High-LET radiation carcinogenesis.

The dose-response curves for the induction of tumors by high-LET radiation are complex and are insufficiently understood. There is no model or formulation to describe the dose-response relationship over a range 0-100 rad. Evidence suggests that at doses below 20 rad the response is linear, at least for life shortening and some tumor systems. Thus limiting values of RBEs for the induction of cancer in various tissues can be determined, but it will require sufficient data obtained at low single doses or with small fractions. The results obtained from experiments with heavy ions indicate an initial linear response with a plateauing of the curve at a tumor incidence level that is dependent on the type of tissue. The RBE values for the heavy ions using 60Co gamma rays as the reference radiation increase with the estimated LET from 4 or 4H to about 27 for 56Fe and 40Ar. The dose responses and RBEs for 56Fe and 40Ar are similar to those for fission neutrons. These findings suggest the possibility that the effectiveness for tumor induction reaches a maximum.