An optical fibre dosimeter based on a terbium-doped gadolinium oxysulphide (Gd<sub>2</sub>O<sub>2</sub>S:Tb) inorganic scintillating detector (ISD) was recently proposed for external beam radiation therapy (EBRT) dosimetry applications. Although it has demonstrated many promising characteristics, an energy dependence was found during percentage depth dose (PDD) measurements. In this study, the response of a Gd<sub>2</sub>O<sub>2</sub>S:Tb based ISD to superficial x-ray energies and 6 MV EBRT photon beams has been measured and compared to absorbed dose values generated using a Monte Carlo (MC) model of a superficial x-ray treatment unit and a clinical linear accelerator treatment head. The relationship between beam energy and the response of the scintillating phosphor was investigated for depth dose and beam profile measurements. An overresponse was observed during physical measurements in the kV range that is indicative of an energy-dependent variation in scintillation efficiency of the Gd<sub>2</sub>O<sub>2</sub>S:Tb. This study indicates that this intrinsic energy dependence may result in a significant increase in signal at kV energies relative to MV irradiation, which was not accounted for in the MC model.