Determination of the Site and Extent of the Lesion and the Surrounding Healthy Tissues
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To establish a plan of radiotherapy, it is first necessary to determine whether or not a lesion exists, how large it is, and how it extends to the surrounding tissues including bones or soft tissues. In case a highly radiosensitive healthy tissue exists close to the tumor, it is essential to know its position accurately in order to minimize radiation injury. Success in radiotherapy depends on the ability to give a sufficient dose to the tumor in order to cause destruction, without producing unacceptable damage to surrounding healthy tissue. For example, in radiation therapy for maxillary cancer, not only the location and the extent of the tumor tissue but also the position of the radiosensitive bilateral lens can be clearly determined on a CT image (Fig. IV.1).