A detector system for the purpose of quality assurance in radiation therapy is being developed and measurements in proton fields are presented. It is based on a beryllium oxide probe coupled to an optical fiber. Radioluminescence in the material generates photons, whose number is proportional to the dose absorbed in the probe. The photons are detected by very sensitive time resolving single photon counting heads. Diameter and height of the beryllium oxide probe amount 1 mm each. The probe was exposed to proton fields at a research and clinical accelerator. With rising linear energy transfer, saturation effects in the light signal. In consequence, the measuring signal is not proportional to the reference dose and needs to be corrected especially in the last 10 mm of the proton range. For this purpose, spectral changes of the radioluminescence photons, which depend on the linear energy transfer of the protons, were used to set up a correction factor. Hereby, it was possible to achieve a deviation of 2.3 % compared to an ionization chamber. Furthermore, beam profiles and linear energy transfer across the profile were studied.