A different perception of the linear, nonthreshold hypothesis for low-dose irradiation.

Abstract

Two equally useful dosimetric quantities, both of which are called dose, are used in toxicology. With radiation measurement, only one-the energy per unit mass D-is called dose. The other-the total energy in the irradiated system-is here distingished from D by assigning it the name collective energy, E. The collective energy is a more complete statement of dose because it is the product of the energy concentration D and the mass irradiated m. Especially in radioepidemiology, in which e is the total energy imparted to all persons irradiated, the quantity m must be specified because it is situation specific and thus highly variable. At present, radioepikemiological dose-response curves are given only in terms of the toxicological model-i.e., the fraction (probability) of radiation-attributable cancers occurring as a function of D. Because this relation does not involve the number of persons at each value ofD, it fosters the illusion that any dose, no matter how small, can result in cancer. However, we show that if the dose-response relationship is expressed in terms ofthe absolute number of attributable cancers as a function of E, cancer occurs, on average, only if the collective energy exceeds a relatively large minimum value, the ma de of which will be ated. Therefore, we conclude that the nonthreshold aspect of the linear hypothesis is misleading and quite probably invalid. For example, in or around a facility in which exposure of humans to relatively low values of D occurs, attributable cancers are most unlikely to appear unless the e to the irradiated population exceeds this minimum value. Dose-response curves for ionizing radiation were first developed to predict its acute, early effects on normal organs and mammals (humans) and on cancers in connection with radiotherapy. These relationships determined the fraction (probability) of persons responding quantally as a function of absorbed dose [a quantal response is a definably either/or change of state such as an irreversible level of injury, the appearance of a cancer, or death (1)]. The purpose and usefulness of these functions was to estimate these probabilities, thus permitting the radiotherapist to prescribe a dose, or course of doses, that would maximize the probability of controlling the tumor while minimizing the chance of unacceptable collateral damage to normal tissues that were unavoidably irradiated. The curves corresponded to those used generally in toxicology and therapeutic medicine-i.e., they had a -threshold and were nonlinear.Subsequent findings in cell systems and in human radioepidemiological studies suggested that cancer could result from relatively small absorbed doses. However, it was evident that at least the initial portion of the dose-response curve was often consistent with a linear, nonthreshold relationship. Nonetheless, the identical variable quantities developed earlier in connection with the toxicological model were carried over directly to describe the linear, nonthreshold relationships (2,3). It is the function based on...

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