It is provided a method for determining arc costs. The method comprises the steps of: determining a plurality of beam orientations； evaluating a set of at least one cost function comprising an intermediate exposure cost function that is evaluated by performing the substeps of: projecting the at least one target volumes on a beam plane； determining an alignment angle based on a collimator angle value； finding any intermediate area in the beam plane along the alignment angle between areas of the at least one target volume projection； determining a value of the intermediate exposure cost function. The method further comprises the steps of: finding a plurality of arcs, wherein each arc comprises a sequence of a plurality of beam orientations； and calculating, for each arc in the plurality of arcs, at least one arc cost based on the cost function values of the beam orientations of the arc.
