A data processing method of determining a radiotherapy treatment plan for radiotherapy treatment of a treatment body part of a patient's body, the method being executed by a computer and comprising the following steps: a) acquiring treatment target position data comprising treatment target position information describing the position of a treatment target (TT1,..., TT3) to be treated by radiotherapy in the treatment body part; b) acquiring statistic model target region position data comprising statistic model target region position information describing the position of a model target region (MT11,..., MT13, MT21,..., MT23) in a model body part (M1, M2) corresponding to the treatment body part, wherein the statistic model target region position data has been generated based on model target position data comprising model target position information describing the positions of model treatment targets (MT11,..., MT13, MT21,..., MT23) in a plurality of model bodies which display a medical condition corresponding to the medical condition to be treated, wherein the statistic model target region data comprises spatial target region cluster data comprising spatial target region cluster information describing at least one spatial cluster of the model treatment targets (MT11,..., MT13, MT21,..., MT23) in the plurality of model bodies, wherein the spatial target region cluster data is determined based on combining sets of medical image data comprising the model target region position information for each individual one of the plurality of human bodies; c) determining, based on the treatment target data and the statistic model target region position data, irradiation region position data comprising irradiation region position information describing the position of an irradiation region (IR1,..., IR3) to be treated by irradiation with treatment radiation in the treatment body part. a) acquiring treatment target position data comprising treatment target position information describing the position of a treatment target (TT1,..., TT3) to be treated by radiotherapy in the treatment body part; b) acquiring statistic model target region position data comprising statistic model target region position information describing the position of a model target region (MT11,..., MT13, MT21,..., MT23) in a model body part (M1, M2) corresponding to the treatment body part, wherein the statistic model target region position data has been generated based on model target position data comprising model target position information describing the positions of model treatment targets (MT11,..., MT13, MT21,..., MT23) in a plurality of model bodies which display a medical condition corresponding to the medical condition to be treated, wherein the statistic model target region data comprises spatial target region cluster data comprising spatial target region cluster information describing at least one spatial cluster of the model treatment targets (MT11,..., MT13, MT21,..., MT23) in the plurality of model bodies, wherein the spatial target region cluster data is determined based on combining sets of medical image data comprising the model target region position information for each individual one of the plurality of human bodies; c) determining, based on the treatment target data and the statistic model target region position data, irradiation region position data comprising irradiation region position information describing the position of an irradiation region (IR1,..., IR3) to be treated by irradiation with treatment radiation in the treatment body part.