Radiation Therapy Planning Using Computerized Interactive Portal Simulation

Abstract

This presentation demonstrates the role of computerized portal simulation (beam’s eye view displays) for defining appropriate beam portals. With this technique, computerized tomography (CT) data input and portal film input are used to design and verify the geometrical placement of radiation fields, beam blocks and compensators. Computer dose calculations utilizing a three dimensional beam model are performed for the beam’s eye view (i.e. on planes perpendicular to the central axis) and are used to assist in the plan evaluation and optimization.

For the geometrical localization, both CT data and Simulator-based imaging (radiographic film) are used as input to the treatment planning system. Beam’s eye views of the target and neighboring anatomy are computer generated. Computer generated color grahics are used to combine grey scale images of digitized radiographics or CT derived portal reprojections with computer reconstructed anatomical surface displays to highlight the important structures. The beam parameters can be modified interactively and beam shielding blocks and compensators can be designed directly using the graphical display.

Computer dose calculations are based on Van de Geijn’s projective beam model and include corrections for blocks, compensators, and inhomogeneities. This model provides rapid dose computation so that the beam parameters can be quickly varied and the resulting effects in dose distribution observed. Techniques to graphically compare different plans or to compare dose predictions with measured data will also be presented.