Gated detection of the output of a fiber-optic-coupled radiation dosimeter effectively eliminated the direct contribution of Cerenkov radiation to the signal. The radiation source was an external beam radiotherapy machine that provided pulses of 6-MeV x rays. Gated detection was used to discriminate the signal collected during the radiation pulses, including Cerenkov interference, from the signal collected between the radiation pulses due only to phosphorescence from the Cu(1+)-doped glass detector. Gated detection of the long-lived phosphorescence of the Cu(1+)-doped glass provided real-time dose measurements that were linear with the absorbed dose and that were accurate for all field sizes studied.