The aim of this study was to investigate the over-response of an inorganic optical fibre sensor (OFS) when measuring percentage depth dose curves (PDDs) with respect to an ion chamber by means of physical measurements and Monte Carlo (MC) simulations. The sensor was constructed by filling a cavity (700 μm diameter and 7 mm deep), which was made in a PMMA (polymethyl methacrylate) plastic optical fibre, with an inorganic scintillating material: terbium doped gadolinium oxysulphide (Gd<sub>2</sub>O<sub>2</sub>S:Tb). The MC software packages BEAMnrc and DOSXYZnrc were used to develop a MC model of an Elekta Versa HD linear accelerator (linac), which was then used to simulate the Gd<sub>2</sub>O<sub>2</sub>S:Tb scintillator. The results of the PDD measurements showed a depth dependence of the OFS, however the percentage differences between the ion chamber and the OFS measurements showed that as the radiation field size decreases, the difference between the two measurements decreases from 16.5% to 5.1% for 10x10 cm<sup>2</sup> and 2x2 cm<sup>2</sup>, respectively. The MC simulation of the sensor showed a good agreement compared to physical measurements at shallow depth in the phantom; however, discrepancies were observed at depth, which was less pronounced for 4x4 cm<sup>2</sup> than for 10x10 cm<sup>2</sup>. The results of this study indicate that including Cerenkov radiation measurements is essential to accurately quantify the overresponse and the higher discrepancy between the measured and simulated PDD profiles of the OFS.