SC 1-20: Biological Effectiveness of Low Linear-Energy Transfer Radiation as a Function of Energy

Members only

The Committee is preparing a report on unresolved questions in evaluating the risk of cancer from exposure to low linear energy transfer (LET) radiation (i.e., photons and electrons) that addresses the dependence of relative biological effectiveness (RBE) on energy. A quantitative determination is relevant for estimating cancer risk from exposure to low-LET radiation at lower energies in mammography, other medical imaging procedures, and from various occupational and public radiation sources. Recent reviews of radiobiological studies and human epidemiology data have provided estimates of RBE that vary widely. In particular, available evidence indicates that the biological effectiveness of lower-energy low-LET radiation may be two or more times greater than for higher-energy low-LET radiation. However, the biological systems used in these experiments may not be strictly applicable to human cancer and reassessment is, therefore, necessary.

Specialists in the areas of microdosimetry, DNA damage, cellular radiobiology, animal studies, and human epidemiology are independently evaluating the variation in RBE for low-LET radiation as a function of energy. Each specialist is deriving a probability distribution function that will ultimately be combined into a single function through a Bayesian statistical model. The result of this effort will be a report that provides guidance for the assessment of cancer risks for low-LET radiation over the energy ranges associated with medical, occupational or other human exposures. NCRP is also aware of the ongoing work on lower-energy low-LET radiation being prepared for risk calculations in the revisions of the EPA Federal Guidance Report 13 and will be preparing complementary guidance accordingly.

A full Committee meeting was held January 30 and 31, 2013 with the following actions:

  • presented and discussed the draft sections for each line of evidence (i.e., microdosimetry, DNA damage, cellular radiobiology, animal studies, human epidemiology);
  • developed, presented and discussed probability distribution functions for each line of evidence;
  • for the first sample effort, generated conditional probabilities for a selected RBE scenario (~20 keV, tissue deeper than 1 mm);
  • developed and discussed refinement of the desired RBE scenarios; and
  • further developed the report outline.

It was agreed to:

  • develop probability distribution functions for the following energy ranges:
    • photons <5 keV (focus on 1.5 keV)
    • photons (50 to 150 keV
    • beta decay of tritium (18.6 keV maximum; 5.7 keV average)
  • use photons (0.5 to 2 MeV) as the reference radiation; and
  • further refine the draft sections for each line of evidence.

In May 2013, the result of the sample probability assessment for RBE (for human cancer) for the 15 to 30 keV energy category conducted as an exercise at the January 30–31, 2013 meeting was distributed to the members for information.

A consolidated draft was prepared and distributed to the members on September 12, 2013. A full Committee meeting was held September 26–28, 2013 with the following actions:

The energy categories of interest are now:

  • photons of energy 1.5 keV: energy range where RBE has been observed to increase the most in experimental studies
  • electrons produced by beta decay of tritium (H3): average energy (5.7 keV)
  • photons of energy 15 to 30 keV: relates to mammography
  • photons of energy 40 to 60 keV: relates to computed tomography
  • photons of energy >60 to 150 keV: relates to orthovoltage x rays

The reference radiation is photons of energy 0.5 to 2 MeV.

A schedule was developed for further refinement of the sections on lines of evidence, intra-committee review, and conduct and interaction on the probability assessments for the various energy categories.

A meeting was held on December 3–4, 2015. A consolidated draft (dated November 20, 2015) and related documents was distributed prior to the meeting. During the first day each line of evidence was reviewed by each section leader. Discussion then centered on the probability density functions (PDFs)] of the quantity r i that resulted from the authors’ analyses of each line of evidence (for each energy range selected for evaluation). During the second day, the elicitation procedure was conducted to evaluate the PDF (for each energy range) of ρ (i.e., a factor relating to enhancement of risk as a function of photon/electron energy that may apply to human cancer). A list of potential expert reviewers was generated by the SC 1-20 membership. The next main steps are:

  • December 2015 (when available)… results of elicitation analyses to members
  • December 2015 through January 30, 2016 … section updates due
  • February 28, 2016 … full draft to all members for review
  • End of June 2016 … review draft ready for PAC 1/expert review

As of March 31, 2016, a complete draft (dated March 25, 2016) was sent to the SC 1-20 members for review, with a deadline of April 29, 2016 for comments. This last Committee member review is in preparation for producing the draft for PAC 1 / expert review in the second quarter of 2016.

NCRP is grateful to the Centers for Disease Control and Prevention for financial support of this work.

The membership of SC 1-20 is:

Simon S


Simon S

received a BS in Physics from the University of Texas, an MS in Radiological Physics from the University of Texas Health Sciences Center in Dallas, and a PhD in Radiological Health Sciences from Colorado State University. Early in his career, he worked in medical physics and was the first treatment planner for clinical trials of treatments of solid tumors with negative pi-mesons at the Los Alamos Physics Meson Facility. Later specializing in environmental radioactivity, he directed the first nationwide monitoring program of the Marshall Islands for residual contamination from nuclear testing. He also participated in the radiological monitoring of numerous other nuclear test sites worldwide including Johnston Island, French Polynesia, and Algeria and has lead, or participated in, health risk studies of fallout exposures in Utah, the Marshall Islands, and Kazakhstan.

