American scientists have come to the conclusion that classical models for assessing the oncogenicity of galactic cosmic rays underestimate their stochastic effects at long exposure times.
Manned space missions pose a high risk to the health of the crew. Thus, microgravity can impair vision, and cosmic rays can provoke cancer. With distance from low-Earth orbit and the radiation belt, for example, as part of the colonization of Mars, the radiation energy and, as a consequence, the likelihood of radiation sickness, apparently, will increase. At the same time, such programs do not provide for short-term implementation: one group of cosmonauts will need more than 900 days to study the Red Planet. Therefore, scientists are looking for opportunities to predict the effects that galactic cosmic rays may have on the human body.
The assessment of this risk is associated with a number of limitations. First, it is unclear how the effects of radiation in space are related to the composition of the radiation. In particular, the effect on living organisms and the linear transfer of heavy ions of low energies, but high intensity - helium particles, protons and delta electrons - remains insufficiently studied. Secondly, the existing models, including NASA, are designed to reveal the direct effects of radiation. They allow predicting a deterministic result associated with a specific dose threshold. Stochastic effects are less descriptive, although they can appear even after several years.
In the new work, specialists from the University of Nevada have built the first structural model of the tracks of galactic cosmic radiation particles and their stochastic effects in the framework of the spread of cancer cells. The calculations were based on the data of experiments on modeling in female mice of type B6CF1 tumors of the Gardera gland, which were carried out from 1985 to 2016. The design of these experiments corresponds to the assumed conditions of cosmic radiation: the animals were irradiated simultaneously with several (more than four) types of particles at low (up to 0.2 warming) doses. To predict the tracks and growth of damaged tissue at low doses, the authors extrapolated a NASA model.
The predicted number of cells sensitive to deterministic (TE) and stochastic (NTE) effects of cosmic rays, depending on the charge number of particles per year (at a depth of five centimeters from the body surface and behind the aluminum surface). Red triangles correspond to the number of cells potentially exposed to delta rays with a dose of less than 0.1 milligray / © Francis A. Cucinotta et al., Scientific Reports, 2017
Using the risk function, they calculated the dynamics of pathology taking into account the type and fluence (transfer) of radiation, particle charge, and kinetic energy per body weight. During the exposure, one year was taken with an absorbed dose of up to 0.2 g. The analysis showed that stochastic effects predict tumor growth much better at low radiation doses (less than 0.1 gray). The model is consistent with experiments: when more than one heavy ion, such as iron-56, acts on the nucleus of a healthy cell, the progression of the disease is sharply accelerated. Moreover, cells that have not undergone the primary, according to calculations, receive small doses of secondary (delta-electrons) radiation.
Despite low doses, up to 10 milligray, secondary radiation has a significantly greater effect on neighboring cells than was thought, scientists say. According to them, in general, stochastic effects at long exposure to low doses of cosmic radiation suggest a two-fold or more increase in cancer risk compared to known values. In the interests of future manned missions, the assessment models require further research.
The article was published in Scientific Reports.
Promotional video:
Denis Strigun