Technical Aspects of Positron Emission Tomography/Computed Tomography Fusion Planning

Emerging technologies in radiation therapy computers and delivery systems allow surgically precise conformal radiation treatment that was not possible with previous generations of equipment. The newest treatment systems can compensate for tumor target motion as well as shape dose distributions to conform precisely to delineated target volumes. These sophisticated technologies now drive the development of imaging modalities able to generate equally high-resolution and lesion-specific roadmaps that are the foundation of these highly accurate radiation plans. Positron emission tomography/computed tomography (PET/CT) is currently becoming a routine imaging tool for radiation oncology because of its combined benefits of positron imaging and high-resolution anatomic display. The improved staging and lesion delineation provided by PET, combined with the 3D anatomic display provided by CT, now allows better treatment stratification and more precise targeting. Additionally, respiratory-gated 4D CT and 4D PET/CT have been used in the simulation process for respiratory-gated radiation therapy. Successful integration of PET/CT into the radiation therapy planning process requires an understanding of how therapy plans are derived and the process by which the patient receives therapy, because these dictate the method of image acquisition. The radiation oncologists, too, must understand the technology of positron imaging to adapt these functional images based on intensities rather than pixels to their targeting process. Modifications to the PET/CT scanner and room are necessary to image the patient in the reproducible position required for treatment planning. Although the impact of these efforts on patient outcome has yet to be determined, the benefit of better treatment choice, due to improved staging, and more precise targeting with less normal tissue exposure resulting in improved quality of life will likely promote PET/CT to the gold-standard for targeted therapies.