Health Effects of Exposure to Low Levels of Ionizing Radiations Time for Reassessment? (1998) / Chapter Skim
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5. Radiobiological Principles and Risk Modeling
5. Radiobiological Principles and Risk Modeling
Pages 33-40
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From page 33... ...
Both are closely related to the problem of threshold versus linear nonthreshold dependence on dose for radiation-related excess cancer rates. LINEARITY VERSUS CURVTLINEARITY One of the major disagreements between radiobiologic observations and current risk models concerns the shape of the dose-response curve for gamma radiation.
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From page 34... ...
al,: linear coefficient, that is, excess relative risk (ERR) per gray at low doses; A: 'crossover dose,' which equals ratio of linear and quadratic coefficient in the dose dependence for y-rays.
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From page 35... ...
In past analyses, it has been common to treat neutrons as a minor contributor to the observed health effects and, in line with this assumption, to use the crude approximation of a simple dosemodifying factor. Instead of total absorbed dose, D = Dy + Dn' the approach uses a weighted dose, Dw = Do, + wDn assuming that the RBE of neutrons is constant.
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From page 36... ...
that numerical values of the neutron RBE cannot be derived from data on the atomic-bomb survivors. But the added computations with the meaningful linear-quadratic dependence ale desirable because, in contrast with earlier approaches, they can provide an inverse relation between photon and neutron risk estimates that results if various values of the RBE of neutrons are considered (Kellerer and Nekolla 1997~.
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From page 37... ...
the computation provides an upper bound to the absolute risk coefficient for neutrons which would be obtained if the RBE of neutrons were infinite, that is, if the ' In these sections, R (in the present exploratory computations with a linear-quadratic model) , w (the constant neutron weight factor in the earlier Radiation Effects Research Foundation calculations)
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From page 38... ...
This interrelation is disregarded in present use where the risk estimate for photons is taken to be substantially independent of the assumed RBE of neutrons and the neutron risk estimate is obtained as the product of the unchanged photon risk estimate and the assumed RBE of neutrons. A scaling factor from excess relative risk to absolute risk has been used here that corresponds-in line with the mode!
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From page 39... ...
Dependence of the gamma-ray and the neutron colon dose on distance from hypocenter in Hiroshima according to DS86 (solid lines) and neutron dose tentatively modified according to Straume and others (1992)
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From page 40... ...
Light and heavily shaded areas show 95% confidence and standard error regions of combinations of photon and neutron risk estimates. The calculations illustrate that the estimates of risk coefficients for protons and neutrons are inversely related; high RBE of neutrons cannot be excluded on the basis of the data but imply reduced risk estimates for photons.
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