  We have developed a model for proton depth dose and lateral distributions
based on Monte Carlo calculations (GEANT4) and an integration procedure of the
Bethe-Bloch equation (BBE). The model accounts for the transport of primary and
secondary protons, the creation of recoil protons and heavy recoil nuclei as
well as lateral scattering of these contributions. The buildup, which is
experimentally observed in higher energy depth dose curves, is modeled by
inclusion of two different origins: 1. Secondary reaction protons with a
contribution of ca. 65 % of the buildup (for monoenergetic protons). 2. Landau
tails as well as Gaussian type of fluctuations for range straggling effects.
All parameters of the model for initially monoenergetic proton beams have been
obtained from Monte Carlo calculations or checked by them. Furthermore, there
are a few parameters, which can be obtained by fitting the model to measured
depth dose curves in order to describe individual characteristics of the
beamline - the most important being the initial energy spread. We find that the
free parameters of the depth dose model can be predicted for any intermediate
energy from a couple of measured curves.
