Letter to the Editor.

Abstract

Sir, Subject: Quantification of stochastic effects: can we be too exact? There is little doubt that exposure to ionising radiation results in a certain health detriment. Usually, in normal situations, the exposure is so low that no clinically observable effects are recognised. In this case, only stochastic or late effects, especially cancer, may occur with a probability proportional to the exposure above the natural radiation background. Many attempts have been made to define a universal quantity to quantify the exposure to a person in order to assess this probability regardless of the type of radiation or irradiation conditions, including non-homogeneous exposure to individual organs. The quantity of effective dose has been adopted for this purpose. Extensive efforts have been made to associate its value with whole-body stochastic effects. In principle, this has been achieved by the introduction of relevant weighting factors taking into account the type of radiation concerned and the tissues or organs exposed. The radiation weighting factor and tissue weighting factor convert the relevant conventional physical quantity, called the absorbed dose, into a sort of bio-physical quantity expressed in sievert. These factors are based largely on epidemiological studies. Thus, they have often been changed (and presumably these modifications will continue in the future too) in order to match the value of the effective dose with the impact of the exposure in terms of stochastic effects based on the latest scientific findings. A similar concept, relying on the quantity of equivalent dose, has also been adopted for the stochastic effects occurring in an individual organ or a tissue that has been exposed to ionising radiation.