A GENERALIZED PENCIL BEAM ALGORITHM FOR OPTIMIZATION OF RADIATION-THERAPY

An iterative pencil beam algorithm for optimization of multidimensiona l radiation therapy dose plans has been developed. The algorithm allow s the use of both physical and radiobiological treatment objective fun ctions and allows arbitrary sampling such as straight Cartesian grids with linear or nonlinear sampling functions or random sampling. The al gorithm can account for and optimally combine almost all the degrees o f freedom at an advanced radiotherapy clinic, such as different beam m odalities and spectra, beam directions, beam fluence distributions, an d time-dose fractionations. The algorithm allows for external charged and neutral beams as well as intracavitary and interstitial sources to be optimally combined. A quantity termed the generalized fluence vect or is introduced, combining fluences and energy fluences from external beams as well as the radiation source densities of intracavity and in terstitial sources or external source distributions. The positivity co nstraint on the generalized fluence can therefore be applied directly during the optimization procedure. The convergence properties and the required iteration time of the algorithm are discussed. Several exampl es with combinations of photon and electron beams of different energie s and directions of incidence are presented. The optimization has been made with the treatment objective to maximize the probability of achi eving tumor control without causing severe complications in healthy no rmal tissues.