Correction for distortion in a beam outline transfer device in radiotherapy CT-based simulation

In using a CT scanner as a radiation therapy simulator, it would be helpful to be able to transfer the beam outline from the computer plan to the patient's skin. A beam outline transfer device has been constructed and installed on a Siemens' DRH CT scanner gantry. The planned treatment beam geometry from a 3-D computerized simulation and planning system can be projected onto the patient's skin surface accurately and efficiently. The positioning accuracy achieved is within +/- 0.1 cm over a 20 cm x 20 cm field. Integrating the device into the CT scanner, simplifies the device and reduces the cost over an externally mounted device. Two unsuccessful methods to correct the projection distortions are also mentioned. In order to achieve the reported beam outline transfer accuracy, a system based on our empirically derived calibration procedure is described.