Thesis: In case of radiological accident involving an external source, several techniques are used in order to determine the dose received by the victim and to allow a better medical management of the victim. One of them is the numerical dosimetric reconstruction. To perform numerical dosimetric reconstructions, the Ionizing Radiation Dosimetry Laboratory from IRSN has developed since ten years a tool named SESAME which combines voxel phantoms and the Monte Carlo computer code MCNP(X). The aim of this thesis is to develop new implementations in SESAME to take into account the morphology and the posture of the victim and to allow the reconstruction of accidents which occured in external radiotherapy treatment. At first a new procedure, which takes into account the posture of the victim at the moment of the accident has been developed in SESAME. This procedure is based on the NURBS format to create modified voxel phantoms and has been numerically and experimentally validated. The second part of the work deals with a feasibility study in order to implement in SESAME tool a functionality dedicated to the reconstruction of accidents in external radiotherapy. Two modelling of a linear accelerator Clinac 2100C with 6MV and 25 MV nominal voltage have been performed with the MCNPX computer code and have been validated. Then, they have been optimised in order to reduce the calculation time to make them compatible with a reconstruction of accident.