Although we did not measure apoptosis or necrosis, these outcomes may influence the responses seen in these cell lines. There is clear evidence of a bystander effect in response to high doses of photons when we used the same serum and cell lines as for the neutron experiments. This result indicates that the methods used in our study are capable of detecting a bystander effect if such an effect exists. To the best of our knowledge there is no other factor that could have prevented neutrons from inducing a bystander effect in these cells, assuming a bystander effect even exists. Our findings are in agreement with previous studies that have reported the lack of a bystander effect on neutron exposure using clonogenic cell survival assay in a human skin cell line and zebrafish. However, other studies have reported contrasting results. Watson et al. found that transplantation of a mixture of neutron irradiated and unirradiated bone marrow cells into mice induced instability in the descendants of unirradiated cells as confirmed by measuring chromosomal aberrations, indicating that neutrons induce a bystander effect. However, since the gamma component in neutrons was 25%, it is possible that the observed bystander effect was due to the contaminating photons, which the authors did not rule out. Kinashi et al. studied a neutron-induced bystander effect in boron neutron capture therapy with a cell survival assay as well as cloning and sequencing methods. They reported an increase in the frequency of mutations in the hypoxanthine-guanine phosphoribosyltransferase locus in cells located near the irradiated cells. These results suggest that a neutron bystander effect may be comprised of gene mutations. The inability of fast neutrons to induce a cytogenetic bystander effect as shown here may be due to different types of damage induced at the molecular level compared to photons. Cellular recognition of DNA damage and the subsequent repair processes may differ between neutrons and photons. Furthermore, due to the lower levels of oxidative damage and free radical production by neutrons compared to photons, some of the critical bystander signaling pathways may not be activated. There is also the possibility that a neutron-induced bystander effect, if any, might depend on cell type, the endpoint being evaluated, and the energy of the neutrons. Neutrons, depending on their energy, might be more effective in controlling certain tumor types where conventional photon therapy is ineffective because the oxygen enhancement ratio, i.e. the differential radiosensitivity between poorly oxygenated and well-oxygenated cells, is reduced with neutrons. Unlike low-LET radiation, for high-LET radiation there is also a reduction in the differential radiosensitivity of cells related to their position in cell cycle. Recently, radiation-induced bystander cells were shown to rescue irradiated cells through intercellular feedback.