The 3-dimensional treatment planning system visualizes the spatial interrelationships of the radiation beam, tumor and adjacent organs within the patient. It is possible to rotate and vary the scale of the display to better comprehend the extent of these structures. By viewing the display as if from along the radiation beam, one can design shaped treatment fields which best suit the 3-dimensional nature of the disease. With this system, it is possible to reduce the volume of normal tissue which would typically be irradiated if 2-dimensional treatment planning techniques and assumptions were employed.