The relative biological effectiveness of proton and ion beams

The understanding of the biological effectiveness of proton and ion beams plays an important role in two very different scientific fields: radiotherapy and space research. Due to the many similarities of both types of radiation, the question is addressed, if the experience gained in radiotherapy with proton and ion beams is of relevance for the assessment of risks due to galactic radiation in manned space flight. The basic characteristics of the relative biological effectiveness (RBE) of proton and ion beams is reviewed, including the dependence on particle type, LET and dose as well as on biological systems and end points. The relevance of these factors for risk assessment in space flight is outlined. The concept of clinical RBE is reviewed and the RBE database gained in vivo is briefly outlined. Emphasis is put here on the most important endpoint for radiation protection, which is carcinogenesis. The database for risk assessment due to galactic cosmic rays is nearly completely based on animal experiments conducted at ion beam therapy facilities. The data especially for carcinogenesis due to heavy ions are very scarce and the estimation of risk factors is connected to large uncertainties. The database gained by the clinical application of ion beams is still too limited to draw any direct conclusion for space research. This may change in the next decade due to the increasing patient numbers that will be treated in the new ion beam facilities. Until relevant data for late effects (esp. carcinogenesis) become available from an analysis of patients treated at ion beam facilities, animal experiments seem to be the only reasonable way to obtain more accurate and systematic data which are needed for a reduction of the uncertainties in the assessment of risks due to galactic cosmic rays in space flight.