Californium-252 in radiotherapy: RBE-dose rate relationships

In a recent article on the potential of Californium-252 in radiotherapy (Hall and Rossi, 1975) the authors reviewed the experimental, physical and biological work that has been done up to 1974. They observed that the RBE values of Cf neutrons relative to γ rays tend to scatter around a common line which indicates a decrease in RBE as a function of increasing dose rate. RBE values for several biological systems such as cells cultured in vitro, pig skin, mouse jejunum, lymphocytic leukaemia cells and mouse lethality, appear to show the same relationship as a function of dose rate. The authors also mentioned that the fit is not perfect because of the well-known RBE variations among different tissues and cell systems. The few points some distance from the line were discussed in more detail and an explanation was provided.