Direct Monte Carlo calculation of absorbed dose in a moving and deforming object

The knowledge of accumulated dose in a specified tissue on a functional subunit´s basis is of crucial importance for the application of biological models to estimate control and complication probabilities. It is known that geometrical uncertainties of the patient´s anatomy lead to differences between the planned and delivered dose distributions. In this work, the absorbed dose in a moving and deforming object is calculated by means of a direct Monte Carlo calculation. The Monte Carlo code EGS4/BEAM was modified to incorporate temporal dynamics of the simulation geometry. Lateral one-dimensional dose distributions were studied in a moving and deforming water slab adjacent to an air interface. A linear motion and a simple deformation of the water volume were investigated. The position of the boundary of the water volume was changing as a function of particle history. The Monte Carlo code is able to directly calculate the dose in the local coordinates of the moving object. The results show that a convolution algorithm to determine the resulting dose distribution is not sufficient for highly inhomogeneous situations and if internal deformations are present