NUMBER AND ORIENTATIONS OF BEAMS IN INTENSITY-MODULATED RADIATION TREATMENTS

The fundamental question of how many equispaced coplanar intensity-mod ulated photon beams are required to obtain an optimum treatment plan i s investigated in a dose escalation study for a typical prostate tumor . Furthermore, optimization of beam orientations to improve dose distr ibutions is explored. A dose-based objective function and a fast gradi ent technique are employed for optimizing the intensity profiles (inve rse planning). An exhaustive search and fast simulated annealing techn iques (FSA) are used to optimize beam orientations. However, to keep c omputation times reasonable, the intensity profiles for each beam arra ngement are still optimized using inverse planning. A pencil beam conv olution algorithm is employed for dose calculation, All calculations a re performed in three-dimensional (3D) geometry for 15 MV photons. DVH s, dose displays, TCP, NTCP, and biological score functions are used f or evaluation of treatment plans. It is shown that for the prostate ca se presented here: the minimum required number of equiangular beams de pends on the prescription dose level and ranges from three beams for 7 0 Gy plans to seven to nine beams for 81 Gy plans. For the highest dos e level (81 Gy), beam orientations are optimized and compared to equia ngular spaced arrangements. It is shown that (1) optimizing beam orien tations is most valuable for a small numbers of beams (less than or eq ual to 5) and the gain diminishes rapidly for higher numbers of beams; (2) if sensitive structures (for example rectum) are partially enclos ed by the target volume, beams coming from their direction tend to be preferable, since they allow greater control over dose distributions; (3) while FSA and an exhaustive search lead to the same results, compu tation times using FSA are reduced by two orders of magnitude to clini cally acceptable values. Moreover, characteristics of and demands on b iology-based and dose-based objective functions for optimization of in tensity-modulated treatments are discussed. (C) 1997 American Associat ion of Physicists in Medicine.