Since 1957, broad proton beam radiotherapy with a spread out Bragg peak has been used for cancer treatment. More recently, studies on the use of proton therapy in the treatment of non-small cell lung cancer (NSCLC) were performed and although the benefit of using protons for the treatment of NSCLC is recognized, more work is needed to gather additional data for the understanding of cell response. Human A549 cell survival was evaluated by colony forming assay 11 days after 10 keV/μm proton beam irradiation at 0.1 and 1 Gy/min. The residual energy of the proton beam at the location of the irradiated cells was 3.9 MeV. In parallel, early effects on the cell viability and DNA damage were assessed and DNA synthesis was measured. The survival curve obtained was fitted with both the linear and the induced-repair models, as a hyper-radiosensitivity was evidenced at very low doses. Above 0.5 Gy, a linear shape was observed with the α parameter equal to 0.824 ± 0.029 Gy⁻¹. In addition, early cell death and cell proliferation arrest were enhanced. Moreover, a clear correlation between DNA damage and surviving fraction was observed. Finally, comparisons with X ray results indicate that proton irradiation at 10 keV/μm enhanced the tumor radiosensitivity with a significant dose-dependent decrease in the survival fraction. The RBE value of 1.9 ± 0.4 obtained for a 10% survival support this observation.