Interstitial brachytherapy using permanent seeds is a common modality for the treatment of early stage prostate cancer. It consists of inserting hundreds of radioactive seeds (size of a grain of rice) in the prostate by means of transperineal needles. In this procedure, dose delivery to healthy surrounding organs at risk (OAR) and dose homogeneity within the prostate are of prime concern. Placement errors should therefore be minimized to avoid unacceptable area underdosage or overdosage. At present, brachytherapy seeds can be loaded with three different radionuclides: iodine-125 (¹²⁵I: 28.37 keV; 59.40 days), palladium-103 (¹⁰³Pd: 20.74 keV; 16.991 days), and cesium-131 (¹³¹Cs: 30.45 keV; 9.689 days). Long or short term morbidity is the main drawback of ¹²⁵I and ¹³¹Cs due to their deeper penetration in the normal tissues. However, both provide a good homogeneity of the dose distribution within the prostate. By contrast, ¹⁰³Pd offers a short penetration depth that reduces the dose to OAR. Nevertheless, it could result in cold spot (underdosage) where a recurrence of the cancer could appear. A compromise had to be found between good implant uniformity and low dose to OAR. We propose therefore to study if the combination of two radionuclides inside the same seed could be a solution. Two mixtures were considered: ¹⁰³Pd_0.75-¹²⁵I_0.25 and ¹⁰³Pd_0.25-¹³¹Cs_0.75. The subscripts denote the fractions of internal activity of each radionuclide. The work is subdivided into three steps. First we adapt the AAPM TG-43U1 dosimetry formalism used by the physician to make multiple-radionuclides sources compatible with Treatment Planning Systems (TPS). Then the dose distributions around the bi-radionuclide seeds are determined. Second, the prescription doses for both sources are derived using the linear quadratic model for tumor cell surviving fraction. They were computed using mono-radionuclide implants as benchmarks. Finally, treatment plans and Dose-Volume Histograms parameters have been computed on real patients virtually implanted with bi-radionuclide seeds and the results were compared with the mono-radionuclide one. These parameters can then be used to evaluate the Normal Tissue Complication Probability (NTCP) of urethra, the most exposed organ at risk in prostate brachytherapy. First, dosimetry results show that, from a pure physical point of view (i.e. without tissue reponse), the dose distributions of both mixtures lies in between that for ¹⁰³Pd and ¹²⁵I/¹³¹Cs. The compromise between homogeneity and reduced dose at large distance can be reached. Second, the averaged prescription doses for the Pd-I mixture are 142⁺¹⁵_-16Gy and 142⁺⁶_-8Gy using ¹⁰³Pd and ¹²⁵I as benchmarks, respectively. The values for the Pd Cs mixture are 128⁺¹³_-13Gy and 115⁺⁶_-7Gy, using ¹⁰³Pd and ¹³¹Cs, respectively, as benchmarks. Finally, urethral NTCP results fall in the 19 to 23% range. However, they are affected by large uncertainties, making the comparison difficult. At present, no conclusion could be drawn about the efficiency of bi-radionuclide brachytherapy in comparison with mono-radionuclide using the available models. Permanent seed prostate brachytherapy suffers a lot from the lack of precision on radiobiological modelling parameters. A better knowledge of their values could significantly improve the predicting models and therefore lead to better treatment outcome.