An Allometric Examination of the Relationship Between Radiosensitivity and Mass

AN ABSTRACT OF THE THESIS OF David Paul Bytwerk for the degree of Master of Science in Radiation Health Physics presented on October 4, 2006 Title: An Allometric Examination of the Relationship Between Radiosensitivity and Mass Abstract approved: Kathryn Higley The results of a review of six decades of existing literature for 50% lethal dose data across a range of phyla are presented. The collected 50% lethal dose data is limited to adult organisms subjected to acute doses of gamma radiation. The data collected is examined to determine whether useful allometric relationships relating lethal dose and body size can be established. Comparative radiosensitivity is examined where the mechanism of death is the same, and across broader scales where the mechanism of death varies. Various power law fits to graphs of lethal dose vs. mass show a clear increase of radiosensitivity with mass across a number of orders of magnitude, but within an order of magnitude in mass it is difficult to make any useful predictions. The conclusion of this preliminary investigation is that allometric relationships can be useful in providing order of magnitude estimates of radiosensitivity based on mass, but must be used carefully and only as indicators of a general trend. Copyright by David Paul Bytwerk October 4, 2006 All Rights Reserved An Allometric Examination of the Relationship Between Radiosensitivity and Mass by David Paul Bytwerk A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented October 4, 2006 Commencement June 2008 Master of Science thesis of David Paul Bytwerk presented on October 4, 2006 APPROVED: Major Professor, representing Radiation Health Physics Head of the department of Nuclear Engineering and Radiation Health Physics Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. David Paul Bytwerk, Author ACKNOWLEDGEMENTS I owe so much to so many that this acknowledgment section will by necessity be incomplete, but there are a few credits in particular that should not go without explicit mention. I owe a great deal to my advisor, Dr. Kathryn Higley, without whose encouragement, wise counsel, and occasional prodding, this thesis would not have been completed. Dr. Higley is an excellent advisor and I am privileged to have worked and continue to work with her. I would like to thank my committee members, Dr. Binney, Dr. Hamby, and Dr. Krane for being willing to serve and prompting me to think more deeply about my subject matter. I also thank my parents, Randall and Sharon Bytwerk, who have loved and supported me throughout this trial and many before it. In academics and in life I try to do justice to the high standards they have modeled for me all my life. TABLE OF CONTENTS Page 1 INTRODUCTION………………………………………………. 1 2 LITERATURE REVIEW……………………………….............. 6 2.1 Nature and Origin of the LD50………………………... 6 2.2 Radiation LD50 Data in the Literature………………… 9 2.3 Strategies for Predicting Species Radiosensitivity …….. 10 2.4 Allometry………………………………………...……. 15 2.4.1 Some Allometric Explanations………………….. 18 2.4.2 Some Allometric Examples……………………... 21 2.4.3 Allometric Cautions and Criticisms…………….. 27 3 METHODS………………………………………………………. 32 3.1 Database Criteria……………………………………….. 32 3.2 Defining Species Mass and Radiosensitivity…………… 37 3.3 Data Analysis………………………………………….... 40 4 RESULTS & DISCUSSION…………………………………….. 42 4.1 Database Contents………………………..……………. 42 4.2 Allometric Regressions…………….…………………... 45 4.3 Discussion…………………...………………………….. 51 5 CONCLUSIONS AND FUTURE WORK………...…................. 53 BIBLIOGRAPHY…………………………………………………... 54 LIST OF FIGURES Figure Page 2.1 Dose-survival time relationship for man ………………………………….…...8 2.2 Radiosensitivity vs. Mass……………………………………………………..11 2.3 Radiosensitivity vs. ICV for female rodents…..……………………………...13 2.4 Radiosensitivity vs. ICV for male rodents……………………………………13 2.5 Radiosensitivity vs. ICV for mammals other than rodents…………………...14 2.6 Radiosensitivity vs. ICV for insects……………………………….........…….14 2.7 Ranges of radiosensitivity…………………………………….....……………15 2.8 A collection of allometric relationships plotted together……………………..17 2.9 Mouse to elephant curve. The exponent to this fit is .734……………………22 2.10 Heusner’s BMR vs. body mass proposal……………………………………..24 2.11 Mass vs. home range size……………………………………………………..25 2.12 Optimal Cruising Speed vs. Mass…………………………………………….26 2.13 Three allometric problems……………………………………………………29 4.1 Rodent radiosensitivity data plotted against mass………….………….…….37 4.2 Mammalian radiosensitivity plotted against mass………….………….…….37 4.3 Avian radiosensitivity plotted against mass………….………….