A beam model for three-dimensional radiotherapy planning.

A comprehensive beam model for three-dimensional radiotherapy planning is described. Input of patient data is either from an arbitrary spaced set of computed tomography (CT) scans throughout the treatment volume or from outlines traced on a digitizer. Any gantry, head or table rotation is allowed as well as any beam portal shape including internal blocking. Account is taken of beam modification due to the presence of the beam flattening filter, wedges and compensators. Both portal image and beam's eye view displays are readily available. A dose calculation algorithm is presented which uses a differential scatter-air ratio method to calculate the scatter dose in the presence of inhomogeneities. The beam model and algorithm form the basis of an interactive three-dimensional planning system running on a UNIX workstation, and performance times for dose calculation are given.