A technique for calculating range spectra of charged particle beams distal to thick inhomogeneities.

A method was developed for calculating range spectra of charged particles after passing through an inhomogeneous structure whose thickness was comparable to the range of the incident particles. It was shown that the spectra are strongly affected by the influence of multiple Coulomb scattering at interfaces parallel to the beam direction of two media with different relative stopping power. The calculations are in agreement with Monte Carlo simulations. The degraded Bragg peak was calculated on the basis of the computed range spectra behind the inhomogeneity interface. The method can be included into charged particle treatment planning systems where broad pencil beams are used to predict the deteriorated Bragg peak behind inhomogeneity interfaces more precisely.