Silica-based optical fibers made by Modified Chemical Vapor Deposition (MCVD) technique have been doped with different concentrations of germanium (Ge). After characterization of the thermoluminescence (TL) process in each of these fibers, their real-time responses were studied by radioluminescence (RL) under X-rays. First, the same counterintuitive trend was observed for the intensity of the two phenomena as a function of the Ge content, namely the decrease in TL and RL signals with increasing Ge content. This behavior was explained by the existence of thermally disconnected traps. Then, the RL response as a function of the dose-rate of the fiber doped with 1% Ge was judiciously chosen and investigated in detail. The results show that up to Total Ionizing Dose (TID) of 1.9 kGy(SiO$_2$ ), this response is not only insensitive to the accumulated dose effect but also exhibits an afterglow decay at the end of the radiation much faster than that of a Ce-doped fiber, a very useful feature where irradiations are repetitive as in the medical field of flash-therapy. In addition to the medical field where the Ge-doped fiber seems to be quite suitable, slightly larger core diameters would also allow this same fiber to meet the challenge of high sensitivity in the environmental radiation monitoring around nuclear power plants or radioactive waste storage areas.