Elmore, E., Lao, X-Y., Kapadia, R. and Redpath, J. L. Threshold-Type Dose Response for Induction of Neoplastic Transformation by 1 GeV/nucleon Iron Ions. Radiat. Res. 171, 764ndash;770 (2009).

Neoplastic transformation of HeLa × skin fibroblast human hybrid cells by doses of 1 GeV/nucleon iron ions in the range 1 cGy to 1 Gy to exposed cultures has been examined. The data indicate a threshold-type dose–response curve with no increase in transformation frequency until doses above 20 cGy. At doses <10 cGy, not all exposed cells receive a direct traversal of an iron-ion track core, but all exposed cells receive up to several mGy of low-LET radiation associated with the δ-ray penumbra. It is proposed that the threshold-type response seen is a consequence of an adaptive response associated with the δ-ray exposure. For comparison purposes, the dose response for ¹³⁷Cs γ rays over the same dose range was examined using the same experimental procedure. As we have shown previously, the dose response for ¹³⁷Cs γ radiation was J-shaped. The iron ions were 1.5 to 1.7 times more biologically effective than the γ radiation over the dose range examined.