|Comments on the 2006 Draft ICRP Recommendations
In the National Institute of Radiological Sciences (NIRS), a working group*) was established to discuss the Draft Recommendation of the ICRP. Comments are provided below based on the consensus among members of the working group. We hope an improved new recommendation will be made taking into account our comments as well as others.
*) Hidenori Yonehara, Yoshiya Shimada, Shinji Yoshinaga, Hiroshi Yasuda, Keiichi Akahane, Shinji Tokonami,Satoshi Yoshida, Kazuo Sakai
Specific comments were also contributed by the following staffs in NIRS:
Yasushi Ohmachi, Yasuhiko Yoshimoto, Kiriko Miyamoto, Shoichi Fuma, Yoshito Watanabe, Kanae Nishizawa, Tetsuo Ishikawa
1. Precise estimates are available in the draft for cancer risks, based on recent data on cancer incidence among atomic bomb survivors. However, there are several issues which should be discussed carefully, including tissue weighting factors and nominal risk estimation which are shown below.
To avoid confusion it is preferable to keep the names and definitions of the present dosimetric quantities.
It is good that the radiation weighting factor (wR) of neutrons has been made consistent with qE over a broad range of energy. On the other hand, the wR values of protons and heavy ions still have large gaps with their qE values. These gaps are problematic particularly for dosimetry of space radiation; astronauts are often exposed to solar protons with energies of a few MeV (referred to in paragraph 106) which make up the dominant part of exposures during extravehicular activities.
2. The explanation about application of the effective dose in section 4.5.6 (paragraphs 139-144) is much appreciated. However, it is unclear in what situations the equivalent dose would apply. Can we use the equivalent dose to evaluate non-uniform exposure in which radiation energy deposits very locally into a small part of a tissue? Or in this case do we need to multiply the RBE value of a stochastic effect by the local absorbed dose?
3. We appreciate that the Commission put emphasis on continuity with the policy in Publication 60 with respect to the principles of protection, including justification, optimization of protection and limitation of maximum dose. Categorizing exposure situations as "planned", "emergency" or "existing" makes it easier to understand measures of protection. The description of the concepts of "practice" and "intervention" was almost lost, but the concepts still remain in the draft.
The principle of justification is considered to be the most important principle. However, the concrete requirements for general planned situations are insufficient, although the requirements for medical exposure of patients are described in detail. In the case of general situations, justification should be judged not only for comprehensive procedures by the government but also for daily practices by individuals.
Optimization is also very important as well as the justification. We feel the process of optimization should be performed on every occasion including not only planning protective action but also for individual daily practice. The dose constraint is taken as the starting point of the optimization process in the draft. For existing situations, the intervention level might be the target dose level of the optimization process. The target level has more relevance to the effects of the optimization process than dose constraint has. However, there is no description of how to set a target level for optimization processes in the planned situations. Although dose limitation still has an important role in dose criteria for radiation protection, dose constraint is described in the draft as the most fundamental level. Dose constraint should be considered a useful tool in the optimization process and should not be the most fundamental level. The characterization and functions of dose constraints in the three situations should be different. The common definition and characterization of dose constraints might be very simple and should be described more briefly. The use of dose constraints in each situation should be described in a different section.
4. It is good that the new draft includes material on medical radiation protection summing up current ICRP publications in a separate chapter, and that there is no drastic change from Publication 60.
The description of radiation therapy is insufficient compared with that of diagnoses. More detailed contents are needed on radiation protection therapy.
5. In light of some publications of ICRP (Publication 60 and Publication 75), it is inappropriate to neglect exposure to cosmic radiation on long haul flights. It is definitely a representative exposure to natural sources and the range of the effective dose and dose rate should appear as fundamental information for general readers. Also, it should be mentioned that assessments of cosmic radiation exposure for aircraft crews are already carried out in selected countries in accordance with past publications.
6. We appreciate the Commission's efforts to add the new chapter about the protection of the environment in the Recommendation. However, we found that the modifications to the previous draft (2005) are conservative, and thus several sentences are too vague. We think the following points should be reconsidered.
1.) ANNEX B, which appeared in the previous draft (2005), has been removed. It should be restored. Since the concept of the protection of the environment is introduced into the Recommendation as a new chapter for the first time, additional explanation in the form of an annex is necessary.
2.) There are several sentences excusing the adoption of this topic.
(352) line 5: But it also recognizes that .......
(357) line 10: not compromise the radiation protection .......
