Journal of Environmental Radioactivity 87 (2006) 1e14 www.elsevier.com/locate/jenvrad Choosing an alpha radiation weighting factor for doses to non-human biota Douglas B. Chambers a,*, Richard V. Osborne b, Amy L. Garva a a SENES Consultants Limited, 121 Granton Drive, Unit 12, Richmond Hill, Ontario L4B 3N4, Canada b Ranasara Consultants Inc., P.O. Box 1116, 7 Pine Point Close, Deep River, Ontario K0J 1P0, Canada Received 19 May 2005; received in revised form 19 October 2005; accepted 25 October 2005 Available online 27 December 2005 Abstract The risk to non-human biota from exposure to ionizing radiation is of current international interest. In calculating radiation doses to humans, it is common to multiply the absorbed dose by a factor to account for the relative biological effectiveness (RBE) of the radiation type. However, there is no international consensus on the appropriate value of such a factor for weighting doses to non-human biota. This paper summarizes our review of the literature on experimentally determined RBEs for internally deposited alpha-emitting radionuclides. The relevancy of each experimental result in selecting a radiation weighting factor for doses from alpha particles in biota was judged on the basis of criteria established a priori. We recommend a nominal alpha radiation weighting factor of 5 for population-relevant deterministic and sto- chastic endpoints, but to reflect the limitations in the experimental data, uncertainty ranges of 1e10 and 1e20 were selected for population-relevant deterministic and stochastic endpoints, respectively. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Biota; Alpha; Radiation; Deterministic; Stochastic; Relative biological effectiveness; Radiation; Weighting factor 1. Introduction The risks of ionizing radiation to non-human biota (biota) are of considerable current inter- est, and both the International Commission on Radiological Protection (ICRP) and the * Corresponding author. Tel.: þ1 905 764 9380; fax: þ1 905 764 9386. E-mail address: [email protected] (D.B. Chambers). 0265-931X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2005.10.009 2 D.B. Chambers et al. / J. Environ. Radioactivity 87 (2006) 1e14 International Atomic Energy Agency (IAEA), among others, have on-going activities in this area. This paper is based on a report completed for Cogema Resources Inc. (SENES, 2005) to compile and systematically evaluate currently available literature on experimental RBEs and to recommend how these data may best be interpreted to provide a nominal value (or range of values) of (biota) radiation weighting factors that could be used in environmental risk assess- ments. These alpha radiation weighting factors were developed by examining the doseeeffect relationships for a variety of biota in which data were available. We note that a current goal for both international and national agencies (e.g., ICRP) is finalizing a list of reference organisms. Once a list of reference organisms has been agreed upon, further work could be completed on this topic to verify that the alpha radiation weighting factors recommended in this paper are still generally applicable. 1.1. Background Radiation effects on biota depend not only on absorbed dose, but also on the type or ‘‘qual- ity’’ of the radiation. For example, alpha particles and neutrons can produce observable damage at much lower absorbed doses than beta or gamma radiation. Thus, the absorbed dose (in Gy) is multiplied by a modifying factor e alternatively called the relative biological effectiveness (RBE) (strictly, the term RBE is reserved for the experimentally observed values but not all adhere to this constraint), quality factor, radiation weighting factor, ecodosimetric weighting factor e in order to account for the relative effectiveness of the different types of radiation (al- pha particles, neutrons, etc.). At present, there is no generally accepted name for this factor in the context of dose to biota. For purposes of brevity and consistency, this paper refers to this factor, derived from experimentally determined values of RBE, as a (biota) radiation weighting factor. Our intention is that such a factor can be applied in an ecological risk assessment involv- ing internally deposited alpha-emitting radionuclides. The concept of RBE is illustrated in the following equation and can be understood as the ‘‘inverse ratio of absorbed doses of different quality radiations, delivered to the same locus of interest, that produce the same degree of a given biological effect in a given organism, organ or tissue’’ all other factors being equal (NCRP, 1967), namely: Dose of reference radiation needed to produce a given effect RBE ¼ ð1Þ Dose of the given radiation needed to produce ðthe same magnitude ofÞ the same biological effect RBE depends on many factors, including the type of cell or tissue irradiated, dose and dose rate, the distribution of Linear Energy Transfer (LET) or linear energy, the