Introduction

Detrimental effects of exposure to ionizing radiation, including toxicity, burns and tumor induction, became noticeable very early in the history of radiology. Many pioneer radiation researchers died of neoplasms, among them were Thomas Edison's assistant and Marie Curie. This pattern became clear in the 1940s with the observation of a high incidence of leukemia among radiologists. In the 1930s, a high incidence of osteosarcoma was noted among luminous dial painters who were in the habit of licking the point of their paintbrushes, to make them thinner, thus ingesting radium (Rowland et al., 1978). In the 1950s, a high incidence of leukemia was also detected among survivors of the atomic bombings in Hiroshima and Nagasaki (Folley et al., 1952) and among patients treated with X-rays for ankylosing spondylitis (Court Brown & Doll, 1957).

Many animal experiments and epidemiological studies have been carried out since then to elucidate the mechanisms of radiation-induced carcinogenesis (BEIR III, 1980; UNSCEAR, 1977). It is now accepted that ionizing radiation can induce cancer in most tissues, although there are great differences in organ susceptibility. The role of ionizing radiation in human carcinogenesis is probably the best understood and quantified of all exogenous physical and chemical agents (BEIR V, 1989; UNSCEAR, 1988). Available information is based predominantly on studies of large populations receiving high dose exposures of short duration.