The coregistration of planning CT and 18F-fluoro-deoxy-2-glucose (FDG) positron emission tomography (PET) with patient in the same treatment position is the principally well-established tool for improving the target coverage defined and the target planning volume to treat the metabolic target volume. Most of the interest in the coresgistred CT/PET images on volume delineation has focused on conformal radiation therapy of non-small cell lung cancer. In spite of technical difficulties related to the target volume displacements, and the sensitivity and the specificity of FDG-PET images < 100%, the target volume delineation is significantly changed by the coregistration of FDG-PET images and planning CT by either reduction of the radiation volume (excluding atelectasis or mediastinal lymph node) or the increasing of mediastinal lymph node involvement. Image fusion technique reduces the interobserver variability in target volume delineation. Furthermore, after induction chemotherapy image fusion leads to improve the patient management by detecting locoregional progression disease or the presence of metastatic disease. Other anatomic tumor sites are going to investigate such as: head-and-neck cancer, gynecologic cancer, oesophageal cancer, anal cancer, Hodgkin's disease, and non-Hodgkin's lymphoma. The impact on treatment outcome remains to be demonstrated.