Role of beam orientation optimization in intensity-modulated radiation therapy

Purpose: To investigate the role of beam orientation optimization in intens ity-modulated radiation therapy (IMRT) and to examine the potential benefit s of noncoplanar intensity-modulated beams. Methods and Materials: A beam orientation optimization algorithm was implem ented. For this purpose, system variables were divided into two groups: bea m position (gantry and table angles) and beam profile (beamlet weights). Si mulated annealing was used for beam orientation optimization and the simult aneous iterative inverse treatment planning algorithm (SIITP) for beam inte nsity profile optimization. Three clinical cases were studied: a localized prostate cancer, a nasopharyngeal cancer, and a paraspinal tumor. Nine fiel ds were used for all treatments. For each case, 3 types of treatment plan o ptimization were performed: (1) beam intensity profiles were optimized for 9 equiangular spaced coplanar beams; (2) orientations and intensity profile s were optimized for 9 coplanar beams; (3) orientations and intensity profi les were optimized for 9 noncoplanar beams. Results: For the localized prostate case, all 3 types of optimization descr ibed above resulted in dose distributions of a similar quality. For the nas opharynx case, optimized noncoplanar beams provided a significant gain in t he gross tumor volume coverage. For the paraspinal case, orientation optimi zation using noncoplanar beams resulted in better kidney sparing and improv ed gross tumor volume coverage. Conclusion: The sensitivity of an IMRT treatment plan with respect to the s election of beam orientations varies from site to site. For some cases, the choice of beam orientations is important even when the number of beams is as large as 9, Noncoplanar beams provide an additional degree of freedom fo r IMRT treatment optimization and may allow for notable improvement in the quality of some complicated plans. (C) 2001 Elsevier Science Inc.