Abstract: Proton therapy lacks a standard method to verify the proton range during the treatments in the clinical routine. In this context, the monitoring of prompt gamma-rays in a coaxial geometry using a compact detector based on a CeBr3 scintillator coupled to a commercial photomultiplier tube (PMT) could lead to the identification of proton range deviations. Such detection system could be easily integrated in every treatment room. Although measuring in this geometry profits from an advantageous solid angle, the detector is also exposed to an extreme gamma-ray rate, of up to 10 Mcps. In this work, we present the first experimental performance evaluation for the proposed detector by irradiating it at very high bremsstrahlung rates at the gamma ELBE facility. Using a customized active voltage divider to supply voltage to the PMT, the detection system was able to sustain a photon rate higher than 12 Mcps without dead time while keeping gain drifts below 15% in the best configuration, and to achieve a sub-nanosecond time resolution.