A CT-based computerized treatment planning system for I-125 stereotactic brain implants.

A computer program has been developed at the University of California, San Francisco, as an aid in planning and evaluating stereotactic brain implants made with 125I seeds. The program allows images of seeds and catheters to be positioned in the target volume revealed by CT. It then generates and displays the resulting isodose distributions. Catheters may be changed interactively until an optimum implant is achieved. From the geometry of a stereotactic implant frame as measured by CT, the program calculates the approach angles of the catheters in the frame coordinate system. After the seeds are implanted, films made with a fiducial marker box can be used to generate true seed positions and hence true isodoses. This paper describes mathematically the geometrical transformations used by the program, and also outlines its many features and options. In its first 2 years of use the program has proved to be a valuable contributor to improved patient care.