IMPROVEMENTS IN PROSTATE RADIOTHERAPY FROM THE CUSTOMIZATION OF BEAM DIRECTIONS

A methodology for optimizing the beam directions in radiotherapy treat ment planning has been developed and tested on a cohort of twelve pros tate patients. An optimization algorithm employing an objective cost f unction was used, based on beam's-eye-view volumetrics but also employ ing a simple dose model and biological considerations for organs-at-ri sk (OARs). The cost function embodies information about the volume of OARs in a single field and their position relative to the planning tar get volume (PTV). The proximity of the PTV to the surface of the patie nt is also included. Within the algorithm ''importance factors'' were used to model the clinical importance of different organs-at-risk so t hat all organs-at-risk were included in a single objective score. ''Ga ntry-angle-windows'' were introduced to restrict the available beam di rections. The methodology was applied to twelve prostate patients to d etermine the optimum beam directions for three-field direction plans. Orientation-optimized and standard treatment plans were compared via m easures of tumor control probability (TCP) and normal tissue complicat ion probability (NTCP). Standard plans had fixed beam directions where as orientation-optimized plans contained beam directions chosen by the algorithm. The beam-weights of both the orientation-optimized and sta ndard plans were optimized using a dose-based simulated annealing algo rithm to allow the improvements by optimizing the beam directions to b e studied in isolation. The results of the comparison show that optimi zation of the beam directions yielded better plans, in terms of TCP an d NTCP, than the standard plans. When the dose to the isocenter was sc aled to produce a rectal NTCP of 1%, the average TCP of the orientatio n-optimized plans was (5.7 +/- 1.4)% greater than that for the standar d plans. In conclusion, the customization of beam directions in the tr eatment planning of prostate patients using an objective cost function and allowed gantry-angle-windows produces superior three-field direct ion plans compared to standard treatment plans. (C) 1998 American Asso ciation of Physicists in Medicine.