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The purpose of this study is to establish protocols for simulation and external-beam radiotherapy of pituitary tumors and to examine the performance of a new 3-dimensional treatment planning system (3-D RTP). The asymmetric collimation feature of a dual-energy photon beam linear accelerator allows the user to set up half-beam treatment fields posterior to the orbits with no divergence to anterior structures. Treatment planning was performed with a relatively new Food and Drug Adminisration (FDA)-approved 3-D RTP system. Computerized tomographic (CT) scans used by this system to generate isodose distributions and dose-volume histograms were obtained directly from the scanner which is connected via ethernet cabling to the 3D treatment planning system. These were used for evaluating the dose distribution to the treatment volume, clinical target volume, gross tumor volume, and certain critical organs. Using 6 and 18 MV photon beams, different configurations of standard treatment techniques for pituitary tumors, i.e., a 190° flying wedge arc, 3-fields (an anterior plus a lateral wedged pair), and a pair of lateral fields, were studied and the resulting dose distributions were analyzed. Real-time dose measurements on patients using diode dosimetry were made and compared with computed dose values. With regard to minimizing radiation dose to surrounding structures, (i.e., lens, temporal lobes, spinal cord, etc.) the 190° flying wedge arc technique gave the best isodose distributions, followed by 3 fields (an anterior plus a lateral wedged pair). The opposing lateral fields gave the highest integral dose to the temporal lobes. The dose measured on the patient during the treatment agrees to within ±2% with the computed dose. The protocols presented in this work for simulation, immobilization, and treatment planning of patients with pituitary tumors provide the optimum dose distributions in the target volume with reduced irradiation of surrounding non-target tissues, and can be easily implemented in a radiation oncology department. The presence of a real-time dose-measuring system plays an important role in verifying the actual delivery of radiation dose. © 1995 Wiley-Liss, Inc.