Purpose : In radiotherapy treatment, it is necessary to position the patient before each session to ensure that the beams target the tumor while sparing the surrounding healthy tissue. This step is made possible via different imaging techniques like the "electronic portal imaging device" (EPID) which is the most common approach. In this configuration, the imaging source is the same as the treatment source, while a flat panel detector is automatically placed under the patient via an articulated arm. In this context, our study was focused on the Monte Carlo modeling of such an EPID in order to simulate both planar and 3D CBCT images. Methods : The work presented here follows a preliminary study in which the treatment part of a clinical LINAC (Siemens Oncor) was modeled in GATE v6.2. The objective was to calculate the dose delivered by complex treatments like IMRT. The objective of the study is to extend this model by incorporating the imaging part of the LINAC, which consists of a flat panel detector (Perkin Elmer XRD1640). For this purpose, geometrical and physical data provided by the manufacturer were first accurately modeled in GATE. As a second step, 2D and 3D images simulations were performed according to clinical protocols: projections over a 200° arc, 1° increments, and 41cm×41cm field of view. IMRT QA and, RANDO anthropomorphic phantom and head-and-neck patient CT were used as input objects for simulations. Results : From the obtained series of 2D projections a 3D reconstruction was performed in each case, and the dimensions of the reconstructed structures were compared with actual objects. Conclusion : We have developed within the GATE platform a complete and detailed model of a 160MLC LINAC with its MVCBCT flat panel. Future work will consist in simulating epid-based transit dosimetry.