A method for determining arc costs of respective potential arcs of a treatment plan for use in radiation therapy targeting at least one target volume (3a-c), the method being performed in a treatment planning system and comprising the steps of: determining a plurality of beam orientations, wherein each beam orientation comprises respective values of a couch angle, a collimator angle and a gantry angle, each beam orientation defining a beam direction through the collimator; evaluating, for each beam orientation, a set of at least one cost function, wherein the at least one cost function comprises an intermediate exposure cost function that is evaluated by performing the substeps of: projecting the at least one target volumes (3a-c) on a beam plane being a normal plane to the beam direction of the beam orientation; determining an alignment angle based on the 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, such that there is at most one intermediate area crossed by each line parallel to the alignment angle; determining a value of the intermediate exposure cost function based on the any intermediate area; the method further comprising 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. determining a plurality of beam orientations, wherein each beam orientation comprises respective values of a couch angle, a collimator angle and a gantry angle, each beam orientation defining a beam direction through the collimator; evaluating, for each beam orientation, a set of at least one cost function, wherein the at least one cost function comprises an intermediate exposure cost function that is evaluated by performing the substeps of: projecting the at least one target volumes (3a-c) on a beam plane being a normal plane to the beam direction of the beam orientation; determining an alignment angle based on the 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, such that there is at most one intermediate area crossed by each line parallel to the alignment angle; determining a value of the intermediate exposure cost function based on the any intermediate area; the method further comprising 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. projecting the at least one target volumes (3a-c) on a beam plane being a normal plane to the beam direction of the beam orientation; determining an alignment angle based on the 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, such that there is at most one intermediate area crossed by each line parallel to the alignment angle; determining a value of the intermediate exposure cost function based on the any intermediate area; the method further comprising 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. 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.