Cancer in Space Review (CiSR)

Astronauts have to endure various conditions and stressors in the space environment. These so-called space-associated factors can have drastic effects on the human body, on the tissue and organ level as well as on a cellular and molecular level. The complex interplay of these stressors is not fully understood, yet we know that they have detrimental effects on astronaut health. Microgravity and radiation are the two stressors that could have the most extensive effect on our cells, with disruption of signalling pathways, induction of DNA damage and changes to our DNA repair mechanisms. This accumulation of DNA damage could eventually lead to tumor initiation or carcinogenesis. In this review, we explain the effects of microgravity and radiation, the basic principles of cancer, as well as the differences between the radiation found in outer space compared to the radiation we know here on Earth. Furthermore, we try to elucidate the mechanisms through which the microgravity and radiation environment of outer space affects cancer initiation and cancer progression and if these effects are additive rather than synergistic. A summary of recent literature is made on these subjects, with a differentiation based upon the different types of radiation or cancers. However, while our knowledge with regards to these mechanisms has broadened greatly in the last couple of decades, a full understanding cannot be made yet. Heterogeneity between studies and study setups, as well as different cellular responses depending on cell type are identified as possible confounding factors that could explain the conflicting results found in this review. Further research with a homogenous study setup is needed to fully grasp the intricacies of this subject.