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Latest developments in radiation therapy such as IGRT (image guided radiation therapy) and IMRT
(intensity modulated radiation therapy) promise to spare organs at risk by applying better dose
distribution on the tumor. For any effective application of these methods the exact positioning of the
patient and the localization of the exposured organ is crucially. With respect to the filling of rectum
and bladder the prostate can move several millimeters up to centimeters. That implies the need of
daily determination and correction of the position of the prostate before irradiation. We calibrated
a scan head of a B-mode US machine (Ultramark 9, advanced Technology Laboratories, USA) by
means of an optical tracking system (Polaris, NDI, Can). 2D/3D registration was accomplished
by minimizing an adapted mutual information function. Before the registration procedure was
started CT respectively US images were preprocessed by applying various filters and masks. For
registration between the tracking system and the coordinate system of the Linac (defined by the
positioning laser system) a block with three drilled holes was used. Since the coordinates of the
holes are known in both coordinate systems a simple point-to-point registration fulfills the task. The
complete system setup was evaluated by means of a water-filled balloon embedded in a gelatinetank.
For additional evaluation of the 2D/3D registration we used real patient data. For evaluation
of the 2D/3D registration on patient data the prostate was drawn in by a physician on the US
image and the reformatted CT slice. Then, the Hausdorff distance between the two structures was
calculated. The target registration error (TRE) with respect to the balloon experiments amounted
to be 2.1mm±1.2mm for 10 targets. The US calibration was accomplished with an error of 0.8mm±
0.2mm (five calibrations). With respect to the 2D/3D registration we found an Hausdorff distance
of 2.6mm. The results imply that the method is sufficiently accurate and robust. It can be easily
applied to older linear accelerators at low costs.
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