Toward a Pixelated Solid State Detector for In-Vivo Dosimetry in Radiotherapy

A large area pixelated silicon array detector named \MP987 has been developed for in-vivo dosimetry. The detector was developed to overcome the non-water equivalent response of EPID (Electronic Portal Imaging Device) dosimetry systems, due to the shortfalls of the extensive corrections required. The detector, readout system and software have all been custom designed and operate independently from the linac. The MP987 array is secured directly above the EPID, to be used in combination with the 6 MV imaging system. During the course of this thesis many aspects of in-vivo dosimetry were looked at, including: electrical and dosimetric characteristics of silicon diodes, feasibility of a dual detector system, angular dependence of silicon and synthetic diamond detectors and the suitability of different detector readout systems for multichannel arrays in external beam radiotherapy. In the final chapter of this thesis dosimetry characterisation measurements of percentage depth dose (PDD), dose rate dependence, radiation damage, output factors (OF), profile measurements, linearity and uniformity were performed with the MP987. Additionally, the first pre-clinical tests with this novel detector are presented, including, a transit dosimetry characterization and a collapsed IMRT (intensity modulated radiation therapy) study. Both PDD and OF measurements had a percentage difference of less than 2.5% to the reference detector. A maximum change in sensitivity of 4.3 ± 0.3% was observed after 30 kGy of gamma accumulated dose. Transit dosimetry measurements through a homogeneous phantom had a measured dose within the uncertainty of the TPS calculations, for field sizes between 3 × 3 cm2 and 10 × 10 cm2. A four-fraction collapsed IMRT plan on a lung phantom had absolute dose pass fractions between the MP987 and TPS (treatment planning system) from 94.2% to 97.4%, with a 5%/5 mm criteria. The ability to accurately measure dose at a transit level, without the need for correction factors derived from extensive commissioning data collection procedures, makes the MP987 a viable alternative to the EPID for in-vivo dosimetry. This MP987 is this first of its kind to be successfully developed specifically for a dual detector application

Controls of shear zone rheology and tectonic loading on postseismic creep

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B10404, doi:10.1029/2003JB002925.Postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes. In the postseismic time interval, GPS is most sensitive to creep in the lower crust or upper mantle activated by earthquake-generated stress perturbations. In these regions, deformation may be localized on an aseismic frictional surface or on a ductile shear zone. These two hypotheses imply specific rheologies and therefore time dependence of postseismic creep. Hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere. I present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied. In the absence of tectonic loading during the postseismic time interval, the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1/n. For a frictional surface, postseismic creep follows the same relaxation law in the limit 1/n → 0. A rough estimate of the apparent stress exponent can be obtained from continuous GPS records. Application to data collected after the 1994 Sanriku earthquake yields 1/n ∼ 0.1, which is consistent with dislocation creep mechanisms. However, the records of two other subduction zone events, the 2001 Peru event and the 1997 Kronotski earthquake, and a continental strike-slip earthquake, the 1999 İzmit earthquake, require negative 1/n. Rather than characterizing the shear zone rheology, these negative exponents indicate that reloading of the shear zone by tectonic forces is important. Numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1/n < 0, although the actual stress exponent of the rheology is positive. While this prevents rheology from being tightly constrained by the studied GPS records, it indicates that reloading is important in the postseismic time interval. In other words, the stress perturbation induced by an earthquake is comparable to the stress supported by ductile shear zones in the interseismic period.This work was supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the USGS, complemented by NSF grants OCE- 9907244, OCE-0327588, EAR-0337678, and a grant from the Deep Ocean Exploration Institute at WHOI to Greg Hirth

Safeguarding Imperiled Biodiversity and Evolutionary Processes in the Wallacea Center of Endemism

Wallacea—the meeting point between the Asian and Australian fauna—is one of the world's largest centers of endemism. Twenty-three million years of complex geological history have given rise to a living laboratory for the study of evolution and biodiversity, highly vulnerable to anthropogenic pressures. In the present article, we review the historic and contemporary processes shaping Wallacea's biodiversity and explore ways to conserve its unique ecosystems. Although remoteness has spared many Wallacean islands from the severe overexploitation that characterizes many tropical regions, industrial-scale expansion of agriculture, mining, aquaculture and fisheries is damaging terrestrial and aquatic ecosystems, denuding endemics from communities, and threatening a long-term legacy of impoverished human populations. An impending biodiversity catastrophe demands collaborative actions to improve community-based management, minimize environmental impacts, monitor threatened species, and reduce wildlife trade. Securing a positive future for Wallacea's imperiled ecosystems requires a fundamental shift away from managing marine and terrestrial realms independently

Safeguarding Imperiled Biodiversity and Evolutionary Processes in the Wallacea Center of Endemism

Wallacea—the meeting point between the Asian and Australian fauna—is one of the world's largest centers of endemism. Twenty-three million years of complex geological history have given rise to a living laboratory for the study of evolution and biodiversity, highly vulnerable to anthropogenic pressures. In the present article, we review the historic and contemporary processes shaping Wallacea's biodiversity and explore ways to conserve its unique ecosystems. Although remoteness has spared many Wallacean islands from the severe overexploitation that characterizes many tropical regions, industrial-scale expansion of agriculture, mining, aquaculture and fisheries is damaging terrestrial and aquatic ecosystems, denuding endemics from communities, and threatening a long-term legacy of impoverished human populations. An impending biodiversity catastrophe demands collaborative actions to improve community-based management, minimize environmental impacts, monitor threatened species, and reduce wildlife trade. Securing a positive future for Wallacea's imperiled ecosystems requires a fundamental shift away from managing marine and terrestrial realms independently

A Large Area Pixelated Silicon Array Detector for Independent Transit In Vivo Dosimetry

A large area pixelated silicon array detector named &ldquo;MP987&rdquo; has been developed for in vivo dosimetry. The detector was developed to overcome the non-water equivalent response of EPID (Electronic Portal Imaging Device) dosimetry systems, due to the shortfalls of the extensive corrections required. The detector, readout system and software have all been custom designed to be operated independently from the linac with the array secured directly above the EPID, to be used in combination with the 6 MV imaging system. Dosimetry characterisation measurements of percentage depth dose (PDD), dose rate dependence, radiation damage, output factors (OF), profile measurements, linearity and uniformity were performed. Additionally, the first pre-clinical tests with this novel detector of a transit dosimetry characterization and a collapsed IMRT (intensity-modulated radiation therapy) study are presented. Both PDD and OF measurements had a percentage difference of less than 2.5% to the reference detector. A maximum change in sensitivity of 4.3 &plusmn; 0.3% was observed after 30 kGy of gamma accumulated dose. Transit dosimetry measurements through a homogeneous Solid Water phantom had a measured dose within error of the TPS calculations, for field sizes between 3 &times; 3 cm2 and 10 &times; 10 cm2. A four-fraction collapsed IMRT plan on a lung phantom had absolute dose pass fractions between the MP987 and TPS (treatment planning system) from 94.2% to 97.4%, with a 5%/5 mm criteria. The ability to accurately measure dose at a transit level, without the need for correction factors derived from extensive commissioning data collection procedures, makes the MP987 a viable alternative to the EPID for in vivo dosimetry. This MP987 is this first of its kind to be successfully developed specifically for a dual detector application