1. A method of calculating a dose distribution for a patient for use in a radiation therapy treatment plan, the method comprising: acquiring an image of a volume within the patient; defining a radiation source; defining a reference plane oriented between the radiation source and the patient; generating a radiation therapy treatment plan, the plan including a plurality of rays that extend between the radiation source and the patient volume; calculating a three-dimensional dose volume for the patient volume from the plurality of rays that intersect the reference plane without first having to independently calculate a dose distribution on each of the plurality of rays; and displaying the three-dimensional dose volume.