|PART 3 OF COMMENTS IN 6 PARTS
Comments on Draft for Consultation
2005 RECOMMENDATIONS OF THE INTERNATIONAL
COMMISSION ON RADIOLOGICAL PROTECTION
David C. Kocher
SENES Oak Ridge, Inc.
Center for Risk Analysis
Oak Ridge, Tennessee, U.S.A.
Quantities Used in Radiological Protection
Paragraph (36), end of last sentence. Examples of “specific data” might be given including, for example, organ-specific risks and relevant data on biological effectiveness of different radiation types.
Paragraph (37). This is a helpful discussion.
Paragraph (38), first sentence. What does the Commission mean by “low dose”? Is it defined in terms of an absence of certain kinds of radiation effects (e.g., cell killing), does the Commission have in mind a particular upper bound of a “low” dose, or what?
Paragraph (39). I think that these discussions overstate the degree of correspondence between effective dose and risk of stochastic health effects. Yes, there is some correspondence, but it is only approximate. There are several issues here including that (1) radiation weighting factors are based on the Commission’s definition of detriment, rather than risk itself, (2) tissue weighting factors are rough approximations, as appropriately acknowledged by the Commission, and (3) effective dose often corresponds poorly to risk in cases of intakes of long-lived alpha emitters which are tenaciously retained in the body (use of committed effective dose is the culprit here). A minor issue is that effective dose takes into account risks of hereditary effects as well as cancers, and these two kinds of risk are not experienced in the same individuals. Thus, I think that the Commission needs to be more careful not to overstate the correspondence between effective dose and risk of stochastic effects.
Paragraph (40). Although the change from “deterministic effects” to “tissue reactions” does not seem objectionable, why was it made? What is the problem with “deterministic effects” that “tissue reactions” is intended to address? Here is a case where I believe that the Commission has not been suitably candid in explaining a change in its recommendations.
I have two concerns about the last sentence. First, I think that the Commission’s use of “RBE” is somewhat inappropriate, or at least requires explanation, because the strict definition of RBE clearly points to results of specific radiobiological studies under controlled conditions. Thus, any application of an RBE to another situation (such as exposures of humans) is not really an RBE (it is something else). For example, I have used a new term “radiation effectiveness factor (REF)” to describe the biological effectiveness of different radiation types in inducing cancers in humans and used relevant data on RBE to infer REFs; e.g., see NCI/CDC (2003).
The second concern is that the version of the draft 2005 Recommendations which I copied from the Commission’s web site does not include Section 3.6, as advertised here. I urge the Commission to provide recommendations on biological effectiveness of different radiation types in inducing tissue reactions.
Paragraphs (41)-(47). I found these discussions to be quite informative. They raised two questions in my mind. Does the discussion of high-LET radiation in paragraph (46) imply that cell killing could be important even when the average absorbed dose in an organ or tissue is low? If so, how would this affect the relationship between average absorbed dose and induction of stochastic effects discussed in paragraph (48)? In regard to the discussion in paragraph (47), is it worth noting that the ICRU has used lineal energy, which is a stochastic quantity, rather than LET, to emphasize the point that the Commission has made a choice from two possibilities?
Paragraph (48). These discussions (and the following two paragraphs) apply to stochastic effects at low doses. Does the Commission intend to discuss the use of absorbed dose in protection against tissue reactions? Is this discussion in the missing Section 3.6?
Paragraph (51). The Commission undoubtedly spent much time on the matter of what to call the average absorbed dose modified by a radiation weighting factor (equivalent dose in Publication 60), but “radiation weighted dose” somehow seems less than totally satisfactory. On the other hand, given the definition of “dose equivalent” in terms of the quality factor and its use in operational quantities, something like “average dose equivalent” won’t work. If other usable suggestions are forthcoming from commenters, I hope that the Commission will consider them.
Paragraph (51), last sentence, and paragraph (56), first sentence. Although the Commission acknowledges later, e.g., in paragraph (63), that low-energy photons are biologically more effective than high-energy photons, I think that these statements give the wrong impression that x rays and gamma rays are largely equivalent in biological effectiveness.
Paragraph (54). The cautions about inappropriate uses of effective dose are agreeable, and the next-to-last sentence gives a more reasonable statement about the main use of effective dose than the discussion in paragraph (39), on which I commented above. However, the statement in the second sentence after “because” is incomplete because it fails to mention the importance of judgments about radiation weighting factors which, in some cases, do not conform to the preponderance of radiobiological data.
Paragraph (55). This is a valuable discussion.
Paragraph (58). This is a valuable discussion, and I hope that the missing Section 3.6 will include similar discussions in relation to tissue reactions.
