Liu, Z., Mothersill, C. E., McNeill, F. E., Lyng, F. M., Byun, S. H., Seymour, C. B. and Prestwich, W. V. A Dose Threshold for a Medium Transfer Bystander Effect for a Human Skin Cell Line. Radiat. Res. 166, 19–23 (2006).

The existence of radiation-induced bystander effects mediated by diffusible factors is now accepted, but the mechanisms and precise behavior at low doses remain unclear. We exposed cells to γ-ray doses in the range 0.04 mGy–5 Gy, harvested the culture medium, and transferred it to unirradiated reporter cells. Calcium fluxes and clonogenic survival were measured in the recipients. We show evidence for a dose threshold around 2 mGy for the human skin cell line used with a suggestion of increased survival below that dose. Similar experiments using direct γ irradiation showed no reduction in survival until the dose exceeded 7 mGy. Preliminary data for neutrons where the γ-ray dose was kept below the bystander threshold do not show a significant bystander effect in the dose range 1–33 mGy. A lack of a bystander response with neutrons occurred at around 1 Gy, where significant cell killing from direct irradiation was observed. The result may have implications for understanding the role of bystander effects at low doses.