A charged particle irradiation device (10) and method for irradiating a target volume (50), adapted for receiving a treatment plan (70) defining a series of prescribed irradiation points (140) having each a prescribed dose to be delivered is provided. The device includes an irradiation unit (40) having at least one scanning magnet (100; 110), and at least one beam position monitor (130) installed in between the said scanning magnet (100; 110) and said target volume (50) is provided. A controller (80) comprises means for calculating for any said prescribed irradiation point corresponding nominal magnetic settings of the scanning magnet such that a beam (90) is pointing to said prescribed irradiation point when corresponding magnetic settings are applied, and means for calculating corresponding expected position at said beam position monitor (130) according to said first correction.