Calculation of dose distributions in the vicinity of high-Z interfaces for photon beams.

In the vicinity of interfaces between materials of different atomic number Z, extremes in absorbed dose occur for high-energy photon irradiations. The spatial extension of the effects is within the range of 1 cm, which may not be ignorable from the radiobiological point of view. At the front side of a high-Z slab a maximum is observed, whereas at the exit side a small buildup zone of the dose occurs, e.g., for a 5 MV beam, in front of a water/iron interface, the enhancement is about 30% of that to the homogeneous medium. The reduction at the back of the iron slab is about 16% for this energy, but vanishes with increasing energy. For high-energy photons this effect is mainly caused by the strong atomic number dependence of the scattering power for secondary electrons. The amount and extent of the scattering effects have been measured for aluminum and for iron slabs embedded in water or PMMA. The experimental data are in good agreement with Monte Carlo calculated values. Therefore the data form a reliable base to test the performance of commonly used treatment planning algorithms. The convolution or superposition method is used to calculate dose distributions. To account for the Z dependence of the scattering and the stopping power of the secondary electrons, corrections are applied to the energy deposition kernels. The boundary crossing of energy deposition kernels can be treated only in an approximate manner. However, the algorithm developed improves the accuracy of the dose calculation in the vicinity of interfaces significantly.