In radiotherapy treatment portal images and digitally reconstructed radiographs (DRRs) are the most frequently used image-guidance systems to monitor patient set-up during clinical routine. DRR is therefore a critical element in the process of virtual simulation and patient position verification in radiotherapy. The aim of this work was to assess the geometric reliability of the DRR generated by XiO, Pinnacle, Odyssey and Ocentra-Masterplan Treatment Planning Systems (TPS). Instead of the Computed Tomography (CT) images of a quality assurance phantom two synthetic phantoms were created and used to asses the linearity and the ray tracing reliability of the DRR generators. The obtained results demonstrate that the linearity and the reliability of DRR generators was less than 1 pixel for all TPSs.