OPTIMIZING THE PLANNING OF INTENSITY-MODULATED RADIOTHERAPY

A method of computing optimized intensity-modulated beam profiles has been further developed and used to generate highly conformal radiotherapy dose distributions. The features of these distributions are shown to be strongly dependent on the tuning built into the algorithm. The optimization aims to achieve a specified dose prescription with a posteriori computation of probabilistic biological response. A method has been developed to show the effect of stratifying the intensity-modulated beam profiles into a number of finite intensity increments. It is shown that, provided the number of intensity strata is not too small, highly conformal dose distributions can be achieved with a number of fields (e.g. 7 or 9) which is not excessively large. This number, however, depends on the exact shape of the planning target volume (PTV) and its disposition with respect to juxtaposed organs at risk (OARs). These intensity-modulated profiles can therefore be delivered either by apparatus for 'tomotherapy' or by using the multileaf collimator at each gantry orientation to deliver a sequence of fixed fields with different field sizes, constructing the beam profile via finite increments of beam intensity. When the PTV and OARS overlap, due to including a finite margin on the clinical target volume to account for tissue movement, it is shown that the dose delivered to the overlap region provides a limit on what can be achieved with conformal therapy. This problem is encountered, for example, when treating the prostate which lies next to part of the rectum and bladder. Some comment is provided on, but not a solution for, the problem of optimizing field orientation.