Radiographic technique for acquisition of external patient contour

Despite advances in techniques for contour acquisition, most patients are still contoured manually using lead wire. The lead wire method has unpredictable inherent inaccuracies due to the fact that the wire must be removed from the patient to obtain the contour. A simple radiographic method for a quick and accurate contour is presented that eliminates such an operator error. The technique requires that a thin lead wire be placed on the patient's skin in the desired plane as in the manual contouring process. With the wire still in place, the simulator table is rotated 90 degrees, and then the gantry is rotated to any reasonable angle (30-45 degrees) for a simple radiograph. The source-to-film distance (SFD) and gantry angle are noted. The radiograph of the lead wire contour is digitized on a personal computer (PC) and transformed from the film coordinates to the patient coordinates using the SFD and the gantry angle as input parameters. The accuracy and efficacy of this method were tested in four test cases: a square box, and contours in the head and neck, thoracic, and pelvic regions. The accuracy of the calculated contour was within +/- 4 mm for the large-gradient contours and +/- 3 mm in the other cases. A root mean square deviation of < 2 mm was observed between this method and the CT contour. This method can be used in any clinical setting where a simulator and a digitizer with a PC are available. The transformation code is simple and could be placed on any treatment planning computer system.