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The success of radiotherapy critically depends on the imaging modality used for treatment planning and the level of integration of the available imaging information. The use of functional/metabolic imaging provides us much more than a tool to delineate better the boundary of a tumor target. Together with anatomical CT or MRI images, functional imaging affords valuable 3D structural plus 1D metabolic data for both tumor and sensitive structures, valuable for guiding us to design spatially non-uniform dose distributions to deliver high doses to where the tumor burdens are high and differentially spare the sensitive structures according to the functional importance distributions. The integration and utilization of the functional data in radiation therapy treatment planning become increasingly important to improve clinical cancer management. While it is straightforward to modify the radiation portals to accommodate any changes in treatment volume, new methods of dose optimization and medical decision-making must be developed to take full advantage of the metabolic information and IMRT. How to achieve biologically conformal doses, instead of the geometrically conformal dose distribution, presents a new challenge to radiation oncology discipline. Hopefully, with the efforts from multiple institutions, the new approach of imaging, planning and decision-making will be resolved. Ultimately, whether using deliberately inhomogeneous dose distributions obtained under the guidance of functional imaging such as MRSI can improve patient survival and reduce the side effects associated with radiation treatment should be established through extensive clinical trials.

The creation of a comprehensive system for simulation is being driven by both the advancements in imaging tools for characterization of the patient’s diseased and normal anatomy and by the introduction of volumetric imaging systems for daily guidance and verification of delivery. Such a comprehensive system will (i) accelerate the introduction of further developments in imaging to the simulation process, (ii) provide an appropriate infrastructure to support the vast quantity of imaging information that will be streaming from image-guidance approaches such as cone-beam CT systems. Genuine opportunity to bring accurate disease and normal structure characterization together with daily accounting of the dose delivered for better understanding of disease control and complication induction, as well as, permit re-optimization of the treatment’s parameters as therapy progresses.

Four-dimensional CT is an imaging technique that provides information on organ motion during respiration. It provides a more accurate assessment of target shape and trajectory, and similar information on organs at risk. Technological advances in software and hardware for 4D simulation are likely to rapidly become available in the next few years. The ability to generate 3D CT maps of anatomy as a function of respiratory phase has important applications in treatment planning and delivery, including optimization in the presence of motion, aperture design, dose calculations to moving targets, and image guided therapy delivery.