The linear-quadratic (LQ) model of radiobiological effect is well established in conventional, i.e., external beam, radiotherapy. Because the model is derived from sound biophysical principles, it is also emerging as the standard formalism for assessing biological responses for the whole range of radiotherapy treatments. A central feature of LQ methodology is the quantity known as the biologically effective dose (BED), which may be used to quantify the radiobiological impact of a treatment on both tumors and normal tissues. The BEDs commonly associated with conventional therapy may thus be compared to those expected from novel radiotherapy treatments, such as targeted radionuclide therapy. This approach also provides a mechanism for designing targeted treatments which are therapeutically equivalent to external beam treatments. In this paper the LQ methodology is outlined and worked examples are provided which demonstrate the tentative link between targeted radiotherapy doses and those used in conventional radiotherapy. The incorporation of an allowance for relative biological effectiveness (RBE) effects is also discussed. The complexity of the subject and the potentially large number of variables does place a restriction on overall predictive accuracy and the necessary caveats are outlined.