of measuring bone mineral density (BMD) at highly trabecular bone, which can be statistically linked to the risk of fracture. For DXA, precision error (PE) and phantom-based accuracy studies are among the most important routine quality control procedures. A precision study was performed at our institution using International Society for Clinical Densitometry guidelines. Comparing our results with those reported by other investigators, we draw the following general conclusions: the PE was higher for the spine than the hip, which we attribute to the better geometric reproducibility at the hip. The hypothesis that the DXA calculates BMD relative to water was validated. Whether follow-up measurements are performed by the same technologist on the same day-or different technologists on subsequent days-does not appear to have a clinically significant impact on PE or least significant change (LSC). Mixing beam types (i.e., fan and pencil) may affect lumbar PE and LSC measurements more significantly than those of the hip. The use of a single technologist may reduce the PE for the lumbar spine but appears to increase it for the hip. Restricting the patient population to the female gender has the apparent effect of narrowing the gap between lumbar and hip PEs. Finally, the degree of BMD measurement accuracy can be affected by the type of phantom being used (e.g., European Spine Phantom vs Lunar phantom) and the faults in specific DXA edge detection algorithms.

An Analysis of Inter-Operator Registration Variability in Helical Tomotherapy.

This study aims to determine the image registration interoperator variability using the image fusion interface on the TomoTherapy system. The study focused on the registration results among five Ottawa Hospital Cancer Centre personnel comparing 15 retrospective prostate, bladder, and head and neck patients from day 2 megavoltage computed tomography images with planning computed tomography images. Personnel were instructed to manually adjust the fused images after automatic bony registration matching disease-specific criteria. The Pearson correlation matrix was applied to the summed vector lengths of the personnel's overall translations in all three disease sites. The results were compared with a chosen benchmark personnel and were analyzed for significance in correlation. Finally, the overall translations compared with the automatic bony registration were analyzed for any trends in the data. In each disease site, the average correlation from all personnel compared to a benchmark registering prostate, bladder, and head and neck patients were 0.94, 0.94, and 0.64, respectively. Then, analysis of the interuser overall translations illustrated that each disease site had its own trends with regard to interfraction image fusion corrections. Finally, the mean translational variations were assessed over all personnel for each disease site in X (lateral), Y (longitudinal), and Z (vertical) directional planes. The results demonstrate that consistent, accurate image registration is dependent on factors involving overall user experience with TomoTherapy software and user knowledge of human sectional anatomy, among others.

On some behavioral anomalies in an analytical expression for the solid angle subtended by a circular detector from a symmetrically positioned linear source.

We explore the causes of an interesting behavioral anomaly in an analytical result which has been previously derived for the solid angle subtended by a circular detector for a line source in a parallel plane-whose midpoint is coaxial with the detector's center. The anomalous behavior is verified by comparing it to the results produced by a special purpose Monte Carlo code. In addition, the relevant cases of a very large detector, a very long source, a very short source, and a very small detector, are investigated using standard analytical techniques in the theory of limits. We demonstrate that in these limiting cases, the analytical expression correlates well with the Monte Carlo code. These results are presented in analytical, and graphical forms.

On the physical, spectral, and dosimetric characteristics of a new 125I brachytherapy source.

A new 125I source under the name Braquibac has been developed in Argentina for interstitial brachytherapy applications. The aim of this work is to study the new seed's design and to calculate its dosimetric parameters. Radiographic and destructive tests were carried out on inactive seeds to determine the physical characteristics of the source. Values of g(r), Lambda, F(r, Theta), and phi(an)(r), were obtained in water and air by simulation using the MCNP5 Monte Carlo code according to the methodology recommended in TG-43 and updated in TG-43U1. The dose rate constant was determined to be 0.937+/-0.004 cGy h(-1) U(-1) (overall statistical uncertainty +/-2.7%). Sk per unity activity was calculated to be 0.671+/-0.003 cGy cm2 h(-1) mCi(-1) by simulation of the seed in dry air using point detectors. Spectroscopic studies for both the new and the Amersham model 6711 seed were performed using an HPGe planar detector. The emission spectra of both seeds proved to be very similar. The anisotropy of the total photon intensity in air was measured in planes containing the seed's short and long axes using the HPGe detector. The minimum photon intensity for the new seed was 31.14+/-3.10% of the transverse intensity.

