From clinical target volume to biological target volume

The authors' experience with a point matching algorithm for image registration belonging to a commercially available software package for conformal radiotherapy, is reported. The algorithm IFS (Image Fusion System) permits the registration of two image data-sets in two different manners: by use of a stereotactic localization frame, dedicated to brain studies, and by means of point markers that may be internal anatomical landmarks or external fiducials fixed on the patient skin. Position errors were obtained by evaluating the stereotactic coordinates of seven sources detectable by Magnetic Resonance Imaging (MRI) and Computed Tomography (CT), for the first method. The comparison of the geometric centers of cylindrical rods enclosed in a second phantom was employed to evaluate the registration accuracy of the second algorithm. The mean differences in source identification between CT and MRI images are inferior to 1 mm with both techniques, if MRI distortion phenomenon and patient movements are excluded. The software utility of the IFS algorithm to draw, after fusion, a target ROI that is the synthesis of the two information modalities undergoing registration may be a useful tool for the optimization of a radiotherapy treatment planning.