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Purpose: RBE-LET relationships for cell inactivation and hprt mutation in V79 cells have been studied with mono-energetic low-energy proton beams at the radiobiological facility of the INFN-Laboratori Nazionali di Legnaro (LNL), Padova, Italy., Materials and Methods: V79 cells were irradiated in mono-layer on mylar coated stainless steel petri dishes, in air. Inactivation data were obtained at 7.7, 34.6 and 37.8 keV/microm and hprt mutation was studied at 7 7 and 37.8 keV/microm. Additional data were also collected for both the end points with the proton LET already considered in our previous publications, namely 11.0, 20.0 and 30.5 keV/microm., Results: A maximum in the RBE-LET relationship for cell inactivation was found at around 31 keV/microm, while the RBE for mutation induction increased continuously with LET., Conclusions: The proton RBE-LET relationship for cell inactivation is shifted to lower LET values compared with that for heavier ions. For mutation induction, protons of LET equal to 7.7keV/microm gave an RBE value comparable with that obtained by helium ions of about 20 keV/microm. Mutagenicity and lethality caused by protons at low doses in the LET range 7.7-31 keV/microm were proportional, while the data at 37.8 keV/microm suggest that this may not hold at higher LET values.

During the upgrading of the radiobiological facility at the Laboratori Nazionali di Legnaro (LNL) we found that uncorrected values of the proton energy were used in the past. This circumstance prompted us to perform the re-evaluation of the physical parameters for all the proton beams used in our previous radiobiological investigations (Belli et al. 1987) and, subsequently, the re-evaluation of all our previous dose-response curves for inactivation and mutation induction (Belli et al. 1989, 1991). This re-evaluation leads to significant changes in the dose-response curves and in the RBE-LET relationships only at the two lowest energies (highest LET) used. These two points are not reliable for the identification of a peak in RBE-LET relationship for cell inactivation. In spite of that, the extent of the changes is not such as to modify the general conclusion previously drawn, pointing out that there is a LET range where protons are more effective than alpha-particles.