They make the importance of this topic uncertain. We recommend that such sentences be removed and that you concentrate on describing the importance and required future work as a group of scientists. If necessary, such sentences should appear in the beginning of the introduction.
3.) In the previous draft (2005), the importance of environmental radiation protection was explained in paragraph (242) based on the need to comply with the requirements of legislation directly aimed at the protection of wildlife and natural habitats, the need to make environmental impact assessments with respect to the environment generally, and the need to harmonize approaches to industrial regulation. Such requirements and the need to develop common international approaches are important driving forces of the environmental radiation protection. We think that paragraph (242) in the previous draft (2005) should be restored.
7. For the development of a framework for radiation protection of the environment, it is practical to adopt a Reference Animal and Plant approach and evaluate mortality, morbidity and reduced reproductive success (also reductions of the intrinsic rate of natural increase, which is a typical population-level parameter calculated from the age-specific survival rate and fecundity) of reference animals and plants. This approach can assess environmental impacts at population or lower levels, but cannot assess community-level impacts, which include indirect effects due to interspecies interactions and thus are more important for assessment of environmental impacts. This recommendation should therefore state that a primary approach for the assessment of environmental impacts of ionising radiation is evaluation of population- or lower level effects on reference animals and plants, but the evaluation of community-level effects using theoretical models or experimental model ecosystems is also required in some cases. Paragraphs 75-78 of Publication 91 would help the Committee to prepare such a description.
8. The general recommendations of the Commission play a key role in radiation protection, making this document different from other publications. Phrases such as "... is given in the forthcoming ICRP ..." and "... is provided in ..." should be avoided for the sake of consistency of the recommendation. At least brief comments should be described concretely.
• Tissue reactions
Threshold and mortality risks can be reduced by the improvement of medical care. Thus please add the following sentence: "It should be noted that the threshold for death from injury to bone marrow and the cancer mortality risk coefficient, which will be mentioned later, can be reduced by the improvement of medical care".
Paragraph 241, Table 5
Recent studies (eg Nakashima E et al Health Phys 90:154, 2006) show that the threshold for cataracts can be lower than the previous estimates, with central estimates of 0.6-0.7 Sv, although these estimates are not significantly different from 0. The Commission recognizes that the radiosensitivity of the lens of the eye needs to be reconsidered. The recommended dose limits are indicated in Table 5 (page 63). It is better at present to regard cataracts as one of the “tissue reactions” without threshold or stochastic effect. Significant change in concept of protection from induction of cataracts is expected in an additional publication. The dose limit in Table 5 should be described as a "tentative dose limit". More cautious description is required for dose limits for cataracts.
Table 5 or Table 3.4
It would be helpful to show in the tables (eg in Table 5, or Table 3.4 in Annex A) the dose limit for mental retardation of in utero exposures.
• Radiation effects in the embryo and fetus
Paragraph 81, 177 ,263
There is limited information on radiation effects in the embryo and fetus. The Commission states that "The working conditions of a pregnant worker should be such as to make it unlikely that additional dose to the fetus would exceed about 1 mSv during the remainder of the pregnancy" (page 48, 177). Therefore, "in utero exposure should not be a specific protection case in prolonged exposure situations where the dose is well below about 100 mSv" and "Doses below 100 mGy to the developing organism should not be considered a reason for terminating a pregnancy" in paragraphs 81 and 263, respectively, should be changed or deleted.
• Site-specific cancer risk estimation
In the draft, cancer risks are estimated by using recent data on cancer incidence among atomic bomb survivors for the tissues/organs for which a strong association with radiation exposure is established. However, for skin and bone cancers, atomic bomb data are not used, but crude risk estimates per unit exposure shown in ICRP Publication 60 is used. The reason why data on atomic bomb survivors are not used should be clearly explained, or such data should be used for risk estimation.
• Tissue weighting factor
It is well established from biological and epidemiological studies that cancer risks depend profoundly on age at exposure. It is recommended, therefore, that sets of values in association with age at exposure should be shown for tissue weighting factors as well as for nominal risks and detriments.
Epidemiological findings are taken into account in deciding tissue weighting factors. Indeed, the factors for thyroid, salivary glands, and brain are set higher according to the epidemiological findings, but there are no detailed descriptions of the reasons in the main text or Annex A. It is recommended to add explanations of such findings, except for thyroids. (Annex A)
It should clearly be noted that DDREF can be applied only for low-LET radiation.
A linear relationship is assumed for risk estimation, but it is not applied for detrimental effect or tissue response. It is recommended that a remark should be added in the text so that linear relationships are not used for estimation of detriments.
