The ultimate goal of radiotherapy is to result in complete local control of tumor while sparing the surrounding normal tissues as much as possible. Since the development of CT in 1970s, patient's anatomical normal tissues and the site and extent of infiltration of tumor were identified almost accurately. In addition, the isodose distribution of delivered radiation to target tumor was shown in each cross-section. In the treatment planning of head and neck cancers, CT-reconstruction provided almost 3-dimensinonal inter-relationship between tumor and normal tissues. The utilization of imaging system of the CT scanner made it possible to illustrate in superposition the patient structure image, the radiation beams, and the isodose distributions. Thus it was possible to deliver radiation enough to control the local disease, and to avoid unnecessary administration of radiation to normal tissue such as spinal cord. CT-reconstructed image in axial, sagittal, and coronal planes suggested 3-dimensional radiotherapy treatment planning be possible and practical instead of conventional 2-dimensional planning at coronal plane.