Figure 1.

Diagrammatic plot of tumour control probability (TCP) or normal tissue complication probability (NTCP) vs. radiotherapy dose. Sparing normal tissues shifts the NTCP curve to the right (B to C), allowing a lower incidence of normal tissue damage for the same dose (dose 1) or the same level of NTCP for a higher dose (dose 2). This is the basis for an improvement in the therapeutic ratio. This benefit can be the result of any measure which reduces the normal tissue dose, for example including better target imaging, conformal radiotherapy, and improved patient immobilisation.