Paragraph (59). Is it true that radiation weighting factors account for the “detriment” by different radiation types relative to photons? It seems to me that they are based on data on incidence of stochastic effects in radiobiological studies, with no adjustments of the kind used in the Commission’s definition of detriment; i.e., radiation weighting factors essentially account for differences in risks of stochastic effects associated with given absorbed doses of different radiation types, not differences in detriment. Detriment enters the picture only in deriving tissue weighting factors, which have nothing to do with radiation weighing factors.
Paragraph (62). The statement in the second sentence that 60Co gamma rays or “medium to high energy x rays” are the usual reference radiations, together with the statement in the last sentence about significant differences in radiation quality between 60Co gamma rays and “low energy x rays,” give a misleading impression about the biological effectiveness of photons of different energy. Setting aside the problem that “medium to high energy” and “low energy” in regard to x rays are not defined and, therefore, are ambiguous, these statements give the misleading impression that there is no difference in biological effectiveness between 60Co gamma rays and medium to high energy x rays. This misleading impression is corrected in the next paragraph, but I think that more care is needed here. Perhaps a reasonable fix would be to delete “low energy” in the last sentence, or change it to “orthovoltage and lower energy.”
I also think that the Commission could indicate that 60Co gamma rays are the most appropriate reference radiation, essentially because they are close in biological effectiveness to the spectrum of photons to which Japanese atomic-bomb survivors were exposed.
Paragraphs (63)-(67). As indicated in a general comment, I do not agree with the Commission’s recommendation that a radiation weighting factor of one should be used for photons and electrons of any energy. In my view, this position cannot be defended on scientific grounds, and it casts considerable doubt on the idea that the Commission’s system is protective of human health. I accept that the Commission prefers not to change its position, but I believe that explanations of this position in Publication 60 and here are rather poor excuses. The excuse noted in paragraph (63) concerning large uncertainties in estimating radiation risk factors seems particularly lame, because the system of radiological protection is not really based on risk and uncertainties in organ-specific radiation risks aren’t all that important in developing tissue weighting factors and in estimating effective dose. I also believe that it would not be difficult to incorporate changes in radiation weighting factors for photons and electrons in a practical system of radiological protection, as discussed below.
Let me consider photons first. In my view, data which led to discussions of biological effectiveness in paragraph (63) provide sufficient support for a position that the radiation weighting factor for orthovoltage x rays should be in the range of 2 to 3, essentially as recommended in ICRU Report 40 by a joint ICRU/ICRP committee 20 years ago, and that the radiation weighting factor for lower-energy x rays should be in the range of 4 to 6. Of course, photon energies to which these radiation weighting factors apply would need to be specified. A calculation of the effective quality factor in ICRU Report 40 suggests that a value of 2 to 3 could apply at energies of about 30-250 keV and a value of 4 to 6 could apply at energies less than about 30 keV. However, given that there are few (if any) data at energies above about 100 keV, judgment is needed in defining the higher energy range.
How could these radiation weighting factors be used in a practical system of radiological protection? For external exposure to mixed fission or activation products or any other situations where the effective dose would be determined primarily by higher-energy photons or there is a broad spectrum of photons of various energies, a radiation weighting factor of one reasonably could be used. I see no need to account for differences in biological effectiveness depending on photon energy in such cases. Thus, an assumption of a radiation weighting factor greater than one for lower-energy photons would be most useful in assessing doses (effective doses or radiation weighted doses) from exposure to x rays used in medicine. There should not be any difficulty whatsoever in changing how such doses are estimated to reflect more reasonable assumptions about biological effectiveness. Arguments in paragraph (66) do not apply in cases of medical exposure to x rays in the absence of exposure to higher-energy photons or to exposure of employees or members of the public at x-ray facilities; these are common situations which affect a large number of people. It also is conceivable that external exposure to radionuclides which emit only lower-energy photons could occur in some cases. If so, it again would not cause any difficulty to use a higher biological effectiveness in estimating effective dose. External exposure to 241Am in smoke detectors is a good example.
Incorporating higher radiation weighting factors for lower-energy photons in internal dosimetry would require that dose coefficients be recalculated. However, new calculations would need to be performed only once, and they would not present any difficulties when existing codes already calculate absorbed dose from every energy of photons emitted by every radionuclide for every combination or source and target organ or tissue. All that would be required is to segregate calculations for photons by energy and apply the appropriate radiation weighting factor to emitted photons in each energy range. This is little different from impacts of changing the quality factor for alpha particles years ago from 10 to 20. Once this is done, there would be no impact on radiological protection programs.
There is increasing recognition among radiobiologists that x rays are biologically more effective than high-energy gamma rays. This recognition makes the Commission’s position increasingly less tenable. For example, there have been several recent studies of the biological effectiveness of low-energy mammography x rays; e.g., see the paper by Heyes and Mill in Radiation Research, Vol. 162, page 120. I do not see how the Commission can ignore results of these studies in formulating its recommendations on radiological protection.
