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.