……………38 4.4 Amphibian radiosensitivity plotted against mass………….………….………38 4.5 Reptilian radiosensitivity plotted against mass………….………….…...……39 4.6 Fish radiosensitivity plotted against mass……………………………………. 4.6 Insect radiosensitivity plotted against mass………….………….………..…..39 4.7 Vertebrate radiosensitivity plotted against mass………….………….……….40 LIST OF FIGURES (Continued) 4.8 A complete database plot………….………….……..…….…….……………41 4.9 A complete database plot minus insects………….………….……………….41 LIST OF TABLES Table Page 3.1 LD50/30 values for female rats subjected to different doses from…………...34 a Co60 source. 4.1 Breakdown of database content by taxonomic group….……………………..41 4.2 LD50 value database …………………………………………………………42 4.3 Regression Line Equations and R2 values ……………………………..…….50 AN ALLOMETRIC EXAMINATION OF THE RELATIONSHIP BETWEEN RADIOSENSITIVITY AND MASS 1. INTRODUCTION This work was motivated by a shift that is occurring in the discipline of radiation protection. Through most of the 20th century, radiological protection meant the radiological protection of humans. Rules and regulations dealt exclusively with the effect of radiation on h. sapiens, reasoning that “The level of safety required for the protection of all human individuals is thought likely to be adequate to protect other species, although not necessarily individual members of those species. The Commission therefore believes that if man is adequately protected then other living things are also likely to be sufficiently protected.” (ICRP 1977) Those words were written in 1977 and defined the scope of radiological protection in the second half of the 20th century: protect people and the environment will be protected. Recently, though, there have been discussions presaging a change. It is increasingly recognized that there are problems with the traditional approach. (Bréchignac 2002, Gonzalez 2002) In protecting man, at the end of the food chain, it is difficult to prove biota earlier in the food chain are not exposed to dangerous levels of radioactivity. More generally, the traditional approach neglects environments with few or no exposure pathways to humans (Bréchignac 2002). The ICRP weighed in again in 1991, adding a caveat that “individual members of non-human species might be harmed but not to the extent of endangering whole species or creating imbalance between species” (ICRP 1991). Since the ICRP made that change, criticism of the traditional approach has increased, not decreased. In 2002, at the opening address to the Third 2 International Symposium on the Protection of the Environment from Ionising Radiation (SPEIR 3), A. J. González (2002) presented what the IAEA views as the fundamental questions facing the discipline of radiation protection that need resolution: “Is the aim to protect the human habitat or the wider environment? (The current international standards implicitly refer to species in the ‘human habitat’, rather than to species in the ‘environment’.) Is the objective to protect individuals of a given species or the species as a whole? Namely, is it sufficient to protect non-human species as a whole, i.e., collectively? Or should protection be afforded to individual members of the species? And finally, what is the applicable ethic?” In a certain sense, the first question was already answered when it was posed. The subtitle of the symposium it opened was “The Development of a System of Radiation Protection for the Environment,” and most of the articles took for granted that a shift towards considering explicitly environmental impacts was occurring. The topic of discussion is, given that this regulatory shift is occurring, what is the best way to establish a regulatory framework to protect the environment. The majority of the plans proposed have held that protecting non-human species at a community level is sufficient, in contrast to human radiation protection where every individual’s survival is important. Another aspect of this question is the nature of radiation’s effects; if protecting the species is sufficient should effects that set in after an organism has reproduced be of interest? Cancer, and many other radiation-induced effects usually set in after an individual’s reproductive life is complete. Should effects like these, which affect 3 individual animals, but not the species, be regulated? The last questions posed are the trickiest both scientifically and politically, and in the last decade, have received significant attention around the world. There is a growing consensus that the non-human environment needs to be protected, but uncertainty as to regulate that protection. In the United States, the DOE has promulgated a new standard: “A Graded Approach for Evaluating

Load more