Experimental determination of the extrinsic sensitivity and counting efficiency of a nuclear gamma camera using a homogeneous circular planar source.

Two important operational parameters of medical gamma cameras are the extrinsic counting efficiency and sensitivity. Historically, for practical reasons these two parameters have been defined using a point source of radiation at a certain distance from the detector. This definition has the disadvantage of producing measurements of limited clinical relevance since real patients are not point sources of radiation. In this work, we propose a more clinically relevant method of determining efficiency and sensitivity, using a planar, circular, homogeneous source. For this purpose, a gamma camera of 39 cm diameter, and a circular, homogeneous, 36.5 cm diameter Co-57 source with an activity of 5.5+/-0.7%mCi were employed. The source was placed coaxially with the detector at distances of 50, 100, and 150 cm. Data were acquired using a medium and a high-resolution collimator. The efficiency was found to depend directly on the solid angle subtended by the detector. Computed values for the sensitivity are in the range of 5-6 kcts/sec-mCi, and values for the efficiency were found to range between 0.2% and 2%. The results show an approximate 2:1 ratio in sensitivity and efficiency between the medium and high-resolution collimators regardless of source-detector distances.

Peak efficiency calibration for attenuation corrected cylindrical sources in gamma ray spectrometry by the use of a point source.

A theoretical method of determining the gamma-ray peak efficiency for a cylindrical source, based on a modified expression for point sources is derived. A term for the photon self-attenuation is included in the calculation. The method is valid for any source material as long as the source activity concentration is considered to be homogeneous. Results of this expression are checked against experimental data obtained with (241)Am, (57)Co, (137)Cs, and (60)Co sources.

An analytical solution for the solid angle subtended by a circular detector for a symmetrically positioned linear source.

The analytical determination of the solid angle subtended by a circular detector for a line source in a parallel plane, whose midpoint is co-axial with the detector's center, is of some relevance to the medical and nuclear power fields. No report has been found in the literature of a closed-form solution to this problem. We present a first principles solution to the problem along with a graph, which gives some indication of the behavior of the derived expression.

A comparison of different analytical methods of determining the solid angle of a circular coaxial source-detector system.

In this work, seven different analytical expressions for the solid angle subtended by a circular detector for a coaxial circular source are compared. These expressions -- which have been published by different investigators over the course of the last half century -- are compared for previously published geometries, and for geometries planned in future experimental work. As compared to benchmark Monte Carlo calculations, the expressions published by Segre, Knoll, and Ruby, were the most accurate and exhibited the highest degree of self-correlation.

Determination of the activity concentration of a 238 Pu solution by the defined solid angle method utilizing a novel dual diaphragm-detector assembly.

The activity concentration of a (238)Pu solution was measured by the determined solid angle method employing a novel dual diaphragm-detector assembly, which has been previously described. Due to the special requirements of the detector, a new type of source holder was developed, which consisted of sandwiching the radioisotope between two organic films called VYNS. It was experimentally demonstrated that the VYNS films do not absorb alpha particles, but reduce their energy by an average of 22 keV.A mean activity concentration for (238)Pu of 359.10+/-0.8 kBq/g was measured.

A statistical analysis of the initial biodistribution of 153Sm-EDTMP in a canine.

(153)Sm (t(1/2)=46 h) emits a 103 keV gamma photon and two medium-energy beta particles. Five mCi of Samarium-153 ethylenediaminetetramethylenephosphonic acid ((153)Sm-EDTMP) were administered to a clinically normal dog and whole body scans were obtained at 15min, 2h, and 24h post-injection (PI). Regions of interest (ROIs) were drawn representing abdomen, knee, rib, vertebral bodies, bladder, kidney, and liver, in each image. For each ROI, the mean intensity and standard deviation were computed, and a histogram was created. Clinically significant increased uptakes were found in liver and kidney.