Evaluation of risks in medical radiation exposure

The role of radiation is usually evaluated in terms of excess risks. The excessive cancer risk ascribed to radiation is within the same order of magnitude, notwithstanding the dose or the type of radiation employed, namely the excess of 2 to 8 cases per rad, per year, per million population. In such cases, the causal contribution of carcinogenic factors is expressed by the arithmetic difference in disease rates between exposed and no-exposure populations. The risk per rad in the irradiated group is a function of the natural risk in the controls, and the joint effect of two factors is a function of the product of two or more individual effects. The radioisotope therapy for thyrotoxicosis is still a common practice. A large scale followup study that tried to evaluate the subsequent neoplastic development among the patients treated by 131I yielded no excess of thyroid tumors among the treated. The selection of an appropriate control group is even more problematic and almost insurmountable in the evaluation of the other extreme of this spectrum, that is, the long term effect of low dose diagnostic isotopes on the patients with suspected thyroid disease. The apparent lack of excess risk following diagnostic isotope application is due to the difference in the rate and homogeneity of internal and external irradiations. The major danger to the public in disasters such as the accident of nuclear plants is through internal contamination, primarily following the entry of radioactive iodine into the natural life cycle, while the extrapolation from external irradiation may not be simple. (Yamashita, S.)