The present invention relates to a data processing method for determining control data for controlling beam positions which beam positions describe positions which a treatment beam has or would have if emitted by a beam source, the treatment beam being for treating a treatment body part of a patient, said method constituted to be performed by a computer and comprising the steps of: .cndot. providing treatment data which comprise data on a position of the treatment body part； .cndot. providing condition data which describe: constraints on the beam positions and/or constraints on positional changes of the beam positions, wherein said constraints allow for at least a part of the beam positions to lie not in a common plane； .cndot. providing an arrangement of the beam positions on the basis of the treatment data which arrangement fulfils the condition data； .cndot. determining control data for controlling a change of a relative position of the beam source relative to the treatment body part for changing the beam positions and for controlling an emission of the treatment beam from the beam source, the determined control data being constituted to change the beam positions to follow the provided arrangement of the beam positions and to cause continuous emission of the treatment beam during a continuous change of the beam positions while the beam positions follow at least a part of the arrangement.
