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The investigation showed that the issue was caused by a defective collimator prefilter unit.Several potential technical causes were assumed, including mechanical blockage of the filter wheels, warped filter wheels, or malfunctions of sensors, processor boards, or the motor.Any of these could result in incorrect prefilter positioning and trigger an emergency message.Because the applied prefilter position could not be reliably documented during the issue, the exact dose deviation could not be calculated, and only expert-based estimates were possible.Based on log file analysis and expert opinion, an additional dose of approximately 1 gy for the first procedure and 0.5 gy in the second procedure was estimated.The prefilter was exchanged, and the issue was resolved.The occurrence rate of the error pattern was checked.A possible error accumulation or even a systematic error, which leads to a corrective action of the installed base, could not be determined by the investigation.
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