Purpose: To systematically investigate how the choice of beam angles in IMRT of the prostate affects the dosimetry of the PTV and each OAR, as well as how the dosimetry of each structure is correlated with that of other structures. Methods and Materials: Five prostate patients, previously treated with a 5-beam IMRT plan to 86.4 Gy, were selected for this study. Candidate beams were defined in a 360° arc around the isocenter. Treatment plans were generated for each set of 5 beams taken out from the set of the candidate beams. First, all the optimization constraints, as well as all other algorithmic parameters, were kept fixed as in the clinical plan. Subsequently, additional constraints were placed on the rectum and bladder, in order to bring forth any differences between the beam sets. Preliminary Results: The methodology described previously has been manually applied to a single prostate patient for 9 candidate beams, equally spaced every 40°. Altogether 252 (126 x 2) plans were generated for this patient. All plans were normalized so that the maximum dose to the rectal wall is 99%. The envelope DVHs describe all the DVHs obtained from the 126 plans (Fig. 1). The clinical DVHs may or may not lie within the envelope because the clinical beam arrangement was not among the 126 examined. The envelope DVHs for the PTV is fairly narrow, indicating that coverage was not an issue for this patient, regardless of the beam arrangement used. However, the envelope DVHs for the rectum, bladder and, especially, the large bowel are much wider, suggesting that the choice of beam directions may have a significant impact on the treatment plan. The use of additional constraints improves the range of DVHs for the rectum, bladder and, especially, the large bowel. Conclusion: The choice of beam directions may have a significant impact on the dosimetry of the rectum, bladder and, especially, large bowel. Furthermore, the use of additional constraints has the potential to improve current treatment plans. These may, in turn, affect the patient’s quality of life and post-treatment disease management.