Rapid, accurate, three-dimensional location of multiple seeds in implant radiotherapy treatment planning

Abstract

The three-dimensional (3-D) locations of the radioactive seeds in an interstitial implant can be obtained from stereo-shift or orthogonal radiographs. When an implant contains a large number of seeds, however, there are often ambiguities in identifying corresponding seed images on two radiographs. The authors' method achieves accurate spatial locations by using three radiographs whose X-ray source positions are (i) in a plane which does not intersect the implant and (ii) widely separated to achieve adequate parallax. All triangulation is done in an anatomical coordinate reference frame attached to the patient so that patient motion does not introduce errors. The authors achieve independent error-insensitive calibration of each radiograph, rapid and accurate data entry in the local coordinates of each radiograph, 3-D seed location in the patient (anatomical) reference frame, and automatic verification of each calculation.