The capability to calculate the distribution of absorbed dose produced by photon beams and electron beams of the most general characteristics is of vital importance in radiotherapy. Conceptually, this new radiation field model takes as a basis the empirical distributioins along three mutually perpendicular reference lines in a """"""""master field"""""""" and mathematical expressions to describe the effect if variations of field size, depth and focal distance. This concept is applied to the beam-modifying devices as well. The approach is attractive from a theoretical as well as a practical point of view. The investigations include the generalization for irregular fields modified by irregular blocks for photon beams and electron beams, and the influence of inhomogeneities. New developments in the description of the along-the-ray distirbution as well as the field size dependence of the beam profile are being implemented. The extension to electron beams is continuing. Of special interest are the implications of the large number of electron energies and the need for flexible application of different energies and field shapes in conbination with photon fields. The central ray distributions can now be described on the basis of only seven charactertistic depth dose data points, for 60Co to 18 MV x-rays, using the concept of Net Fractional Depth Dose (NFD). The NFD formalism is currently being extended to the description of the influence of inhomogeneities, such as lung tissue.
