The authors describe a five-field technique of megavoltage radiation therapy involving the use of large, irregularly-shaped portals at variable source-skin distances (130–160 cm), developed with the goal of achieving homogeneous dose distributions to the brain, spinal column, gonads, and abdomen simultaneously. A computational model for determining the dose rate at any point within such fields has been derived from a consideration of tissue-maximum ratios, scatter-phantom ratios, and collimator scatter factors studied at these extended SSDs and compared with experimental measurements.