An irradiation apparatus for irradiating by scanning a target volume according to a predetermined dose profile with a scanning beam of charged particles forming an irradiation spot on said target volume, said apparatus comprising: a beam generating device, a reference generator for computing, from said predetermined dose profile, through a dynamic inverse control strategy, the time evolution of commanded variables, these variables being the beam current I(t), the spot position settings x(t),y(t) and the scanning speed settings vx(t), vy(t), a monitor device having means for detecting at each time (t), the actual spot position as a measured position defined by the values xm(t),ym(t) on the target volume, characterised in that said irradiation apparatus further comprises means for determining the differences ex(t), ey(t) between the measured values xm(t), ym(t) and the spot position settings x(t) and y(t), and means for applying a correction to the scanning speed settings vx(t) and vy(t) depending on said differences ex(t), ey(t). The present invention is also related to a monitor for determining beam position in real-time.