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Appendix A
Statement of Task
BACKGROUND
NASA’s current missions to the International Space Station (ISS) and potential future exploration missions
involving extended stays by astronauts on the lunar surface, as well as the possibility of Near-Earth Objects (NEO)
or Mars missions, present challenges in protecting astronauts from radiation risks. These risks arise from a number
of sources, including solar particle events (SPE), galactic cosmic rays (GCR), secondary radiation from surface
impacts, and even nuclear isotope power sources transported with the astronauts. The serious early and late radiation
health effects potentially posed by these exposures are equally varied, ranging from prodromal radiation sickness
to cancer induction. Other possible effects include central nervous system damage, cataracts, heritable effects,
impaired wound healing and infertility. Since opening in October 2003, the NASA Space Radiation Laboratory
(NSRL) at Brookhaven National Lab has enabled NASA to sponsor research focused on understanding and quan -
tifying the radiation health risks posed by space radiation environments. While most aspects of the space radiation
environments are now relatively well characterized, important uncertainties still exist regarding biological effects
and thus the level and types of risks faced by astronauts.
CURRENT STATUS
In response to reviews and recommendations, NASA has developed an integrated research program that con -
sists of openly solicited peer reviewed research to investigate the biomedical effects of simulated space radiation
and provide the needed knowledge base on GCR and SPE radiobiology. While the main focus is on understanding
cancer risks, the program also includes studies on potential degenerative risks to the central nervous system, the
cardiovascular system and other tissues. In cooperation with the U.S. Department of Energy (DoE), NASA oper-
ates the NSRL, the only place in the U.S. where sophisticated biomedical experiments can be conducted using the
heavy ions encountered in the space radiation environment.
For projecting cancer risk for ISS crews and to support trade study assessments of potential lunar, NEO, and
Mars missions, NASA uses the model recommended by the National Council of Radiation Protection and Measure-
ments (NCRP) Report No. 132. NASA also makes an uncertainty assessment in the NCRP model coefficients that
describes errors in low LET human radio-epidemiology data, dose- and dose-rate effectiveness factors (DDREFs),
radiation quality factors, and space physics. For astronaut occupational exposures, the 95 percent confidence level
is used as a supplementary requirement as part of the Permissible Exposure Limit (PEL) of a no greater than
55
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56 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
3 percent increase in the Risk of Exposure Induced Death (REID). The PEL standards are approved by the NASA
Chief Health and Medical Officer.
NASA has updated its Space Radiation Cancer Risk Model based on recent developments including the
following:
• BEIR-VII, UNSCEAR-2006, and other reports in the scientific literature have made new assessments of
human radio-epidemiology data and DDREFs;
• New research results from NSRL have begun to modify the understanding of radiation quality and dose-
rate effects; and
• NASA’s revised evaluation of uncertainty factors.
Because it is used to project the cancer risk for current ISS crews and future explorations missions, this NASA
update to its Space Radiation Cancer Risk Model requires independent review and validation.
PROPOSED STUDY
1. The committee will evaluate proposed updates to the NASA cancer projection model taking into consid -
eration the following:
• Current knowledge of low-LET radiation cancer epidemiology,
• Effects of tissue weighting factors, radiation weighting factors, and DDREFs used in projecting risks, and
• Current uncertainties in Quality Factors, DDREFs and organ dose assessment.
This will be done taking into consideration possible qualitative differences between low LET and heavy
ion biological effects to determine if the use of quality factors are appropriate or inappropriate for GCR risk
assessments.
2. The committee will identify gaps in NASA’s current research strategy for reducing the uncertainties in
cancer induction risks.
