A detector system adapted for monitoring a radiation treatment system comprising a pulsed beam radiation source for treating a body with a given beam intensity and beam configuration, with pulse times and intervals between pulses less than 100 milliseconds, using at least one monitoring radiation source located inside or outside the body, the detector system comprising; a) a detector designed to detect radiation from the monitoring source, and subject to interference radiation from the beam source; and b) control circuitry that creates a data record of radiation received by the detector, to provide information about the body; wherein, when the detector detects radiation in real time during operation of the beam, the data record selectively excludes data for radiation received by the detector during the pulses, as opposed to data for radiation received by the detector between pulses.