Astronauts are exposed to approximately 72 millisieverts (mSv) while on six-month-duration missions to the International Space Station (ISS). Longer 3-year missions to Mars, however, have the potential to expose astronauts to radiation in excess of 1000 mSv. Without the protection provided by Earth's magnetic field, the rate of exposure is dramatically increased. The risk of cancer caused by ionizing radiation is well documented at radiation doses beginning at 100 mSv and above.
Related radiological effect studies have shown that survivors of the atomic bomb explosions in Hiroshima and Nagasaki, nuclear reactor workers and patients who have undergone therapeutic radiation treatments have received low-linear energy transfer (LET) radiation (x-rays and gamma rays) doses in the same 50-2,000 mSv range.
Composition of space radiation While in space, astronauts are exposed to radiation which is mostly composed of high-energy protons, helium nuclei (alpha particles), and high-atomic-number ions (HZE ions), as well as secondary radiation from nuclear reactions from spacecraft parts or tissue.
The ionization patterns in molecules, cells, tissues and the resulting biological effects are distinct from typical terrestrial radiation (x-rays and gamma rays, which are low-LET radiation). Galactic cosmic rays (GCRs) from outside the Milky Way galaxy consist mostly of highly energetic protons with a small component of HZE ions.
Prominent HZE ions: Carbon (C) Oxygen (O) Magnesium (Mg) Silicon (Si) Iron (Fe)
GCR energy spectra peaks (with median energy peaks up to 1000 MeV/Da) and nuclei (energies up to 10000 MeV/Da) are important contributors to the dose equivalent.
Uncertainties in cancer projections One of the main roadblocks to interplanetary travel is the risk of cancer caused by radiation exposure. The largest contributors to this roadblock are: (1) The large uncertainties associated with cancer risk estimates, (2) The unavailability of simple and effective countermeasures and (3) The inability to determine the effectiveness of countermeasures. Operational parameters that need to be optimized to help mitigate these risks include: length of space missions crew age crew sex shielding biological countermeasures
Major uncertainties Source: effects on biological damage related to differences between space radiation and x-rays dependence of risk on dose-rates in space related to the biology of DNA repair, cell regulation and tissue responses predicting solar particle events (SPEs) extrapolation from experimental data to humans and between human populations individual radiation sensitivity factors (genetic, epigenetic, dietary or "healthy worker" effects)
Minor uncertainties Source: data on galactic cosmic ray environments physics of shielding assessments related to transmission properties of radiation through materials and tissue microgravity effects on biological responses to radiation errors in human data (statistical, dosimetry or recording inaccuracies)
Quantitative methods have been developed to propagate uncertainties that contribute to cancer risk estimates. The contribution of microgravity effects on space radiation has not yet been estimated, but it is expected to be small. However as microgravity has been shown to modulate cancer progression, more research is needed into the combined effects of microgravity and radiation on carcinogenesis. The effects of changes in oxygen levels or in immune dysfunction on cancer risks are largely unknown and are of great concern during space flight.
Types of cancer caused by radiation exposure Studies are being conducted on populations accidentally exposed to radiation (such as Chernobyl, production sites, and Hiroshima and Nagasaki). These studies show strong evidence for cancer morbidity as well as mortality risks at more than 12 tissue sites. The largest risks for adults who have been studied include several types of leukemia, including myeloid leukemia and acute lymphatic lymphoma as well as tumors of the lung, breast, stomach, colon, bladder and liver. Inter-sex variations are very likely due to the differences in the natural incidence of cancer in males and females. Another variable is the additional risk for cancer of the breast, ovaries and lungs in females. There is also evidence of a declining risk of cancer caused by radiation with increasing age, but the magnitude of this reduction above the age of 30 is uncertain.
It is unknown whether high-LET radiation could cause the same types of tumors as low-LET radiation, but differences should be expected.
The ratio of a dose of high-LET radiation to a dose of x-rays or gamma rays that produce the same biological effect are called relative biological effectiveness (RBE) factors. The types of tumors in humans who are exposed to space radiation will be different from those who are exposed to low-LET radiation. This is evidenced by a study that observed mice with neutrons and have RBEs that vary with the tissue type and strain.
Measured rate of cancer among astronauts
The measured change rate of cancer is restricted by limited statistics. A study published in Scientific Reports looked over 301 U.S. astronauts and 117 Soviet and Russian cosmonauts, and found no measurable increase in cancer mortality compared to the general population, as reported by LiveScience.
An earlier 1998 study came to similar conclusions, with no statistically significant increase in cancer among astronauts compared to the reference group.
