37 research outputs found
Recommended from our members
Geometric and dosimetric uncertainties in intracranial stereotatctic treatments for multiple nonisocentric lesions
The purpose of this study was to determine the effects of geometric uncertainties of patient position on treatments of multiple nonisocentric intracranial lesions. The average distance between lesions in patients with multiple targets was determined by a retrospective survey of patients with multiple lesions. Retrospective patient imaging data from fractionated stereotactic patients were used to calculate interfractional and intrafractional patient position uncertainty. Three different immobilization devices were included in the positioning study. The interfractional and intrafractional patient positioning error data were used to calculate the geometric offset of a lesion located at varying distances from the mechanical isocenter for treatments of multiple lesions with a single arc, assuming that no intrafractional position correction is employed during an arc rotation. Dosimetric effects were studied using two representative lesions of two sizes, 6 mm and 13 mm maximum dimensions, and prescribed to 20 Gy and 18 Gy, respectively. Distances between lesions ranged from 3.0 mm distant from the planned position. Dosimetric results depend upon the distance from isocenter and the size of the target. Single isocenter treatments for multiple lesions should be considered only when full six DOF corrections can be applied, the intrafractional immobilization precision is well quantified, and a PTV expansion is included for more distant lesions to account for unavoidable residual patient positioning uncertainties. PACS number: 87.55.Qr, 87.53.Ly, 87.55.D
A model for A=3 antinuclei production in proton-nucleus collisions
A simple coalescence model based on the same diagrammatic approach of
antimatter production in hadronic collisions as used previously for
antideuterons is used here for the hadroproduction of mass 3 antinuclei. It is
shown that the model is able to reproduce the existing experimental data on
Tbar and 3hebar production without any additional parameter.Comment: 7 figures. submitted to Eur. Phys. J.
Can diet niche partitioning enhance sexual dimorphism?
Classic evolutionary theory suggests that sexual dimorphism evolves primarily via sexual and fecundity selection. However, theory and evidence are beginning to accumulate suggesting that resource competition can drive the evolution of sexual dimorphism, via ecological character displacement between sexes. A key prediction of this hypothesis is that the extent of ecological divergence between sexes will be associated with the extent of sexual dimorphism. As the stable isotope ratios of animal tissues provide a quantitative measure of various aspects of ecology, we carried out a meta-analysis examining associations between the extent of isotopic divergence between sexes and the extent of body size dimorphism. Our models demonstrate that large amounts of between-study variation in isotopic (ecological) divergence between sexes is nonrandom and may be associated with the traits of study subjects. We, therefore, completed meta-regressions to examine whether the extent of isotopic divergence between sexes is associated with the extent of sexual size dimorphism. We found modest but significantly positive associations across species between size dimorphism and ecological differences between sexes, that increased in strength when the ecological opportunity for dietary divergence between sexes was greatest. Our results, therefore, provide further evidence that ecologically mediated selection, not directly related to reproduction, can contribute to the evolution of sexual dimorphism