Radiation Weighting for Reference Animals and Plants

IRSN comments on TG-72 Draft report

« Radiation weighting for Reference Animals and Plants »

This report reviews data from studies dealing with Relative Biological Effectiveness (RBE) of low energy beta particle emissions from tritium and of alpha particle-emitting radionuclides, for non-human biota.

General comments

The main text is globally concise and well written but suffers from some inaccurate terms or lack of discussion on some points as explained below.

(i) now ICRP is explicitly moving towards a unique system of radiation protection, any report dealing with protection of the environment should preferably refer to protection of one of the categories taken into account in the system of radiation protection ie biota (beside public, workers and patients). This is to check all over the report

(ii) the 5 main points highlighting the finding of the report are clear but:

- the second one and the 4^th one could be reinforced and changed accordingly to the weaknesses expressed below:

• The experimentally-evidenced general pattern that RBE increases when dose rate decreases either for tritium or alpha particles should be indicated.
• For tritium as HTO (this is to be said), the authors concluded that RBE values are centered around 1.5-2 compared to X-rays and 2-2.5 compared with gamma rays. However, the 4^th main point, recommends a RBE of 1 for low-LET radiations and 10 for alpha particles. These conclusions may appear uncorrelated from values given at table B.5, page 59, which shows RBE data possibly rising up to 8.0. It seems that this conclusion is based on an assessment by expert judgment of the data retained. There has apparently been no attempt to statistically process the existing RBE data, which would improve the robustness of the conclusions through a transparent and reproducible reasoning. This choice should at least be justified if not revised. Actually, the authors could have a look to the findings of the EC-funded ERICA project where one public deliverable discussed this aspect for HTO (ERICA, 2006). Interestingly, fitting a log normal distribution to quality-assessed HTO RBE values for non-human species and reproduction endpoint came out with a median RBE of 1.1 (with a CI95% ranging from 0.6 to 1.8). Even more, the recommended RBE value for tritium in the TG72 draft report is not consistent with the conclusions of several published reviews (Tab. B.5), especially when gamma rays are the reference radiation (which is much most relevant for the use of RBE in radiation protection than X-rays)

-another main point should be added regarding the absence of information on plant species. A rationale should be developed to support any extrapolation from what is known to what is unknown and to recommend a transitional value of RBE for plants (low beta and alpha). Otherwise the report title has to be corrected since it deals to Reference animals and Plants.

(iii) the justification of adopting one value across species is not robust since a few number of species largely dominated by mammals have been examined in terms of RBE. Even more nothing is said about plants. One would expect at least some recommendations for research or a better underlying rationale if possible.

Annex A of the report presents the use of RBE in the context of protection of non human biota or the environment. The multiple factors that may influence this quantity, mostly adapted from the context of human radiological protection (use of cultured cells) are presented. It could be interesting to comment on other factors that may complicate the use of RBEs for non-human biota and that increase uncertainties in values (e.g. gender, life stage, variety of species sensitivities). The limits associated to the estimation of RBE for non-human biota may deserve a more detailed discussion (e.g., lack of data available for numerous species). The general context of the use of RBE in Ecological Radiological Risk Assessment tools (e.g. ERICA tool) could also have been slightly expanded.

Annex B presents RBE of tritium beta particles and Annex C RBE of alpha-emitting radionuclides. In these sections, more recent reviews on RBE for non-human biota should be used, as the authors have recalculated RBE based on published papers (Adam-Guillermin et al., 2012) or made an exhaustive review of data, including RBE, of ionizing radiation effects in invertebrates (Dallas et al., 2012). A specific working package has also been dedicated to the estimation of new RBEs for alpha emitters in the EC-funded network of Excellence STAR, with a publically available deliverable on this topic (Oughton et al., 2013). From all these documents, a complementary list of RBEs for non-human species, could be added to the draft report.

Specific comments throughout the document (text to be inserted in red characters, strikethrough text to be deleted)

Page 7, Line 173-174. The sentence should be reformulated to become clearer –“…The extent to which differences in such factors  then need to be taken into account, and how they impact on the final decision on the RBE value to use, will depend on the circumstances of the assessment, as outlined in Publication 124

P8, L196. ..to calculate values of radiation weighted absorbed dose rates….

P8, L212 and throughout the report: the term “reproductive failure” is restrictive in terms of associated endpoints. We think that “reproduction dysfunction” is more appropriate.

P8, L232: use “non human species” instead of “environmental protection “

P9, L248… when there is a possibility of effects in the species or populations

P9, L251-253. This sentence is confusing when ICRP promotes a unique system of radiation protection. It should rephrase accordingly

P10, L298. It is important to say that the statement regarding the uniform distribution of radionuclide in tissue/bodies is correct in case of tritium as HTO. For other tritiated molecules, this is surely not correct. The influence of tritium chemical form could be discussed there.

P10, L301. Respiration is more appropriate than inhalation since it works for any living organism (where inhalation does not – e.g., plants, fish)

P11, L315. Homogenize the spelling of endpoint // end-point throughout the text

P13, L367. There is only one value for invertebrates therefore the 's' should be removed

Page 32, B2. Some precisions should be made in the text: in living tissues, tritium beta particles travel on average 0.6 µm (6 µm being the maximum length of beta particle track in water). The typical cell size may be 10-20 µm for animal cells (although some nerve cells or eggs may be much larger) but plant cells are larger (~ 100 µm).        

References

Adam-Guillermin Christelle, Sandrine Pereira, Claire Della-Vedova, Tom Hinton, Jacqueline Garnier-Laplace. Genotoxic and Reprotoxic Effects of Tritium and External Gamma Irradiation on Aquatic Animals. D.M. Whitacre (ed.), Reviews of Environmental Contamination and Toxicology, Reviews of Environmental Contamination and Toxicology 220, DOI 10.1007/978-1-4614-3414-6_3 (2012)      

Dallas, Lorna J., Miranda Keith-Roach, Brett P. Lyons and Awadhesh N. Jha. Assessing the Impact of Ionizing Radiation on Aquatic Invertebrates: A Critical Review. Radiat. Res. 177, 693–716 (2012)

Oughton Deborah, Christelle Adam-Guillermin, Frédéric Alonzo, Turid Hertel-Aas, Nele Horemans, Alicja Jaworska, Catherine Lecomte-Pradines, Jan L. Lyche, Sandrine Pereira, Hans Christian Teien, Nathalie Vanhoudt. Deliverable 5.3. Radiation Quality: studying sub-cellular modes of action using biomarkers and “omics” tools. STAR report (Contract Number: Fission-2010-3.5.1-269672). (2013)

ERICA (2006). Derivation of Predicted-No-Effect-Dose-Rate values for ecosystems (and their sub-organisational levels) exposed to radioactive substances. Deliverable 5, 88 pages, Contract Number: FI6R-CT-2004-508847. https://wiki.ceh.ac.uk/display/rpemain/ERICA+reports