Abstract: A differential scatter-air ratio calculation procedure is utilized to calculate scatter contribution which when combined with primary radiation yields patient dose. The lengthy polar coordinate integration of the prior art is replaced with a rectangular coordinate integration which to an extent can be performed outside an integration loop. Beam characteristics are entered into a one dimensional matrix BWP. A second matrix RSAR includes the differential scatter air ratio for incremental regions within the patient at a certain depth and radial distance from a scatter calculation point. During a rectangular coordinate integration, a two dimensional coordinate within a patient plane determines an offset into both the one dimensional beam characteristic matrix BWP and the differential scatter-air ratio matrix RSAR. Then values are multiplied to produce a scatter contribution from the incremental region.