Automatic calculation of margins around planning volumes in three dimensions

After the appearance of the ICRU-50 report, the concepts of GTV (gross target volume), CTV (clinical target volume) and PTV (planning target volume) are increasingly being used in RT-planning. The planning system at the DDHCC is CADPLAN. Normally the GTV or CTV is contoured in the CT-slices by the clinician. The planning technician subsequently adds a 3D margin to determine the PTV. If contours in neighbouring CT-slices vary significantly it is virtually impossible to add manually margin to GTV or CTV in the CT-slices such that the overall 3D margin is correct. A computer program has been developed to automate this step in the planning. First the original GTV or CTV contour points are used to place the volume in a 3D matrix. The matrix is filled with values 1 inside, 0 outside and 0.5 at the edge of the volume. The PTV is computed by centring a 3D ellipsoid at every matrix element within the original volume. The shape of the ellipsoid reflects the size of the user-selected margins in each direction. The contours for the PTV are determined by computing the isosurface of value 0.5. The program has been used clinically since the end of 1994 for prostate cancer plannings. The time required to obtain the PTV from the GTV reduces from 4-5 hours for manual drawing of the PTV to 1-2 minutes with a better accuracy for the automatic procedure. Results have also been compared to the planning strategy before the ICRU-50 report, when the PTV was contoured directly by the clinician. Significant discrepancies between the PTV's have been found. Especially in case of conformal therapy, this has a noteworthy influence on the field shape and hence on the dose distribution