The main aim of radiotherapy techniques is to deliver the dose to the target volume while sparing as much as possible the healthy tissue. Dose verifications prior the treatment of the patient are mandatory in order to guarantee high accuracy to the treatment. We have developed a 2D dose imaging system for dose delivery verification in hadron therapy. The system consists of two cascaded Gas Electron Multipliers (GEMs) in an Ar/CF4 scintillating gas mixture. A GEM is a copper clad thin kapton foil with a regular pattern of sub-mm holes. The primary electrons, created in the detector's sensitive volume by the incoming beam, drift in an electric field towards the GEMs and undergo gas multiplication in the GEM holes. During this process light is emitted by the excited Ar/CF4 molecules and detected by a mirror-lens-CCD camera system. The measured 2D light intensity distribution is proportional to the 2D distribution of the energy deposited in the sensitive volume by the hadron beam. When the thickness of a water bellows phantom, mounted upstream the detector, is varied from zero up to beyond the hadron range, a 3D dose distribution can be reconstructed easily. The response of the detector was studied in a proton, alpha and clinical carbon beam. The main result of this work is that the energy dependence of the detector response in high LET (Linear Energy Transfer) beams is quite small compared to state-of-the-art 2D dosimeters with a millimetre resolution. The scintillating GEM detector is a promising device for dose verification, especially in case of high LET beams for checking treatment plans composed of several beam energies. Its fast signal response opens the possibility for online monitoring of a 2D scanning beam as well.