Incorporating functional imaging information into radiation treatment.

Three-dimensional treatment planning systems allow the clinician to define tumor and normal anatomy of computed tomograpy (CT) scans and project the result onto a digitally reconstructed radiograph (DRR) for comparison with a simulation or portal film. Unfortunately, the CT scan does not always show the tumor accurately. First, the tumor may have either been removed surgically, or cytoreduced with chemotherapy before the treatment planning scan was taken. Or, the planning CT may not be the ideal imaging modality for a particular tumor, magnetic resonance imaging (MRI) or positron-emission tomography (PET) being much better. In either case, the nonplanning images provide more reliable data as to the position and extent of tumor than do the CT. A 3D/3D registration between the diagnostic and planning image must then be performed to make the data from both images available for the planning process. Methods of performing accurate 3D/3D registration of dissimilar images have been studied extensively by experts in image processing, but the techniques have not yet been fully adopted by the medical community. In addition, there is no standard way of dealing with the multiple tumor volumes that will be generated by full multimodality treatment planning. This article ends with speculation as to the extent to which multimodality image-based treatment planning can improve cancer treatment rates. New imaging modalities such as magnetic resonance spectroscopy, PET, or functional imaging, tuned to the particular tumor type, might reveal more than just the gross tumor volume seen on CT or MRI. One could imagine radiation treatment to many sites in the body under image guidance that would result in cure of metastatic disease, should the cancer be confined to a reasonable number of discrete sites.