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In current clinical practice, radiation therapy planning (RTP) has often been treated as a two-dimensional (2D) problem, mainly due to the limitations in visualization technology available to date. The slice-by-slice display format makes it difficult to visualize the path of radiation beam not perpendicular to the axis of the CT slices. This discourages consideration of treatment plans that utilize radiation beam out of the transverse plane. Human body anatomical structures are inherently three-dimensional (3D) objects, and tumors and tissues/organs involved in the RTP are all of 3D shapes. A clear understanding of 3D spatial relationships among these structures, as well as the anatomic impact of 3D dose distributions, is essential for designing and evaluating radiation therapy plans. We have recently made an important breakthrough in the high-resolution volumetric 3D display technology and have made an initial attempt to apply it to RTP applications. By "volumetric 3D display", we mean that each "voxel" in the displayed 3D images is located physically at the (x, y, z) spatial position where it is supposed to be, and emits light from that position to form real 3D images in the eyes of viewers. We have demonstrated the feasibility of our system design by building full-scale prototypes and achieved a multi-color, large display volume, true volumetric 3D display system with a high resolution of over 10 million voxels in a portable design. This type of true 3D display system is able to present a 3D image of a patient's anatomy with transparent skin, providing both physiological and psychological depth cues to oncologists in perceiving and manipulating radiation beam configuration in true 3D fashion, thus offering a unique visualization tool to ensure the safety, effectiveness, and speed of the RTP process. The volumetric 3D display technology holds promise to significantly enhance the accuracy, safety, and speed of RTP procedures. Such an "understanding at a glance" ca- - pability is necessary to keep the clinicians from becoming bogged down in details, as he/she would be if provided only with conventional 2D display of CT slices with overlaid isodose lines. The main focus of this paper is to provide technical details on the volumetric 3D display system we developed, and present some initial results on its capability of displaying true 3D images. While the system design framework of applying such technology into RTP is introduced, its full scale clinical applications to RTP is still an ongoing effort and will be reported later in other publications.