Particle therapy apparatus comprising - a particle accelerator to deliver a charged particle beam; - a beam transport system to deliver the charged particle beam according to a main beam axis to a target in an isocenter plane which is perpendicular to the main beam axis; - scanning magnets to scan the charged particle beam over the target; - a beam position detector arranged downstream of the scanning magnets and adapted to detect an X and Y position of the charged particle beam when it crosses the detector; - a beam stopper arranged downstream of the beam position detector, the beam stopper being adapted to stop the charged particle beam when it hits the beam stopper; - a control system configured to drive the scanning magnets to scan the charged particle beam over the target according to a pre-defined treatment field which covers a treatment surface in the isocenter plane, the control system being further capable to drive the scanning magnets to scan the charged particle beam according to a reachable field which covers a reachable surface in the isocentre plane, the reachable surface covering and being larger than the treatment surface, characterized in that the beam stopper is arranged at a position to prevent the charged particle beam from reaching at least a portion of the reachable surface and to allow the charged particle beam to reach any portion of the treatment surface, and in that the control system is configured to control the particle therapy apparatus: - to direct the charged particle beam to the beam stopper and to meanwhile measure an X and Y position of the charged particle beam by using the beam position detector, - to calculate a difference between a desired X and Y position of the charged particle beam and respectively the measured X and Y position of the charged particle beam when directed to the beam stopper, - to scan the charged particle beam over the target according to the pre-defined treatment field by taking into account the calculated difference. - a particle accelerator to deliver a charged particle beam; - a beam transport system to deliver the charged particle beam according to a main beam axis to a target in an isocenter plane which is perpendicular to the main beam axis; - scanning magnets to scan the charged particle beam over the target; - a beam position detector arranged downstream of the scanning magnets and adapted to detect an X and Y position of the charged particle beam when it crosses the detector; - a beam stopper arranged downstream of the beam position detector, the beam stopper being adapted to stop the charged particle beam when it hits the beam stopper; - a control system configured to drive the scanning magnets to scan the charged particle beam over the target according to a pre-defined treatment field which covers a treatment surface in the isocenter plane, the control system being further capable to drive the scanning magnets to scan the charged particle beam according to a reachable field which covers a reachable surface in the isocentre plane, the reachable surface covering and being larger than the treatment surface, characterized in that the beam stopper is arranged at a position to prevent the charged particle beam from reaching at least a portion of the reachable surface and to allow the charged particle beam to reach any portion of the treatment surface, and in that the control system is configured to control the particle therapy apparatus: - to direct the charged particle beam to the beam stopper and to meanwhile measure an X and Y position of the charged particle beam by using the beam position detector, - to calculate a difference between a desired X and Y position of the charged particle beam and respectively the measured X and Y position of the charged particle beam when directed to the beam stopper, - to scan the charged particle beam over the target according to the pre-defined treatment field by taking into account the calculated difference. - to direct the charged particle beam to the beam stopper and to meanwhile measure an X and Y position of the charged particle beam by using the beam position detector, - to calculate a difference between a desired X and Y position of the charged particle beam and respectively the measured X and Y position of the charged particle beam when directed to the beam stopper, - to scan the charged particle beam over the target according to the pre-defined treatment field by taking into account the calculated difference.