RBE-LET relationships for cell inactivation and mutation induced by low energy protons in V79 cells : further results at the LNL facility

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/μm and hprt mutation was studied at 7.7 and 37.8 keV/μm. Additional data were also collected for both the end points with the proto LET already considered in our previous publications, namely 11.0, 20.0 and 30.5 keV/μm. Results: A maximum in the RBE LET relationship for cell inactivation was found at around 31 keV/μm, 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 7.7 keV/μ gave an RBE value comparable with that obtained by helium ions of about 20 keV/μm. Mutagenicity and lethality caused by protons at low doses in the LET range 7.7 31 keV/μm were proportional, while the data at 37.8keV/μm suggest that this may not hold at higher LET values.