In the scope of a radiotherapy application, we have previously developed a methodology to reconstruct the 3D shape of deformable organs from their CT scan sections using particle systems. In this paper, we propose a new technique to track changes in the object's shape from some partial information provided by an ultrasound sensor. The aim of this method is to quickly obtain the shape alteration while preserving organ's volume which can be a strong requirement in medical applications. Our approach is based on the deformation of the initial model by applying physical forces to its surface in order to fit the new data set. The amplitude of this force is calculated to preserve the cohesion of the organ's model. The advantages of our method include: -- updating is faster than initial modelling, -- undersampled data is sufficient for updating. Keywords: deformable model, particle systems, physically based interpolation and modelling, Lennard Jones forces.