This is fundamentally ongoing research and development. 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 radiation field model takes as a basis the empirical distributions along three mutually perpendicular reference lines in a """"""""master field."""""""" Mathematical expressions describe the effect of 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 present investigations include the generalization of the treatment of the influence of inhomogeneities. New concepts in the description of the along- the-ray distribution as well as the field size dependence of the beam profile have been developed and are being implemented. A completely new approach to electron field modeling is well underway and extremely promising. 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 combination with photon fields. For the high energy photon
