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During a pelvic irradiation, the presence of a hip prosthesis is not correctly taken into account in the treatment planning systems and leads to calculate the dose distribution with some approximations. So, Monte Carlo models were elaborated to determine with accuracy the modifications caused by these implants on the dose distribution. Parameters as the geometry of the prosthesis, the composition of different materials used in the manufacturing of these implants and finally the influence of several coatings were studied. The BEAMnrc04 code was used to create mathematical models. Phantoms were elaborated to describe the dose perturbations produced by a hip prosthesis during a pelvic irradiation for one of the beam passing through the implant. Theoretical results were validated with experimental measurements carried out with an ionisation chamber (PTW / type M31002) around a titanium alloy hip prosthesis put in a water phantom (PTW / MP3). Two types of alterations have been shown. First, a dose increase at the two media interface which can be reduced by some coatings was observed. Secondly, a beam attenuation has been highlighted. It depends on the material density and can become very high under the femoral head (up to 45 % and 25 % for a steel and a titanium alloy head, respectively). The theoretical results show the importance of the dose modifications caused by the presence of such implants. Moreover, this study underlines the difficulty to integrate all the specificities of a hip prosthesis when a treatment for a patient with this type of implant is elaborated.