An investigation of changes in relative biological effectiveness (RBE) with depth for X ray beams generated between 100 and 250 kVp using the mouse foot as biological test system.

For the early mouse foot skin reactions an increase in RBE relative to 60Co has been found for the irradiations with X ray beams generated between 100 and 250 kVp at both the surface and depth, respectively. In most cases, the RBE of the late skin reactions for irradiations at the surface and at depth are significantly lower than those for the early reactions. The clinical implications of these results, the effect of a possible hypoxic fraction of cells in the skin, and the choice of a "standard" radiation modality for radiobiological experiments are discussed.