Is there a place for quantitative risk assessment?

Abstract

The use of ionising radiations is so well established, especially in the practice of medicine,that it is impossible to imagine contemporary life without them. At the same time, ionisingradiations are a known and proven human carcinogen. Exposure to radiation in somecontexts elicits fear and alarm (nuclear power for example) while in other situations, untilrecently at least, it was accepted with alacrity (diagnostic x-rays for example).This non-uniform reaction to the potential hazards of radiation highlights the importanceof quantitative risk estimates, which are necessary to help put things into perspective.Three areas will be discussed where quantitative risk estimates are needed and whereuncertainties and limitations are a problem.First, the question of diagnostic x-rays. CT usage over the past quarter of a century hasincreased about 12 fold in the UK and more than 20 fold in the US. In both countries,more than 90% of the collective population dose from diagnostic x-rays comes from the fewhigh dose procedures, such as interventional radiology, CT scans, lumbar spine x-rays andbarium enemas. These all involve doses close to the lower limit at which there are credibleepidemiological data for an excess cancer incidence. This is a critical question; what is thelowest dose at which there is good evidence of an elevated cancer incidence? Without lowdose risk estimates the risk–benefit ratio of diagnostic procedures cannot be assessed.Second, the use of new techniques in radiation oncology. IMRT is widely used to obtain amore conformal dose distribution, particularly in children. It results in a larger total bodydose, due to an increased number of monitor units and to the application of moreradiation fields. The Linacs used today were not designed for IMRT and are based onleakage standards that were decided decades ago. It will be difficult and costly toreduce leakage from treatment machines, and a necessary first step is to refine theavailable radiation risks at the fractionated high doses characteristic of radiotherapy.The dose response for carcinogenesis is known for single doses up to about 2 Svfrom the A-bomb data, but the shape at higher fractionated doses is uncertain.Third, the proliferation of proton facilities. The improved dose distribution made possibleby charged particle beams has created great interest and led to the design and building ofmany expensive proton centres. However, due to technical problems, most facilitiesuse passive scattering, rather than spot scanning, to spread the pencil beam tocover realistic target volumes. This process, together with the methods used offinal collimation, results in substantial total body doses of neutrons. The relativebiological effectiveness of these neutrons is not well known, and the risk estimates aretherefore uncertain. Unless and until the risks are known with more certainty, it isdifficult to know how much effort and cost should be directed towards reducing, oreliminating, the neutron doses. These three examples, where uncertainties inquantitative risk estimates result in important practical problems, will be discussed.