A Nonparametric Method for the Derivation of α/β Ratios from the Effect of Fractionated Irradiations

Multifractionation isoeffect data are commonly analysed under the assumption that cell survival determines the observed tissue or tumour response, and that it follows a linear-quadratic dose dependence. The analysis is employed to derive the α/β ratios of the linear-quadratic dose dependence, and different methods have been developed for this purpose. A common method uses the so-called Fe plot. A more complex but also more rigorous method has been introduced by Lam et al. (1979). Their method, which is based on numerical optimization procedures, is generalized and somewhat simplified in the present study. Tumour-regrowth data are used to explain the nonparametric procedure which provides α/β ratios without the need to postulate analytical expressions for the relationship between cell survival and regrowth delay

Ab initio atomistic thermodynamics and statistical mechanics of surface properties and functions

Previous and present "academic" research aiming at atomic scale understanding is mainly concerned with the study of individual molecular processes possibly underlying materials science applications. Appealing properties of an individual process are then frequently discussed in terms of their direct importance for the envisioned material function, or reciprocally, the function of materials is somehow believed to be understandable by essentially one prominent elementary process only. What is often overlooked in this approach is that in macroscopic systems of technological relevance typically a large number of distinct atomic scale processes take place. Which of them are decisive for observable system properties and functions is then not only determined by the detailed individual properties of each process alone, but in many, if not most cases also the interplay of all processes, i.e. how they act together, plays a crucial role. For a "predictive materials science modeling with microscopic understanding", a description that treats the statistical interplay of a large number of microscopically well-described elementary processes must therefore be applied. Modern electronic structure theory methods such as DFT have become a standard tool for the accurate description of individual molecular processes. Here, we discuss the present status of emerging methodologies which attempt to achieve a (hopefully seamless) match of DFT with concepts from statistical mechanics or thermodynamics, in order to also address the interplay of the various molecular processes. The new quality of, and the novel insights that can be gained by, such techniques is illustrated by how they allow the description of crystal surfaces in contact with realistic gas-phase environments.Comment: 24 pages including 17 figures, related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

EphB2 and EphA4 receptors regulate formation of the principal inter-hemispheric tracts of the mammalian forebrain

Abstract Previously, we have demonstrated that EphB2 activity is required for proper development of the posterior branch of the anterior commissure (ACpp) within the mammalian forebrain. In the present study, using magnetic resonance imaging (MRI), immunohistochemistry, and in vivo stereotactic fluorescence tracing of EphB2, B3, A4 and combinatorial Eph receptor mutants, we have developed a detailed three-dimensional model of how EphB-class receptors interact to regulate commissural formation within the forebrain. The results demonstrate that EphB2 and EphA4 each regulate distinct aspects of axon guidance within the ACpp. Specifically, while EphB2 is required to retard ACpp axons from projecting aberrantly into the ventral forebrain, EphA4 is required to restrict axons from entering the anterior branch of the anterior commissure (ACpa). Together, EphB2 and EphA4 act synergistically to prevent a subpopulation of axons within the anterior branch of the AC from mis-projecting caudally. Analysis of EphA4 null mice using high resolution MRI reveals for the first time that, in addition to errors in midline guidance, loss of EphA4 results in aberrant lateral and ventral displacement of the ACpa tract. In addition, tracing studies in α-chimerin null mice reveal that EphA4-mediated effects are not regulated through this pathway. Taken together, the results demonstrate that each of the principal guidance decisions within both anterior and posterior tracts of the anterior commissure can be accounted for by the individual and combinatorial actions of EphB2/A4 receptors

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