In regard to electrons, I think that there are two problems with the Commission’s recommendation on a radiation weighting factor. First, it ignores an extensive amount of radiobiological data which clearly indicate that tritium beta particles are about 2 to 3 times more effective than high-energy gamma rays in inducing stochastic effects. Indeed, this was recognized back in Publication 2, but was later ignored by the Commission (and for unknown reasons). There is no excuse whatsoever for ignoring this today, because intakes of tritium are always analyzed separately from intakes of other radionuclides by urinalysis and, therefore, there would be no impact on radiological protection programs other than the need to increase effective dose coefficients. On the basis of information on the biological effectiveness of low-energy photons, such an increase in radiation weighting factor also should be applied to other radionuclides that emit electrons of energy less than about 15 keV. This also would have no impact because all such radionuclides need to be analyzed separately by bioassay.
The other problem is that, in presenting the recommended radiation weighting factor for electrons, the Commission no longer includes a caution about applying a radiation weighting factor of one to Auger electrons, which usually have low energies on the order of 1 keV or less. Does the Commission seriously believe that the biological effectiveness of these electrons is the same as that of high-energy photons, and that there is no longer any need to consider higher radiation weighting factors, especially when an Auger emitter is incorporated in DNA? If so, such a position is rather ridiculous.
In general, an increased biological effectiveness of electrons is of concern only in cases of internal exposure, because electrons of energy sufficiently high to deliver an external dose to the skin should have about the same biological effectiveness as higher-energy photons. So, I do not see that including a higher radiation weighting factor for low-energy electrons could impose any hardship or increased complexity in estimating effective dose in any situations.
Cautions in paragraph (67) about low-LET radiations in general are reasonable. However, they fail to note that it is not appropriate to use the Commission’s recommendation on calculating radiation weighted doses from exposure to photons in assessing risks from medical exposure to x rays. To me, neglect of an increased biological effectiveness of lower-energy photons in evaluating the risk-benefit tradeoffs of medical procedures is one of the most important deficiencies in the recommended system of radiological protection.
Paragraphs (68)-(74). I have two concerns about the new recommendations on radiation weighting factors for neutrons. First, the continuous function presented in Publication 60 is retained at neutron energies of 1 MeV and above even though that function was not the actual recommendation in Publication 60 and Publication 60 noted that the continuous function was a matter of calculational convenience with no biological basis.
Second, I wonder whether a continuous function for the neutron radiation weighting factor is practical. Neutron dosimetrists tell me that they often have only a rough idea about the energies of neutrons to which people are exposed. Therefore, it seems to me either that use of a continuous function will place demands on neutron dosimetry that can be met only with great difficulty, or that estimates of neutron effective doses will be based on a pretense that neutron energies incident on the body are known when they aren’t (i.e., there will be an unwarranted appearance of precision in estimating effective doses). Thus, neutron dosimetrists may not be happy with this change. To me, the previous step-function radiation weighting factor was useful in that effective dose could be estimated in a reasonable way without having to know incident neutron energies in detail. For example, fission neutrons could be assigned the radiation weighting factor of 20 for 0.1-2 MeV neutrons without the need to worry about details of fission neutron spectra at locations of exposed individuals.
I would also comment that there is a certain incongruity in the Commission’s recommendations on radiation weighting factors for different radiation types. The Commission has gone to great lengths to deal with fairly low values at some neutron energies but has completely ignored what could be substantially higher values for low-energy photons. This situation becomes even more incongruous when the number of people who are exposed to neutrons (few) and low-energy photons (many) is considered.
Since the draft document does not include a glossary, the quantities H sub E and D sub E in equation (4) need to be defined. How do they differ from H sub T and D sub T defined previously? The quantity QT also needs to be defined.
Paragraph (75). I have no problems with the recommendation on a radiation weighting factor for protons. Here again, however, the Commission is dealing with an effect that appears to be no more than a factor of 2 while completely ignoring what is likely to be a substantially greater effect for low-energy photons, so we have another incongruity. Also, I am struck by the fact that the Commission apparently chose a radiation weighting factor for protons toward the upper end of the range of measured RBEs. I have no objection to this, but a similar approach might be used for other radiation types, especially alpha particles as noted below.
Paragraph (77), last sentence. What is the justification for a statement that a radiation weighting factor of 20 for alpha particles “may be a rough conservative estimate”? Reviews of data on RBE indicate to me that 20 is a good approximation of a central estimate, and that values as high as 60-100 are possible, as noted in Publication 31 (which, interestingly enough, recommended a quality factor of 30, rather than the value 20 adopted in Publication 26).
Especially in the case of alpha particles, but preferably for all radiation types, I think that the Commission should include statements on whether recommended radiation weighting factors are intended to be central estimates or biased conservatively. Summary statements about uncertainties in RBEs that underlie radiation weighting factors also should be provided.