An automatic seed identification technique for interstitial implants using three isocentric radiographs.

A technique for the automatic reconstruction of the spatial coordinates of seeds in an interstitial implant has been developed. Seed coordinates from three isocentric radiographs, an anterior-posterior film, and an orthogonal pair of films taken at gantry angles of +/- 45 degrees from the anterior-posterior direction are digitized and recorded in a random sequence. From this, the three-dimensional coordinates of the seeds in the implant are computed by matching the values of common coordinates along the axis of rotation. This rotate and match technique leads to an accurate and consistent identification and reconstruction of seeds with 90% of them within 2 mm of their actual location. These new computer routines have been added to an existing treatment planning system (AECL TP-11 treatment planning system, version 5A). We present our algorithm and technique along with clinical examples.