Schematic diagram of the dose optimization and calculation geometry used in this work. The central transverse slice of the geometry is shown. The phantom is cubic with a cylinder of elliptical cross section as the target. The major and minor axes of the ellipse are 20 cm and 6.6 cm, respectively, and its length is 7.8 cm, resulting in a volume of 793.3 cm3. Point A is centered in the target volume and is the isocenter for all lateral oblique fields, and for anterior fields delivered with multiple carriage positions. Points B and C, 8 cm apart, are isocenters for cases where the anterior component of the dose distribution is delivered with dual isocenter fields.