The potential impact of bystander effects on radiation risks in a Mars mission

Densely ionizing (high-LET) galactic cosmic rays (GCR) contribute a signifi cant component of the radiation risk in free space. Over a period of a few months-sufficient for the early stages of radiation carcinogenesis to occur -a significant proportion of cell nuclei will not be traversed. There is co nvincing evidence, at least in vitro, that irradiated cells can send out si gnals that can result in damage to nearby unirradiated cells. This observat ion can hold even when the unirradiated cells have been exposed to low dose s of low-LET radiation. We discuss here a quantitative model based on the B aD formalism, an approach that incorporates radiobiological damage both fro m a bystander response to signals emitted by irradiated cells, and also fro m direct traversal of high-LET radiations through cell nuclei. The model pr oduces results that are consistent with those of a series of studies of the bystander phenomenon using a high-LET microbeam, with the end point of in vitro oncogenic transformation. According to this picture, for exposure to high-LET particles such as galactic cosmic rays other than protons, the bys tander effect is significant primarily at low fluences, i.e., exposures whe re there are significant numbers of untraversed cells. If the mechanisms po stulated here were applicable in vivo, using a linear extrapolation of risk s derived from studies using intermediate doses of high-LET radiation (wher e the contribution of the bystander effect may be negligible) to estimate r isks at very low doses (where the bystander effect may be dominant) could u nderestimate the true risk from low doses of high-LET radiation. It would b e highly premature simply to abandon current risk projections for high-LET, low-dose radiation; however, these considerations would suggest caution in applying results derived from experiments using high-LET radiation at flue nces above similar to1 particle per nucleus to risk estimation for a Mars m ission. (C) 2001 by Radiation Research Society.