Answer to Question #8542 Submitted to "Ask the Experts"

Is there a dose/dose rate limit where the quality factor/radiation weighting factor should no longer be used? In 10 CFR 835, for example, units are normally rem. However, for “Very High Radiation Areas” the unit is rad. Where/why does this change occur?

Your questions are good ones that have engendered and continue to engender some confusion among individuals concerned with radiation protection. I shall attempt to address your question of what high dose and dose rate are and to provide my understanding of what is intended in specifying high doses as absorbed doses rather than effective or equivalent doses.

To my knowledge, the question of what values constitute high dose and high dose rate has not been absolutely answered. However, we can make some reasonable inferences from information provided by the International Commission on Radiological Protection (ICRP) (ICRP-103, Recommendations of the International Commission on Radiological Protection, 2007) and others. In the cited ICRP Publication 103, Chapter 3, paragraph 57, referring to the bulk of material to come later in the report, it is stated, “The views developed by the Commission are summarized in this chapter with emphasis on effective doses of up to about 100 mSv (or absorbed doses of about 100 mGy of low-LET radiation) delivered as a single dose or accumulated annually.” In paragraph 60 the report goes on to state that at acute or chronic doses of high- or low-LET radiations below 100 mGy, no tissues have expressed clinically relevant impairment (i.e., deterministic effects usually associated with a threshold dose). In later paragraph 64, the report notes that ICRP has accepted the applicability of the linear no-threshold model for doses less than about 100 mSv. The Health Physics Society published a position paper in 1996 that affirmed the conviction that doses below about 0.05 to 0.10 Sv are insufficient to produce observable health effects. Considering these opinions and recommendations, it seems reasonable to use a value of about 100 mSv as a demarcation between low and high dose.

As to what constitutes high dose rate, I believe it is reasonable to assume that high doses delivered over time intervals of a few minutes, possibly even several hours, could be judged as accruing at a high rate. In reality, most of the important epidemiological data, the Japanese bomb victim data in particular, represent high doses occurring over very short time intervals. Also, many of the high-dose accidents that produced acute radiation effects have occurred over short exposure durations, ranging from near instantaneous to several hours.

The quantities equivalent dose and effective dose are biological dose quantities, with units of Sv, and they have always been intended for use only in routine radiation protection. Effective dose limits are founded on the basis of ensuring that deleterious stochastic effects of radiation exposure are maintained at acceptable levels. The stochastic effect of major concern is cancer, although life-span shortening and genetic effects are also considered. The ICRP recommends against using effective dose for retrospective detailed evaluation of detriment or risk to an individual or for evaluations in epidemiological studies. This is stated in the following paragraph 157 of the cited ICRP report:

(157) Effective dose is intended for use as a protection quantity on the basis of reference values and therefore is not recommended for epidemiological evaluations, nor should it be used for detailed specific retrospective investigations of individual exposure and risk. Rather, absorbed dose should be used with the most appropriate biokinetic biological effectiveness and risk factor data. Organ or tissue doses, not effective doses, are required for assessing the probability of cancer induction in exposed individuals.

In the event of acute high doses to individuals, the immediate concerns are with possible deleterious effects of a deterministic nature (e.g., depression of the blood-forming system, for which the threshold dose is about 0.5 Sv). In such events the radiation weighting factors that were used to establish limits on stochastic effects have no immediate utility. At high doses the ICRP recommends against using effective dose and equivalent dose, as described in paragraph 105 of the cited document:

(105) At high doses, and especially in emergency situations, radiation exposures may cause deterministic effects (tissue reactions). Such clinically observable damage occurs above threshold doses. The extent of damage depends upon the absorbed dose and dose rate as well as radiation quality (see Annexes A and B) and the sensitivity of the tissue. In general, values of relative biological effectiveness (RBE) for tissue reactions caused by high-LET radiations are found to be lower than those obtained for stochastic effects at low doses, and the relative sensitivity of tissues also differs. The quantities equivalent dose and effective dose should not be used to quantify higher radiation doses or to make decisions on the need for any treatment related to tissue reactions. For such purposes, doses should be evaluated in terms of absorbed dose (in gray, Gy), and where high-LET radiations (e.g., neutrons or alpha particles) are involved, an absorbed dose, weighted with an appropriate RBE, should be used (see Annex B).

    Thus, for high acute doses, the doses should be reported as absorbed doses, with SI units of Gy. When high-LET radiation is involved, as stated in the ICRP paragraph above, the absorbed doses to tissues of interest should be modified by the respective relative biological effectiveness factors (RBE). The specific value of the RBEs that apply may be significantly different from the radiation weighting factors (or older quality factors) used in routine radiation protection that have been based on different biological (stochastic) endpoints. As is noted above in paragraph 105, generally RBE values for nonstochastic effects (deterministic effects) are lower than those for stochastic effects.

Your question has given me an opportunity to consolidate some thoughts and information that I had not previously done. I have found this useful, and I hope you find the answer helpful.

George Chabot, PhD, CHP