Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Incidence in exposedpopulationSpontaneousincidence (unexposed)Radiationexposure~ Latency~ (t)Radiationexposure~ Latency~ (t), ,,,,", ,,, ,,,~,," ~',,,,, ,,,,, , ~,, ,#" ...•,,,,,FIGURE 25-16. Comparisonof multiplicative and additiverisk models. Radiationinducedrisk increments areseen after a minimal latentperiod t; x is age at exposure.When the increased risk isassumed to occur over theremaining lifetime, inducedeffects can follow either themultiplicative risk model (A),in which the excess risk is amultiple of the natural agespecificcancer risk for agiven population, or theadditive risk model (B), inwhich a constant incrementin incidence is added to thespontaneous disease incidencethroughout life. Themultiplicative risk model predictsthe greatest incrementin incidence at older ages.An alternative hypothesis is the additive risk model, which predicts a fixed (orconstant) increase in risk unrelated to the spontaneous age-specific cancer risk at thetime of exposure (see Fig. 25-16B).In general, neither of these two simple models appears to adequately describethe risk of cancer after exposure. A modification of the relative risk model, with correctionsfor factors such at type of cancer, gender, age at time of exposure, and timeelapsed since exposure, was recommended by expert committees on radiationinducedcancer risks (see later discussion of BEIR V risk estimates).Risk ExpressionOne way of expressing the risk from radiation in an exposed population is the relativerisk (RR). Relative risk is the ratio of the cancer incidence in the exposed populationto that in the general (unexposed) population; thus, a relative risk of 1.2would indicate a 20% increase over the spontaneous rate that would otherwise havebeen expected. The excess relative risk is simply RR-l; in this case, 1.2 - 1 = 0.2.To be able to detect a relative cancer risk of 1.2 with a statistical confidence of950/0 (i.e., p < ,05) when the spontaneous incidence is 2% in the population (typi-