High doses of ionizing radiation clearly produce deleterious consequences in humans, including, but not exclusively, cancer induction. At very low radiation doses the situation is much less clear, but the risks of low-dose radiation are of societal importance in relation to issues as varied as screening tests for cancer, the future of nuclear power, occupational radiation exposure, frequent-flyer risks, manned space exploration, and radiological terrorism. We review the difficulties involved in quantifying the risks of low-dose radiation and address two specific questions. First, what is the lowest dose of x-or ␥-radiation for which good evidence exists of increased cancer risks in humans? The epidemiological data suggest that it is Ϸ10 -50 mSv for an acute exposure and Ϸ50 -100 mSv for a protracted exposure. Second, what is the most appropriate way to extrapolate such cancer risk estimates to still lower doses? Given that it is supported by experimentally grounded, quantifiable, biophysical arguments, a linear extrapolation of cancer risks from intermediate to very low doses currently appears to be the most appropriate methodology. This linearity assumption is not necessarily the most conservative approach, and it is likely that it will result in an underestimate of some radiation-induced cancer risks and an overestimate of others.

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PrefaceMicrodosimetry originated more than 35 years ago when the senior author studied energy deposition in small irradiated masses and formulated what is now termed Regional Microdosimetry. A.M. Kellerer developed the further concepts of Structural Microdosimetry. Microdosimetry and its applications have been the subject of an extensive literature. This includes several hundred papers which have appeared in the Proceedings of what to date have been eleven Symposia on Microdosimetry. General reviews are contained in chapters of books and in a journal dealing with a broader range of subjects. The International Commission on Radiation Units and Measurements has produced Report 36 on Microdosimetry. The form of these publications limited their scope and in this work it is our aim to provide a more comprehensive account.Dealing extensively with interdisciplinary matters (from atomic and solid state physics to integral geometry and molecular biology) we were confronted with the standard problem of presenting material in a manner that makes it comprehensible to readers with diverse backgrounds, without providing a superficial treatise. Although some readers may find it too difficult to absorb the entire contents of some chapters, they should be able to gain substantial information from introductory sections and from data presented. Occasional repetitions, including identical formulae, have reduced the need for cross-references between chapters.With the exception of chapter III it is recommended to read the book sequentially; this appeared to us to be the logical way of learning microdosimetry. The conceptual framework of microdosimetry is introduced in the first two chapters. At this stage of the presentation the details of energy deposition in matter are unimportant. The fourth and fifth chapter are the twin workhorses of microdosimetry. The fourth chapter is about the art of measuring microdosirnetric spectra. It gives details on the construction and operation of microdosimetric detectors. It also makes aware both the experimentalist and the theorist that what one measures may sometimes be different from the actual pattern of energy deposition. The material in Chapter V, the theoretical companion of the preceding chapter, comes as a result of the tremendous progress made during the past 20 years or so in obtaining cross sections for the interaction of charged particles with matter, and of the subsequent effort made to integrate these into sophisticated Monte Carlo transport codes that simulate the passage of particles through structured or unstructured matter. This chapter also brings forth the two VI Preface complementary descriptions of microdosimetric events: regional microdosimetry and structural microdosimetry. The applications of microdosimetry are described in the last two chapters. The selection of topics here reflects the research interests of the authors and also the need to provide this information within the limitations of a single volume. As a result the list of topics here should be seen as illustrative...

An analytic expression for the tumour control probability (TCP), valid for any temporal distribution of dose, is discussed. The TCP model, derived using the theory of birth-and-death stochastic processes, generalizes several results previously obtained. The TCP equation is [equation: see text] where S(t) is the survival probability at time t of the n clonogenic tumour cells initially present (at t = 0), and b and d are, respectively, the birth and death rates of these cells. Equivalently, b = 0.693/Tpot and d/b is the cell loss factor of the tumour. In this expression t refers to any time during or after the treatment; typically, one would take for t the end of the treatment period or the expected remaining life span of the patient. This model, which provides a comprehensive framework for predicting TCP, can be used predictively, or--when clinical data are available for one particular treatment modality (e.g. fractionated radiotherapy)--to obtain TCP-equivalent regimens for other modalities (e.g. low dose-rate treatments).

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