[Experimental determination of the virtual source point of high-energy electron beams for clinical application]

In radiation therapy, electron beams are applied in many cases without the aid of treatment planning systems. When the geometrical conditions differ from the measurement conditions, the position of the virtual electron source must be known in order to convert the dosimetric data. According to the literature, there are multiple procedures for measuring the position of the virtual electron source, however these partially result in considerably different values. Therefore, the intended purpose of the virtual electron source must be clarifiedfirst, e.g. whether the correction of the field size or the calculation of the output in relationship to the focus-surface distance. Additionally, the scope of application must be clearly defined because the determined value, even for the same measuring method, is strongly dependent on the measurement range used for the evaluation. We investigated all common procedures and developed a new and very time-saving method. The method is presented in this paper, and the results and their impact on the application of electron beams under non-standard conditions are demonstrated and discussed.