Behind : In Response to Drs. Mavroidis and Lind

corresponding response curves of the organs at risk can be easily compared individually or combined in the total complication response curve, PI. In this diagram, the confidence intervals of the dose-response curves are also illustrated. It is apparent that the use of the ̄̄ D concept on the dose axis provides the appropriate dose prescription basis for making such comparisons practical and clinical useful. The normalization using ̄̄ D gives emphasis to the therapeutic window, which characterizes each treatment plan. Such as the dose volume histogram chart is a good illustration of the volumetric dose distribution delivered to the patient, so is the biological evaluation plot (/P ̄̄ D diagram) of a dose plan a good illustration of the expected clinical outcome. Another important clinical use of ̄̄ D is illustrated in the right diagram of Figure 2. Using a certain set of radiobiological parameters of a given radiobiological model, the dose-response curve of a tissue is calculated for a range of uniform doses. Subsequently, the response probability is calculated for every patient using again those parameters and the individual dose distribution delivered. By applying the concept of biologically effective uniform dose on these probabilities, the corresponding ̄̄ D values are found. Plotting these dose-response points on the existing diagram they will by definition fall exactly on the theoretical dose-response curve. To examine whether the theoretical curve reproduce the observed response rates it is enough to compare these values for the region around the prescribed dose used by the center where the patients were treated (using a statistical method such as the chi-square test). If the two values are close enough then the parameters can be used for predicting the treatment outcome for the applied technique. This is a simple way to examine if a set of parameters is compatible with the clinical practice that a center uses.