Determining an irradiation region for radiotherapy based on model patient data and patient image data

The invention relates to a method of determining a radiotherapy treatment plan for radiotherapy treatment of a treatment body part of a patient's body. The method can include acquiring treatment target position data comprising treatment target position information describing the position of a treatment target to be treated by radiotherapy in the treatment body part. Statistic model target region position data is acquired, which describes the position of a model target region in a model body part corresponding to the treatment body part. Based on the treatment target data and the statistic model target region position data, irradiation region position data is determined that describes the position of an irradiation region to be treated by irradiation with treatment radiation in the treatment body part.