Virtual light field projection for CT-simulation

We report a method to project a virtual light field over the patient during CT-simulations of external beam radiotherapy. It can be used to perform, a ll the tasks associated with the physical light field of a conventional sim ulator. The system consists of a three-dimensional sonic digitizer interfac ing with a window-based software on a personal computer. The digitizer can provide the three-dimensional coordinates of any point in space accessible by the digitizer probe. When these coordinates are transformed into the bea m's eye view, the position of the digitized point relative to the beam can readily be displayed. Thus, the system establishes a virtual light field in space, which can be ''seen'' only by the digitizer probe. For any digitize d point, the system can immediately show, by the beam's eye view display, w hether the point is inside the field, outside of the field, or on the field border. Moreover, this virtual held projection allows one to evaluate exte rnal target coverage (or external normal tissue sparing) conveniently and i nteractively. By simply digitizing the concerned area and viewing its posit ion in the beam's eye view display, one can immediately assess the coverage and if necessary, modify the treatment field accordingly. The system also provides an efficient and essential procedure in CT-simulations for marking treatment portals on the patient. By cruising the digitizer probe on the p atient's skin surface under visual and audio guidances, one can promptly fi nd the projection of the field center, held corners, etc,, on the patient. Measurements have been performed to study the accuracy of the GP-12 sonic d igitizer using rigid phantoms. Based on the measured data, the overall accu racy of the portal localization system is estimated to be +/-2 mm. The syst em has been in clinical use for our CT simulator: (C) 1999 American Associa tion of Physicists in Medicine. [S0094-2405(99)00607-0].