[en] The radioluminescence (RL) signal from fiber coupled Al₂O₃:C can be used for real-time in vivo dosimetry during radiotherapy. RL generally provides measurements with a reproducibility of 2% (one standard deviation). However, we have observed a non-random variability of the RL signal which means that the memory of the system is not fully reset by the optically stimulated luminescence (OSL) readout. Here we report an example of how this memory affects the RL signal. Measurements were performed in the range of 0-4 Gy using four Al₂O₃:C crystals, in cycles of irradiation and subsequent readout. We found the memory to be persistent, influencing several successive RL measurements. The induced systematic error was found to be crystal dependent, but proportional to the measurement-to-measurement dose variation (approximately 1.4% per Gy).