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This paper describes the dose delivery system used at the Centro Nazionale di Adroterapia Oncologica (CNAO) for ion beam modulated scanning radiotherapy. CNAO Foundation, INFN and University of Torino have developed and commissioned a Dose Delivery System (DDS) to monitor and guide ion beams accelerated by a synchrotron and to distribute the dose with a 3D scanning technique. The target volume, segmented in several layers orthogonally to the beam direction, is irradiated by thousands of pencil beams which must be steered and held to the prescribed positions until the prescribed number of particles has been delivered. At CNAO, these operations are performed by the DDS. The main components of this system are 2 independent beam monitoring detectors (BOX1 and BOX2), interfaced with 2 control systems performing real-time control, and connected to the scanning magnets and the beam chopper. As a reaction to any potential hazard, a DDS interlock signal is sent to the Patient Interlock System which immediately stops the irradiation. The tasks and operations performed by the DDS are described following the data flow from the Treatment Planning System through the end of the treatment delivery. The ability of the DDS to guarantee a safe and accurate treatment was validated during the commissioning phase by means of checks of the charge collection efficiency, gain uniformity of the chambers and 2D dose distribution homogeneity and stability. A high level of reliability and robustness has been proven by 3 years of system activity. The DDS described in this paper is one among the few worldwide existing systems to operate ion beam for modulated scanning radiotherapy. It has proven to guide and control the therapeutic pencil beams with accuracy and stability showing dose deviations lower than the acceptance threshold of 5% and 2.5% respectively during daily Quality Assurance measurements.