Monitoring of high-energy photon dose in radiation therapy is crucial for radiation protection, as well as to estimate the radiation effects in the operating microelectronic devices. The aim of this work was to investigate the possibility of using the photoactivation technique to monitor the high-energy photon fluence in the Varian 21EX 23MV CLINAC. The 52Cr(γ,n)51Cr and 66Zn(γ,n)65Zn reactions were used. It was found that 51Cr and 65Zn can be used successfully to monitor the fluence for short term (80 days) and long term (110 days), respectively.