"Cancer is a serious disease with high mortality and morbidity. Treatment is often performed with radiation therapy. To assure a good control of the tumour with limited side effect, a very high conformity of the radiation field is needed in the treatments. This led to the development of very complex treatments using external photon beam. There is however another, more cost-effective, way to achieve that very high conformity. For localized tumours, this can be done by inserting radioactive photon sources emitting x-rays or gamma rays, directly inside the tumour. This technique is called brachytherapy and is the focus of this proposal. Because of advantages for dosimetry and radioprotection, all the latest developments in brachytherapy source design are pointing in the direction of the reduction of the photon energy. This presents dosimetric difficulties caused for example by the increase of the importance of tissue composition. The present standard for brachytherapy considers the patient as a 30cm diameter water sphere. Another question concerns the radiobiological effectiveness (RBE) of low energy photons. Although low energy photons have been suspected to have a RBE different than the higher energy photons there are not many studies about the subject and it is not taken into account clinically. This project proposes to study the physics of low energy photons used in brachytherapy and to assess the influence of the patient geometry and composition on the dosimetry and to determine the radiation quality taking into account the non-water equivalence of tissue for low energy sources by performing physical measurements and Monte Carlo calculations. We will develop a fast and accurate way to individualize and optimize the treatment including the RBE effect. Drastically improving the dosimetry for those low energy sources is of utmost importance to bring brachytherapy at the same level of accuracy than what is currently achievable for external beam radiotherapy."