Home  |  Current Program  |  PAC 1  |  SC 1-20

SC 1-20:  Biological Effectiveness of Photons as a Function of Energy

The Committee will prepare a report on important unresolved questions in evaluating the risk of cancer from exposure to photons that addresses: (1) the relative biological effectiveness (RBE) with energy; and (2) the dose and dose-rate effectiveness factor (DDREF). A quantitative determination of both (1) and (2) is necessary for estimating cancer risk from exposure to photons 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 and DDREF that vary widely. In particular, available evidence indicates that the biological effectiveness of lower-energy photons may be two or more times greater than for higher-energy photons. A second issue is under what circumstances should the DDREF be applied or not be applied to relatively higher dose rates such as those used in medical procedures. This would require establishing quantitative criteria for the DDREF with regard to dose and dose rate.

NCRP will undertake a comprehensive evaluation of the variation in RBE as a function of photon energy based on a review of results from cytogenetic studies, carcinogenesis studies with laboratory animals and in vitro test systems, and epidemiological surveys of cancer incidence and mortality in exposed human populations. NCRP will also evaluate the quantitative criteria for the DDREF with regard to dose and dose rate. The result of this effort will be a report that provides an improved scientific foundation for the assessment of cancer risks for photons over the entire energy and dose-rate range associated with medical, occupational, or other human exposures.

The first meeting of SC 1-20 was held on December 9-10, 2010 at the NCRP offices with all members of the Committee in attendance. The following presentations were made:

• Issues in quantifying the effects of low level radiation – D. Goodhead
• Committee reports relating to low-LET RBEs  – J. Puskin
• Recent journal publications on the RBE of low-LET radiation – D. Kocher
• Formal processes for expert judgments – S. Hora

An extended discussion ensued on the various disciplines represented on the Committee and what methods the Committee might utilize to reach consensus recommendations for this report. In particular, the use of a group consensus probability density distribution was discussed.

It was concluded that the SC 1-20 title should be revised to “Biological Effectiveness of Low-LET Radiation as a Function of Energy” to reflect that electrons as well as photons are included. It was also agreed to set up an NCRP SharePoint site for Committee use (a group file for presentations, references, drafts, other documents).

The second interaction with SC 1-20 was held by WebEx on February 17, 2011 and continued the informative specialty background summaries by members with the following four presentations:

• Characteristics of energy deposited in small targets – L. Braby
• Biological effects of low energy radiation – P. Chang
• Radiation biology and genetics: Cytogenetics – J. Tucker
• Epidemiological evidence  – K. Mabuchi and D. Richardson

The third interaction with SC 1-20 was held also by WebEx on March 18, 2011 with two additional informative specialty background presentations by SC 1-20 members:

• Importance of the committee topics to medical radiation dosimetry – E. Vano
• Combining expert judgments about the biological effectiveness of low-LET radiation as a function of energy – S. Hora

The last presentation (combining expert judgments) discussed the application of probability assessment with multiple information sources using a Bayesian approach.

Additional WebEx conferences are planned to discuss member assignments and to determine the next step in exploring the use by SC 1-20 of a group consensus probability density distribution.

As of June 31, 2011, work on SC 1-20 is on hold pending receipt of funding for this NCRP project.

As of September 30, 2011 funding for SC 1-20 is now available and a meeting of the full Committee is being scheduled for the 4th quarter 2011.

A meeting was held on November 30 and December 1, 2011, including three members participating by WebEx.  A review of all the disciplines previously discussed was accomplished to refresh the committee members on the relevant background material. It was agreed to explore an SC 1-20 consensus probability density function (PDF) for an RBE scenario of low-LET radiation of ~20 keV in tissue deeper than the skin layer (>1 mm).

A future Committee meeting will be scheduled most likely in the third quarter 2012 to continue with the following next steps:

• each of five subgroups develop and present PDFs for their line of evidence (i.e., microdosimetry, DNA damage, cellular radiobiology, animal studies, human epidemiology);
• begin the process of generating conditional probabilities for the selected RBE scenario (~20 keV, tissue deeper than 1 mm);
• discuss other RBE scenarios; and
• further develop report outline.

SC 1-15 Radiation Protection and Science Goals
for Short-Term Lunar Missions >

SC 1-16 Uncertainties in the Estimation of Radiation Risks
and Probability of Disease Causation >

SC 1-17 Second Cancers and Cardiopulmonary
Effects After Radiotherapy >

SC 1-21 Multiplatform National Approach for Providing Guidance on
Integrating Basic Science and Epidemiological Studies
on Low-Dose Radiation Biological and Health Effects >

^ Back to Top

Home | About NCRP | Our Mission | Current Program | News & Events | Publications | Members | Related Organizations


© 2011, National Council on Radiation Protection & Measurements. All rights reserved.
Vision: The national scientific authority and a worldwide resource for radiation protection.
7910 Woodmont Avenue, Suite 400, Bethesda, Maryland 20814-3095
voice: 301-657-2652  |  fax: 301-907-8768  |   Executive Director, D.A. Schauer
Design and Consulting Provided by Capital Idea Ventures, Inc.