In 2000, Dr. Simon joined the National Cancer Institute's Radiation Epidemiology Branch as an expert in dose reconstruction and presently heads the Dosimetry Unit in that group. Steve is a member of NCRP and has been an Associate Editor of Health Physics for 20 y. In 2011 during the Fukushima crisis, Steve was deployed by the U.S. Department of Health and Human Services to the U.S. Embassy in Japan to assist with the protection of American citizens.

steven l. simon, Chairman



has been a Research Professor at Texas A&M University since 1996. His previous experience includes Biology and Chemistry Department Staff Scientist from 1971 to 1991 and Radiation Physics and Chemistry Section Manager from 1991 to 1995 at Battelle, Pacific Northwest National Laboratory. He received his BA degree from Linfield College and PhD from Oregon State University in 1972.

Dr. Braby is a former member of the NCRP Board of Directors and a member of several NCRP scientific committees (SC) including SC 88 on Fluence as a Basis of a System of Radiation Protection for Astronauts, SC 1-7 on Research Needs for Deep Space Missions, chair of SC 1-11 on Safety Considerations for Pulsed Fast Neutron Surveillance Systems, SC 6-1 on Uncertainties in Measuring External Beam Irradiation, SC 1-20 on the biological effects of low energy x rays, and Chairman of SC 6-5 on Safety of Cargo Inspection Systems Using High Energy Photons. He was also Chair of the International Commission on Radiation Units and Measurements (ICRU) working committee on characterizing low level radiation exposure, and member of ICRU working committee to prepare a report on microdosimetry.


leslie a. braby
ChangPolly Y. Chang
Goodhead D

Dudley T. Goodhead

Goodhead Dis Emeritus Director of the Medical Research Council (MRC) Radiation and Genome Stability Unit, Harwell, United Kingdom. His Unit carried out basic research on the relationship of genome stability to human health, including how DNA may be damaged by radiation and the cellular repair systems act to restore normality. He is now an independent consultant and assists agencies in evaluating and guiding their radiation research programs in the European Union and the United States. His personal research has been mainly on the biophysics of radiation effects, with particular emphasis on microscopic features of radiation track structure at the atomic, molecular, and cellular levels and their consequent radiobiological and health effects. He gained his PhD in particle physics at the University of Oxford. Early career positions were at the Universities of California, London and Natal and at the MRC Radiobiology Unit. He has served on a variety of national and international committees on evaluation of radiation risks, including the Committee on Medical Aspects of Radiation in the Environment in the United Kingdom, two National Research Council committees [on health risks of exposure to radon (BEIR VI) and evaluation of the National Aeronautics and Space Administration’s (NASA) space radiation cancer risk model] and working groups of the International Agency for Research on Cancer (on carcinogenic risk of gamma rays, neutrons, and internally deposited radionuclides) and the Royal Society (on risks from depleted uranium) and consultancies to the United Nations Scientific Committee on the Effects of Atomic Radiation and the International Atomic Energy Agency. He was chair of the Committee Examining Radiation Risks of Internal Emitters in the United Kingdom and is currently acting chair of the Advisory Group on Ionizing Radiation of Public Health England. In the 2002 Queen’s Birthday Honours List, he was awarded the Order of the British Empire for services to medical research. His other awards include the Weiss Medal from the Association for Radiation Research, Failla Medal from the Radiation Research Society, Douglas Lea Lecturer from the Institute of Physics and Engineering in Medicine and Biology, Bacq and Alexander Award from the European Society of Radiation Biology, Honorary Fellowship from the Society of Radiological Protection, Warren K. Sinclair Lecturer from NCRP, and Gray Medal from the International Committee on Radiation Units and Measurements. For the past 3 y he has directed the NASA Space Radiation Summer School at Brookhaven National Laboratory.

Dudley T. Goodhead

Stephen C. Hora

HoraSis the Director of University of Southern California (USC) Center for Risk and Economic Analysis of Terrorism Events (CREATE), the nation's first Department of Homeland Security (DHS) Research Center of Excellence.

Dr. Hora is a prominent decision analyst who has led several CREATE studies and is an experienced academic leader who served as the University of Hawaii-Hilo's Interim Vice Chancellor for Academic Affairs from 2005 to 2007. He is a Professor of Management Science and Statistics at UH-Hilo. Hora earned both his DBA and his BS from USC.

Stephen C. Hora



has extensive experience in areas of environmental health physics and assessments of dose and risk from exposure to ionizing radiation. He has special expertise in evaluations of dose and risk assessment models for regulatory and decision-making purposes, and he has written and lectured extensively on the issue of "risk harmonization" in regulating radionuclides and hazardous chemicals in the environment. Dr. Kocher's work on assessing the biological effectiveness of different radiation types and their uncertainties was an important contribution to the development of the Interactive Radio-Epidemiological Program, and he was principal investigator on work for the Defense Threat Reduction Agency to develop improved models and methods of uncertainty analysis for use in dose reconstructions.

