The GATE package is used to perform Monte Carlo hadrontherapy simulations of a cancer treatment combined with the complete description of an associated positron emission tomography (PET) imaging device for dose monitoring. This study aimed to demonstrate that the GATE platform has the capability to perform realistic simulations in the field of hadrontherapy, combining both dose and imaging system. We defined the simulation configuration as a carbon ion pencil beam scanning of a thorax CT phantom together with a complete PET imaging system. Two main positron emitters resulting from nuclear 12C reactions are considered in this study: 11C and 15O. We studied the produced data to analyse the interest in using a full PET system simulation instead of the usual Gaussian smoothing applied on the positron emitters map. We found differences in the distal position of the signal falloff of 20% between full PET system simulation and the Gaussian model. We also studied the influence of the 15O isotope in PET images and found the contribution to falloff of this isotope to be negligible (4%), which suggests the inclusion of 15O isotope in the simulation is not necessary. Finally, we analysed the impact of dose delivery on PET image quality and found a difference of 20% on the PET estimation falloff between doses of 10 Gy and 1 Gy. This study shows that GATE, implemented on a computing system with large number of CPUs (>;1000), has the potential to be used for quantitative evaluation of imaging protocols for radiation monitoring.