Abstract: A method and system for simulating and verifying a radiation therapy beam prior to treatment. The system also permits the accurate positioning of the beam with respect to the patient. The system utilizes a contour on a prescription image which defines the location and shape of a desired radiation treatment area to generate a transmissive projection which projects the exact field contour onto the patient during simulation. The patient can then be marked with this field contour prior to treatment in the simulation apparatus. With the patient placed in the treatment apparatus the accurate positioning of the patient can be achieved by aligning the patient's skin markings with a light projected through a multi-leaf collimator onto the skin. Furthermore, the correct functioning of the multi-leaf collimator can be verified in this manner since incorrect leaf positions will show up in the light projected onto the patient's skin.