Dr. Kocher has served on several advisory groups for Federal agencies and on committees of the National Academy of Sciences (NAS) and NCRP, including committees that produced an NAS report on "A Review of the Dose Reconstruction Program of the Defense Threat Reduction Agency" and committees that produced NCRP reports on "Risk-Based Classification of Radioactive and Hazardous Chemical Wastes" (Report No. 139), "Approaches to Risk Management in Remediation of Radioactively Contaminated Sites" (Report No. 146), and "Performance Assessment of Near-Surface Disposal Facilities for Low-Level Radioactive Waste" (Report No. 152). He also was a member of a committee of the U.S. Department of Energy (DOE) that produced a DOE Standard on "A Graded Approach for Evaluating Radiation Doses to Aquatic and Terrestrial Biota." Dr. Kocher has been a frequent consultant to the International Atomic Energy Agency on issues of radioactive and hazardous chemical waste management and protection of the environment.

david c. kocher

Kiyohiko Mabuchi

MabuchiKis the Deputy Branch Chief, Radiation Epidemiology Branch, Acting Chief of the Chernobyl Research Unit, and Senior Scientist, is engaged in epidemiological studies of thyroid disease and leukemia risks following the Chernobyl nuclear reactor accident and the continuing follow-up studies of cancer in the Japanese atomic-bomb survivors. He received an MD from Osaka University Medical School, and an MPH/ DrPH from the John Hopkins University School of Hygiene and Public Health. He joined the Division of Cancer Epidemiology and Genetics, Radiation Epidemiology Branch in 2000 after serving as Chief of the Department of Epidemiology at the Radiation Effects Research Foundation, Hiroshima, Japan, where he led the long-term epidemiological studies of health effects of radiation exposure in cohorts of atomic-bomb survivors and their children. He has been a member of several international radiation committees, including the International Commission of Radiological Protection, the United Nations Scientific Committee on the Effects of Atomic Radiation, U.K. National Radiological Protection Board’s Advisory Group on Ionizing Radiation and also formerly a member of the Executive Committee of the International Association of Cancer Registries.

Kiyohiko Mabuchi



is the Director for the Center of Science and Technology in the Radiation Protection Division, U.S. Environmental Protection Agency (EPA). He has been at EPA since 1985, heading a group with responsibility for developing models for EPA's assessment of radiation doses and risks. From 1982 to 1998, he worked on similar issues at the U.S. Nuclear Regulatory Commission (NRC). Prior to this, he was first a Postdoctoral Fellow and then a faculty member in the Department of Radiation Biology and Biophysics at the University of Rochester, where he performed research on ion transport into mitochondria and ion binding to phospholipid membranes. His academic degrees include a BA from Johns Hopkins and a PhD from Harvard, both in Physics. He has represented EPA on a number of interagency committees, including a committee established to advise on health protection measures for the U.S. population after Chernobyl and the Executive Committees for the Committee on Interagency Radiation Research and Policy Coordination and the Joint Coordinating Committee for Radiation Effects Research.

He served on the scientific committee (SC) for NCRP Report No. 160, Ionizing Radiation Exposure of the Population of the United States and is currently a member of the NCRP SC 1-20, which is addressing the issue of enhanced relative biological effectiveness for low-energy photons and electrons. He was also a member of a committee of experts who developed the "Late Health Effects Uncertainty Assessment" component for the Probability Accident Consequence Uncertainty Analysis, a joint report of NRC and the Commission of the European Communities.

jerome s. puskin
No Image David Richardson
No Image James D. Tucker

Eliseo Vano

VanoEis full Professor of Medical Physics at the Faculty of Medicine of the Complutense University in Madrid and head of the Medical Physics Service at the San Carlos University Hospital. He is Chairman of the Medical Working Party on Medical Exposures of the Article 31 Group of Experts of the European Atomic Energy Community Treaty and Chairman of the Committee on Protection in Medicine of the International Commission on Radiological Protection.

Eliseo Vano



is a consultant, currently concentrating on the preparation of scientific reports produced by NCRP in all subject areas. From 1982 to 1995, he was Director, Office of Health Physics at the Center for Devices and Radiological Health, U.S. Food and Drug Administration. He also served in a number of scientific and management positions related to radiation protection during his 33 y career as a Commissioned Officer in the U.S. Public Health Service, from 1962 to 1995. He received a BS in Chemical Engineering (University of Maryland, 1961), an MS in Environmental Engineering (Rensselaer Polytechnic Institute, 1966) and a PhD in Nuclear Engineering (University of Maryland, 1971). His technical work has concentrated on radiation dosimetry, particularly with regard to x rays used for medical diagnosis, epidemiological studies of exposed populations, and public radiation emergencies.

He is a Distinguished Emeritus Member of NCRP (after serving as a Council member for 18 y), and an Emeritus member of Committee 3 (Protection in Medicine) of the International Commission on Radiological Protection [after serving on Committee 3 for 28 y (1985 to 2013)]. He was also a member of the International Commission on Radiation Units and Measurements report committee that produced Report 74, Patient Dosimetry for X Rays Used in Medical Imaging.

marvin rosenstein, Staff Consultant


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Last modified: March 9, 2017