Abstract: Evaluating precisely the temporal variations of tumor volumes is very important for at least three types of practical applications: pharmaceutical trials, decision making for drug treatment or surgery and patients follow-up. In this paper, we present a volumetric analysis technique, combining precise rigid registration of 3D medical images, non-rigid deformation computation and flow field analysis. Our analysis technique has two outcomes: the detection of evolving lesions and the quantitative measurement of volume variations. The originality of our approach is that no precise} segmentation of the lesion is needed but the approximative designation of a region of interest, which can be automatized. We distinguish between tissue transformation (image intensity changes without deformation) and expansion or contraction effects reflecting a change of mass within the tissue; a real lesion being generally the combination of both effects. The method is tested with synthesized 3D image sequences and applied, in a first attempt to quantify in-vivo a mass effect, to the analysis of a real patient case with Multiple Sclerosis.