Animal data on tumor induction and life shortening (Annex A, page 40)
Radiation-induced thymic lymphomas and ovarian tumors are useful models for the analysis of the molecular mechanism of carcinogenesis. Thus, we would like to ask to change the word "atypical" into "inappropriate for the risk estimation of low dose radiation".
• Collective dose
The Commission judges that it is not appropriate to calculate the hypothetical number of cases of cancer that might be associated with very small radiation doses received by large numbers of people over very long periods of time. "Very small radiation dose" here is a very vague expression. It is recommended that a concrete value be specified where collective doses can be used.
The usefulness of collective doses in a specific situation is shown, but many people cannot judge which situation is good for such calculations. It is recommended that detailed examples of the situations under which collective dose is useful should be described.
• Dose quantities
The organ absorbed dose (DT,R) is vaguely defined. This quantity is a protection quantity because it is not measurable and is defined as the average of absorbed dose over tissue "mass" (not "volume" as written in paragraphs 96-97). The equation for calculation of DT,R as given by ICRU (eg ICRU report 57) should be added in this section.
The operational quantities surely aim to provide an estimate or upper limit for the value of the protection quantities, but this aim is not established well for high-energy particles (neutrons and ions) that deposit energies in a deep part of the tissue. It is desirable to state this fact with some future perspectives; eg we could say that the gap between E and H*(10) of neutrons would be smaller to some extent by the use of the new wR values presented in this draft.
• The system of radiological protection of human
The definition of a single source is important when dose constraints would be set. However a description of the application of the concept of single source to the concrete case should be described in an additional publication.
There are some cases in which it would be very difficult to judge that sufficient net benefit is produced to exposed individuals or to society. The justification process procedure should be described in an additional publication.
Justification should be judged not only for comprehensive procedures by the government but also for daily practice. Descriptions of justification in individual practice should be added in this paragraph.
Abnormally high levels of natural background radiation are classified with the band of 20-100 mSv. It is difficult to distinguish abnormal level from normal level. Criteria or concrete example should be provided.
It is stated that not all aspects of optimization can be codified. It would be possible for the operating managers at each site to codify the procedures for optimization. To make a rule for optimization which is appropriate to the conditions of each site would be very effective. In order to clarify the responsibility of the manager at the site for optimization, requirements should be provided in the legislation that requires the manager to make a rule for optimization.
• Medical exposure of patients
The title of the section is "Diagnostic reference levels". However, the section includes a description of optimization of radiotherapy. These subjects are quite different, so they should be described separately.
The title of the section is "The optimization of protection for patient comforters and carers". However the section has a description of "Volunteers for research" and "Medico-legal exposures". These issues should be described separately, or the title should be changed.
• Exposure to natural sources
Only the result by Darby et al (2005) of 16% / 100 Bq m-3 is emphasized in the present section. Risk estimates from North American (Krewski et al) and Chinese (Lubin et al) pooled analyses should be referenced as well because any consistency between the different pooled analyses can be checked.
In ICRP Publication 65, 1500 Bq m-3 at workplaces and 600 Bq m-3 at homes are defined not as the constraint but as the action level. This sentence should be rewritten.
The constraints in the cases of exposure to radon emanating from a radium source under regulation and radon emanating from a uranium waste source in the distant future need to be stated clearly.
Thoron should not be neglected. Thoron interference to radon measurements has recently become an emerging issue because radon concentrations are overestimated by some common radon detectors used in laboratory work and field measurements. In particular, this frequently happens in radon measurements in dwellings. We have such data in an epidemiological study in Gansu (China). Since thoron problems will definitely become hot as future topics, the current statement is not adequate.
Ref. 1 Tokonami et al Health Phys. (2001)
Ref. 2 Tokonami et al Radiation Res. (2004)
• Protection of the environment
We think that the following sentence should be added to line 11 of this paragraph to explain why it is necessary to consider a wide range of exposure and environmental situations:
..., irrespective of any human connection with them. In some situations, radiation exposure to humans would be minimal, but other members of the flora or fauna could receive considerable exposure. The Commission notes that...
(From Paragraph 82 of Publication 91 with minor modification)
We think that secondary reference organisms are important for assessment of local environments. The following sentences should be therefore added to the end of this paragraph:
... and the potential consequences of such effects. It is, however, unlikely that the sole use of such a limited set of reference organisms would serve to satisfy all assessment needs. Therefore, the ICRP's reference set could be supplemented by information on locally characteristic types of other reference organisms for particular ecosystems.
(From Paragraph 130 of Publication 91 with some modification)