[Excerpt] "Despite the nearly universal adoption of the linear nonthreshold dose response model (LNT) as the primary basis for radiation protection standards for the past half century, the LNT remains highly controversial and a contentious topic of discussion among health physicists, radiation biologists, and other radiological scientists. Indeed, it has been pointed out that the LNT has assumed the status of a paradigm, synonymous with an ideal, standard, or paragon or perhaps to some, a sacred cow. Reduced to its very basics, the LNT postulates that every increment of ionizing radiation dose, however small, carries with it a commensurate increase in the chance or risk that the exposed individual will suffer some undesirable radiation effect, and that the risk thus incurred is directly proportional or linearly related to the dose. The specific effects are termed “stochastic,” which has been defined as “of a random or statistical nature.” Stochastic or probabilistic effects of radiation may occur as a result of low doses and are generally taken to be cancers (including leukemias) and genetic defects in the progeny. The severity of these radiation-induced stochastic effects, should they occur, are independent of the dose that produced them; thus, even though the likelihood or probability of an occurrence may be small to negligible, any and all manifestations of a radiation induced stochastic effect will have equal severity.
By contrast, higher doses of radiation are known to produce characteristic somatic or deterministic effects including erythema, epilation, sterility, diminution of blood cell counts, cataracts and, in very high exposures, acute and chronic radiation syndromes. Such frank biological effects are nonstochastic in nature (in fact, they were at one time termed “nonstochastic effects”) and will always be manifested once a particular minimum dose – i.e., a “threshold” – has been received. The severity of the effect is related to the dose. Below the threshold dose there will be no demonstrable effect; as the dose increases beyond the threshold, so does the severity of the effect, or the degree of harm.
The purpose of the above discussion is to illustrate the underlying controversy and confusion that surrounds the LNT today, as well to underscore the lack of precision that sometimes accompanies the arguments of both the proponents and opponents of the LNT. Given that the LNT is a low dose phenomenon, there needs to at least be consensus on what is low dose, and such a consensus needs also to include consideration of other relevant and important factors such as the dose rate and specific stochastic end point (i.e., type of cancer or mutation). With this as a backdrop, the historical development and gyrations that led to the LNT as it is currently applied (or, some would say, misapplied) in radiological protection can be examined in the context of current scientific thinking with respect to radiation effects. It is not the purpose of this paper to endorse any particular position or to take sides but rather to present the story in a factual and fair minded manner. Hopefully, what follows will successfully achieve this goal. Thus, this paper will briefly review the scientific bases and supporting studies that led to the development and acceptance of the LNT in health physics. It will briefly touch on such topics as hormesis and other studies, such as the classic work of the late Robley Evans, that clearly demonstrate a threshold and nonlinear response for certain stochastic effects such as osteogenic sarcoma, along with the plethora of studies that suggest or have been interpreted to indicate that for at least some end points (i.e., cancers), response to ionizing radiation is consistent with the LNT model."
Ronald L. Kathren, Historical Development of the Linear Nonthreshold Dose- Response Model as Applied to Radiation, 1 Pierce L. Rev. 5 (2002), available at http://scholars.unh.edu/unh_lr/vol1/iss1/5