A radiation therapy dose distribution method starts with selecting a treatment type. Then an organ at risk (OAR) distance to target map is determined. The OAR distance to target map comprises distances to a target organ for portions of an OAR. The OAR distances are determined from at least one segmented patient organ image. A cohort average dose distance to target histogram is selected. A dose value to the portions of the OAR are assigned to form a first 3D dose distribution map. The dose values are from the selected cohort average dose distance to target histogram. A second 3D dose distribution map is determined based on a field arrangement determined by the treatment type and the first 3D dose distribution map. A dose distance to target histogram is calculated using the second 3D dose distribution map and the distance to target map.