Table 1.

Radiation characteristics of particle therapy and cosmic radiation and effects on the immune system.

	Particle therapy	Cosmic radiation
Target tissue	Very localized → tumor	Whole body, healthy tissue
Exposure	Several sessions (fractionation)	Chronic
Source	Particle accelerators	Galaxy, the Sun
Particle type	Protons or heavy ions such as C (also H, Li, O, etc.)	Protons, H ions, HZE particles (e.g., Fe), and electrons
Maximal energy	~200 MeV for protons and ~600 MeV for C ions	~1,000 MeV for protons and ~600 MeV for heavy ions
Dose	Target tissue: high – 60–80 Gy-eq	~662 mSv (in a round trip to Mars, without considering the time spent in the planet)
Secondary irradiation	Problem of neutrons leakage	Secondary cosmic rays due to vehicle shielding
Short-term immune effects	Alterations in tumor cells and their environment leading to immunogenic cell death	aSignificant downregulation of different components of the immune system [more affected—B cell > T cells (CD8+ > CD4+) > NK cells—less affected]
Long-term immune effects	Systemic immunogenic response affecting specific tumor cells both local and distant (abscopal effects)	aMainly unknown. Immune system downregulated response may persist
Potential outcome	Improve survival of cancer patients	Immune dysregulation → major health risk for astronauts during exploratory missions

H, hydrogen; C, carbon; Li, lithium; O, oxygen; Fe, iron; NK, natural killer.

^aFrom animal studies.