Abstract: A computer system to project and localize radiation treatment portals, i.e., field center, field borders, on patient skin without using a light field is disclosed. The system includes a multimedia software program that interacts with a three-dimensional digitizer which acquires the coordinates of any point in space that is accessible by the digitizer probe. The system constructs and employs coordinate system transformations from digitizer coordinates to room coordinates to radiation beam coordinates. For any point digitized, the invention system calculates the position of this point relative to the radiation field, and displays or signals to the user whether the digitized point is in the radiation field, outside of the field or on the field border. This is accomplished through a screen display of the radiation beam's eye view, an audio signal and/or LED light indicators mounted on the digitizer probe.