Answer to Question #2121 Submitted to "Ask the Experts"

I have been reading your risk assessments for lifetime increased risk of cancer as a function of effective dose received. The exposure from medical and dental x rays is often compared with that received through natural (background) sources. However, is there a larger health effect from acute high doses versus low-level, constant exposure? For instance, one can imagine that DNA repair enzymes are up-regulated in a response commensurate to steady-state background exposure, but that these mechanisms are swamped by a single intense exposure hundreds of times the natural background exposure rate in a medical or dental x ray. Do the risk estimates therefore need to be adjusted depending on dose rate and not just cumulative exposure?

There are clear dose-rate dependencies of radiation bioeffects. All relate to differences between large doses at high dose rates and either very small doses at relatively high rates (such as diagnostic radiation exposures) or chronic low-dose-rate exposures (such as background). Virtually all of our risk estimates were derived from populations exposed to large doses at high rates, such as the Japanese atomic bomb survivors. Authoritative bodies have recommended dose-dose rate effectiveness factors (DDREF) of about 2 to convert these risk estimates to ordinary populations who are not exposed to such large doses. That is, risk of cancer per unit dose from the Japanese survivors has been on the order of 10% per sievert. This estimate has then been divided by the DDREF to get the best estimate of risk from small doses or chronic radiation, about 5% per sievert. All available data indicate no difference in risk between small dose increments such as diagnostic exposure in the healing arts and chronic exposure such as background. There is evidence that the DDREF of 2 is conservative, i.e., that it really is much greater than 2. There is further evidence that the linear no-threshold hypothesis used to estimate radiation risks does not apply to small doses. Thus, there are problems with our current risk estimates for small doses. However, for radiation protection purposes, most authorities recommend continued use of present data, feeling that it is better to err on the conservative side. We must always remember the ALARA principle: as low as reasonably achievable, taking economic and social factors into account. That is, we should eliminate radiation risks if it can be done at little or no cost. However, we should not invest scarce resources in attempts to reduce radiation risks that may be trivial while ignoring other risks that may be much greater.

S. Julian Gibbs, DDS