effect of interest, and other factors. The concept of RBE is complex and involves many considerations and sources of uncertainty and a full discussion of the factors influencing the use of experimental RBE data for application as (environmental) radiation weighting factors for non-human biota is beyond the scope of this paper. However, a few important issues are briefly noted below. Amongst many other factors, RBE depends on LET, which is the amount of energy absorbed by the target tissue per unit path length. Low LET radiations such as X-rays, gamma rays or electrons of any energy have an average LET of about 3.5 keV/mm (of water) or less (NCRP, 1967, 1990). Gamma rays from 137Cs or 60Co and 250 kVp X-rays have been used as the ‘‘stan- dard’’ or ‘‘reference’’ radiations. It is important to understand, in looking at the literature, that 60Co gamma rays are less effective than 250 kVp X-rays in producing radiobiological effects. At high doses, the difference is small (RBE ¼ 0.86 for 60Co relative to X-ray as the standard); however, the difference is larger at lower doses (ICRU, 1986). For many purposes, the D.B. Chambers et al. / J. Environ. Radioactivity 87 (2006) 1e14 3 difference in the relative effectiveness of 60Co gamma rays and 250 kVp X-rays can be taken as about a factor of 2 (NCRP, 1990; ICRU, 1986; ICRP, 1989). In many systems, the RBE increases with increasing LET until the LET reaches about 100 keV/mm and then begins to decline. This phenomenon is shown for example in the impair- ment of regenerative capacity of cultured human cells inactivated by monoenergetic particles (Nikjoo et al., 1999). The peaking of the RBE at an LET of about 100 keV/mm occurs for sev- eral complex reasons; however, in general it only requires a few tens of keV of energy to break a single strand of DNA and a single alpha particle with an LET of 100 keV/mm is sufficient to produce a double strand break which is prone to imperfect repair and may result in the death of the cell. Thus at LETs greater than about 100 keV/mm, there is sufficient energy to ensure a dou- ble strand break in target DNA and the breaks induced by the additional energy deposition can- not kill an already terminally damaged cell. The dosimetry of internally deposited alpha emitters remains the source of many questions. Consider for example, the studies shown in Table 1. According to ACRP (2002), some of the results were analyzed on the basis that the alpha-emitting radionuclides were uniformly distrib- uted throughout the organ of interest. However, in the dose ranges reported (0.1e10 mGy), only a few cells would have received very high doses; the vast majority of cells would have received no dose at all. The effect of this inhomogeneity on the dose results in those estimates of the RBE is to bias towards high values. For example, as noted by the ACRP (2002), Samuels (1966) high value of 377 was based on a single data point in a study on cell-killing in mouse oocytes following injection of the alpha source 210Po. An assumption of a uniform distribution gave an alpha dose of only 0.1 mGy to the entire mouse ovary. Samuels himself was skeptical of this result, and recommended an RBE not higher than the values of 50e100. 1.2. Recent evaluations of alpha RBE for non-human biota Over the past decade, a number of authors have reported evaluations of published data on RBE (e.g., ACRP, 2002; DOE, 2002; EC and HC, 2001; Copplestone et al., 2001; UNSCEAR, 1996; FASSET, 2003; NCRP, 1991; Trivedi and Gentner, 2000). Nominal values for an alpha radiation weighting factor from these reviews are summarized in Table 2. In considering these Table 1 Summary of selected studies with alpha RBE values greater than 20 (ACRP, 2002) RBE Author(s) Endpoint Comment 377 Samuels (1966) Cell-killing in mouse oocytes Based on single point; author urged caution, suggested only 50e100 250e360 Jiang et al. (1994) Fetal hemopoetic stem cell Assumed uniform dose distribution. deficit in mice A repeat experiment gave 150 245 Rao et al. (1991) Spermhead abnormalities Assumed uniform dose distribution. in mice Poor statistics 50e100 Brenner et al. (1991) Lens opacification in rats For argon ions. Relevance to survival not clear 65 Brooks et al. (1995) Micronuclei in rat lung Conversion from WLMa to mGy fibroblasts alpha dose was suspect 37e60 Martin et al. (1995) Transformation of Syrian Poor statistics hamster embryo cells a WLM e Working level month. 4 D.B. Chambers et al. / J. Environ. Radioactivity 87 (2006) 1e14 Table 2 Radiation weighting factors for internal alpha radiation for deterministic effects in non-human biota (Relative to low LET radiation) Source Nominal value Comment NCRP (1991) 1 Built-in conservatism in dose model IAEA (1992) 20 Keep same as for humans Barendsen (1992) 2e10 Non-stochastic effect of neutrons and heavy-ions UNSCEAR (1996) 5 Average for deterministic effects Trivedi and Gentner (2000) 10 Deterministic population-relevant endpoints Copplestone et al.