In radiotherapy planning, computed tomography (CT) images are used to calculate the dose in the patient. However, a high density hip prosthesis can cause streaking artefacts in CT images, which make dose calculations for nearby organs inaccurate. This study aim to quantify the impact of a hip prosthesis on 6 MV photon beam dose distribution using the Monte Carlo (MC) simulation. To quantify the radiation dose at the hip prosthesis accurately, image processing techniques were used to generate CT images free from streak artefacts. MATLAB software was used to produce computer-generated phantoms consisting of bone, titanium, stainless steel and CoCrMo. Percentage depth dose (PDD) and beam profile were used to analyse the impact of the hip prosthesis on the dose distribution of the photon beam. PDD showed that the absorbed dose was reduced as the density of the material increased, and the dose was reduced by as much as 49% when the photon beam struck the highest density material (CoCrMo, 8·2g/cm3). However, dose was increased at the tissue-hip prosthesis interface (depths of 4 and 19cm). As the depth increased, the absorbed dose decreased due to attenuation of photons by the tissue and the metal.