AbstractID: 8990 Title: Daily Target Localization Based on 3D Image Correlation Target localization has become increasingly important with the advent of IMRT due in part to small margins and the high dose gradients outside the target volume. There are several localization techniques that have been used for prostate treatment so far. Recently, the potential use of a CT gantry on rails in the treatment room has been explored in several studies. The goal of our study is to develop an automated procedure for daily treatment table shifts calculated based on two CT datasets: planning CT data and treatment CT data. The method suggested in this study is a 3D image correlation of small regions of interest (ROI) within the two datasets. The relative position of the two ROI with respect to each other is determined by the maximum value of the correlation integral, calculated for all possible locations of the ROI within the two datasets. After the best match is found the shifts are given by the vector connecting the treatment isocenter and the planning isocenter (both determined by the radio opaque marks on the patient’s skin.) The results have been compared with shifts calculated through manual fusion. The differences are less then 2 mm in the lateral and AP directions, and less than the slice thickness in the longitudinal direction. The impact of image noise on the performance of the algorithm has been tested. The results show that the algorithm is reliable in the prostate region with noise levels as high as 20%.