The present invention relates to a method for determining data referred to as target change data which can be used for performing radiotherapy treatment, the target change data describing information on the change of position of a target included in a body of a patient, the method being performed by a computer and comprising the following steps: acquiring determination rule data describing a rule for mapping an indicator change set to the target change data, the indicator change set being a set which comprises more than one element, the elements of the indicator change set respectively representing information on respective changes of positions of indicators, at least two of the indicators respectively indicating a change of position of different body structures referred to as indicator structures, acquiring set data describing an acquired indicator change set; and determining the target change data based on the determination rule data and the set data by applying the rule for mapping to the acquired indicator change set.