The invention relates to a data processing method of determining a radiotherapy treatment plan for radiotherapy treatment of a treatment body part (P) of a patients 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 (P) 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 (P) 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 (P).
