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In PET/CT thoracic imaging, respiratory motion has been reported as a limiting factor reducing image quality and biasing lesion volume measurement. One solution consists in performing respiratory gated PET acquisitions. The aim of this study was to evaluate the impact of respiratory gating on Monte-Carlo realistic PET data, simulated using the 4D-NCAT numerical phantom on the GATE platform. To obtain reconstructed images as close as possible to those obtained in clinical conditions, a particular attention was paid to apply the same type of reconstruction and correction processes on the simulated data as on real clinical ones. The whole set of simulations required a CPU time of 140 000 h generating 1.5 To of data, including simulations of 147 respiratory gated and 49 ungated thoracic exams. Comparison of the displacement volume (DV) measurements using conventional PET acquisitions versus respiratory gated acquisitions was performed, using an automatic iterative segmentation method and a fixed 40% threshold. The segmentation of gated and ungated frames using the 40% fixed threshold needed time consuming initial manual exclusion of noisy structures and so not considered as an automatic method. This step was not necessary when the automatic iterative method was used. Accuracy on DV measurement using the automatic approach was largely improved on gated compared to ungated images. This improved accuracy might have a significant impact when patient treatment is performed using ungated external radiotherapy.