The paper reports on the first demonstration of in-situ, real-time dosimetry realized with an enhanced back-scattering optical fiber and a high-resolution optical back-scattering reflectometry measurement. This work is devised to overcome the current problems in monitoring radiotherapy treatments, in particular the difficult evaluation of not only the actual x-ray dose that is accumulated on the target volume, but also the distribution profile of the ionizing radiation beam. The experiments have been conducted by evaluating the radiation- induced spectral shift of the Rayleigh back-scattering along the fiber under test during x-ray exposure, in a radiation chamber. The sensing region is a section of aluminum-doped silicate fiber, that overcomes the poor sensitivity to radiation of standard, germanium-doped, silicate fibers for telecom applications. The preliminary results show that it is possible to remotely track the x-ray dose at high dose rates (700 Gy/min) and at rates closer to therapeutic values (22 Gy/min). A linear relationship between accumulated dose and spectral shift has been found. This research aims at developing a dose sensor with the most demanding features of small form factor, spatial profiling and remote interrogation.