This paper reviews the generalized application of the linear quadratic (LQ) model of radiobiological effect to targeted radiotherapy. Special attention is given to formulations for normal tissue responses and these are applied, in particular, to the kidney. Because it is derived from self-consistent bio-physical principles, the LQ model currently remains the standard formalism for assessing biological responses for the whole range of radiotherapy treatments. A central feature of the model is the derivation of biologically effective doses (BEDs), which may be used to quantify the impact of a treatment on both tumors and normal tissues. BEDs are routinely derived for conventional external-beam treatments. The likely limits of targeted radiotherapy may, thus, be assessed by comparing the expected normal-tissue BEDs for such treatments with those known to be just tolerable in conventional therapy. The main parameters required in the model are defined, and data are provided which demonstrate the tentative link between targeted radiotherapy doses and those used in conventional radiotherapy. The extension of the LQ method to targeted radiotherapy involves using parameters for which the numerical values may not be accurately known at present. This places a restriction on the overall predictive accuracy of the model and the necessary caveats are, therefore, outlined.