Approaches for setting acceptable risk levels The various approaches to setting acceptable levels of radiation risk are summarized below:
Unlimited Radiation Risk – NASA management, the families and loved ones of astronauts, and taxpayers would find this approach unacceptable. Comparison to Occupational Fatalities in Less-safe Industries – The life-loss from attributable radiation cancer death is less than that from most other occupational deaths. At this time, this comparison would also be very restrictive on ISS operations because of continued improvements in ground-based occupational safety over the last 20 years. Comparison to Cancer Rates in General Population – The number of years of life-loss from radiation-induced cancer deaths can be significantly larger than from cancer deaths in the general population, which often occur late in life (> age 70 years) and with significantly less numbers of years of life-loss. Doubling Dose for 20 Years Following Exposure – Provides a roughly equivalent comparison based on life-loss from other occupational risks or background cancer fatalities during a worker's career, however, this approach negates the role of mortality effects later in life. Use of Ground-based Worker Limits – Provides a reference point equivalent to the standard that is set on Earth, and recognizes that astronauts face other risks. However, ground workers remain well below dose limits, and are largely exposed to low-LET radiation where the uncertainties of biological effects are much smaller than for space radiation.
NCRP Report No. 153 provides a more recent review of cancer and other radiation risks. This report also identifies and describes the information needed to make radiation protection recommendations beyond LEO, contains a comprehensive summary of the current body of evidence for radiation-induced health risks and also makes recommendations on areas requiring future experimentation.
Current permissible exposure limits
Career cancer risk limits Astronauts' radiation exposure limit is not to exceed 3% of the risk of exposure-induced death (REID) from fatal cancer over their career. It is NASA's policy to ensure a 95% confidence level (CL) that this limit is not exceeded. These limits are applicable to all missions in low Earth orbit (LEO) as well as lunar missions that are less than 180 days in duration. In the United States, the legal occupational exposure limits for adult workers is set at an effective dose of 50 mSv annually.
Cancer risk to dose relationship The relationship between radiation exposure and risk is both age- and sex-specific due to latency effects and differences in tissue types, sensitivities, and life spans between sexes. These relationships are estimated using the methods that are recommended by the NCRP and more recent radiation epidemiology information
Principle of 'as low as reasonably achievable' The as low as reasonably achievable (ALARA) principle is a legal requirement intended to ensure astronaut safety. An important function of ALARA is to ensure that astronauts do not approach radiation limits and that such limits are not considered as "tolerance values". ALARA is especially important for space missions in view of the large uncertainties in cancer and other risk projection models. Mission programs and terrestrial occupational procedures resulting in radiation exposures to astronauts are required to find cost-effective approaches to implement ALARA.
Evaluating career limits Organ (T)Tissue weighting factor (wT)Gonads 0.20Bone Marrow (red) 0.12Colon 0.12Lung 0.12Stomach 0.12Bladder 0.05Breast 0.05Liver 0.05Esophagus 0.05Thyroid 0.05Skin 0.01Bone Surface 0.01Remainder* 0.05*Adrenals, brain, upper intestine, small intestine,kidney, muscle, pancreas, spleen, thymus and uterus.
The risk of cancer is calculated by using radiation dosimetry and physics methods.
For the purpose of determining radiation exposure limits at NASA, the probability of fatal cancer is calculated as shown below:
The body is divided into a set of sensitive tissues, and each tissue, T, is assigned a weight, wT, according to its estimated contribution to cancer risk. The absorbed dose, Dγ, that is delivered to each tissue is determined from measured dosimetry. For the purpose of estimating radiation risk to an organ, the quantity characterizing the ionization density is the LET (keV/μm). For a given interval of LET, between L and ΔL, the dose-equivalent risk (in units of sievert) to a tissue, T, Hγ(L) is calculated aswhere the quality factor, Q(L), is obtained according to the International Commission on Radiological Protection (ICRP). The average risk to a tissue, T, due to all types of radiation contributing to the dose is given by or, since , where Fγ(L) is the fluence of particles with LET = L, traversing the organ, The effective dose is used as a summation over radiation type and tissue using the tissue weighting factors, wγ For a mission of duration t, the effective dose will be a function of time, E(t), and the effective dose for mission i will be The effective dose is used to scale the mortality rate for radiation-induced death from the Japanese survivor data, applying the average of the multiplicative and additive transfer models for solid cancers and the additive transfer model for leukemia by applying life-table methodologies that are based on U.S. population data for background cancer and all causes of death mortality rates. A dose-dose rate effectiveness factor (DDREF) of 2 is assumed.
Evaluating cumulative radiation risks The cumulative cancer fatality risk (%REID) to an astronaut for occupational radiation exposures, N, is found by applying life-table methodologies that can be approximated at small values of %REID by summing over the tissue-weighted effective dose, Ei, as
where R0 are the age- and sex- specific radiation mortality rates per unit dose.
For organ dose calculations, NASA uses the model of Billings et al. to represent the self-shielding of the human body in a water-equivalent mass approximation. Consideration of the orientation of the human body relative to vehicle shielding should be made if it is known, especially for SPEs
Confidence levels for career cancer risks are evaluated using methods that are specified by the NPRC in Report No. 126 . These levels were modified to account for the uncertainty in quality factors and space dosimetry.