Answer to Question #13633 Submitted to "Ask the Experts"

Is there a quantity called "lifetime equivalent dose" or "lifetime eye dose" or "lifetime hand dose" similar to the quantity "lifetime effective dose"? If yes, can the same formalism used to estimate the lifetime effective dose be used to estimate the others?

The brief answer to your question is "Yes and No." I’ll discuss the "No" part first. This applies to tissue reactions that are not cancer and not genetic effects, such as possible effects to the skin, eye, or any other individual tissue. The National Commission on Radiation Protection and Measurements (NCRP) Report No. 116, "Limitation of Exposure to Ionizing Radiation," has recommended a cumulative effective dose limit of 10 mSv times the worker's age in years; for a retiring 75-year-old worker, the allowed "lifetime" effective dose limit would be 750 mSv. The ICRP has recommended a lifetime worker effective dose limit of about 1 Sv. However, there are presently no recommendations as to the adoption of lifetime limits for the nonstochastic effects, now referred to by the ICRP as tissue reactions. Such effects were first referred to as nonstochastic effects and later as deterministic effects and are radiation-induced effects that are typically characterized as exhibiting a threshold response—i.e., some minimum dose is required to produce the effect.

This is unlike the situation for stochastic effects, such as cancer and genetic effects for which the probability of an effect is assumed to be proportional to the dose, but the severity of the effect is independent of the dose. Additionally, we assume that the damage produced is cumulative, recognizing, though, that some repair may occur between separate exposures. For tissue reactions, a particular effect, such as skin erythema requires a minimum dose (although the magnitude may naturally vary somewhat among individuals) to produce observable reddening. At doses below this threshold, some damage to cells may occur but may not be sufficient to produce the effect, and some or all of the damage may be repaired following exposure. At doses above the threshold the effect may worsen in severity.

It has been recognized for some time, and discussed earlier in International Commission on Radiological Protection (ICRP) Publication 41, Nonstochastic Effects of Ionizing Radiation that it is not feasible to attempt to quantify the extent of damage from nonstochastic effects at low dose rates; this makes it impossible to establish an acceptable relationship between the severity of the effect and the dose, and this restricts our ability to recommend a lifetime dose limit to a specific tissue. For most tissues that are subject to a 0.5 Sv y^-1 limit (for the lens of the eye the annual limit is recently at 0.15 Sv), a 25 Sv lifetime dose might be inferred for a working lifetime of 50 years, although this is not considered as a real limit and is not the same as specifying a lifetime dose independent of the annual limit.

Regarding the use of equivalent dose (the product of the absorbed dose and a radiation weighting factor), as is discussed somewhat in the 2007 ICRP Publication 103, the relative biological effectiveness factors for inducing tissue effects are typically smaller than the values that are used when stochastic effects are being considered. This has the consequence that the typical radiation weighting factors that would be used to calculate equivalent dose when considering tissue effects would be too large for exposures to high LET radiation. Thus, for high LET radiations, equivalent dose is really only well-suited to considering the contributions of particular tissue doses to the effective dose, in which case the equivalent dose to a tissue is multiplied by its tissue weighting factor and added to the sum of all such values for significantly irradiated tissues.

This is where the "Yes" part of our answer might apply—to the part of your question that specifically mentions equivalent dose in that it would be a part of the effective dose determination, which is subject to a lifetime limit. Under current ICRP recommendations, for a case such as irradiation of a single tissue, as when radioiodine is taken up internally, and the thyroid is the only significantly irradiated tissue, the stochastic dose lifetime limit would still be the effective dose limit, all of which would be associated with the thyroid dose. The thyroid would be protected from nonstochastic effects because of the low limit for effective dose. I suspect that this aspect is not what you were really interested in, and the reality is that there exists no lifetime dose limit for nonstochastic tissue reactions.

The ICRP has noted that the current dose limits that apply to stochastic effects, for which effective dose is the controlling quantity, are sufficiently low that it is unlikely that any individual tissue will be irradiated to a dose sufficient to produce any discernible tissue reactions.

I expect the above is not exactly what you were hoping for, but it represents the current thinking, and I don’t see much likelihood of the near-term adoption of lifetime dose limits based. on specific tissue reactions

George Chabot, PhD, CHP