Thesis: In nuclear medicine, radiopharmaceuticals are distributed in the body and thus, each organ can deliver a fraction of emitted energy in tissues. Therefore, dose calculations must be assessed accurately and realistically to ensure the patient radiation protection. Absorbed doses were until now based on mathematical standard models and electron transport approximations. The International Commission on Radiological Protection (ICRP) has recently adopted voxel phantoms as a more realistic representation of the reference adult. The main goal of this thesis was to study the influence of the new reference models and Monte Carlo methods on the major dosimetric quantities. In addition, the contribution of patients' specific geometry to the absorbed dose was compared to a standard geometry. Particular attention was paid to the bone marrow which is characterized by a high radiosensitivity and a complex microscopic structure. An accurate alpha dosimetry was assessed for bone marrow using microscopic images of several trabecular bone sites. The results showed variations in the absorbed fractions as a function of the particles" energy, the skeletal site and the amount of fat within marrow cavities, three parameters which are not taken into account in the values published by the ICRP. Finally, the heterogeneous activity distribution of the radiopharmaceuticals was considered within the framework of the treatment of a hepatocellular carcinoma with selective internai radiotherapy using Yttrium-90 through the analysis of dose-volume histograms. The developments made in this thesis show the importance and the feasibility of performing a personalized dosimetry for nuclear medicine patients