The purpose of this bachelor-thesis is the evaluation of the interobserver variability in the contouring of the rectum and of its impact on the dose during primary irradiation of the prostate. For this intention, an empirical data evaluation is carried out at the Fachhochschule Campus Wien. In the primary radiooncological therapy of the prostate carcinoma the application of the curative dose is performed precisely in the target volume. However, a radiation dose on the organs at risk can technically not be prevented. The careful contouring of the organs at risk in the context of three-dimensional conformal irradiation planning is therefore unavoidable in order to minimize this undesirable accompanying dose. As part of this paper the interobserver variability of students of the Fachhochschule Campus Wien are evaluated in the contouring of the rectum and bladder in primary irradiation of the prostate and compared to a gold standard. The quantification method of the interobserver variability is carried out using the Dice similarity coefficient, which has shown to be a suitable metric tool in Bachelor Thesis 1 by Alexandra Bauer (2017). The observers’ deviations in direction are investigated and their influence on the dose is analysed. Following, the empirical results obtained are compared with correspondent data from current literature. The interobserver variability of the rectum turns out to be larger than that of the urinary bladder. The greatest variability in the rectum can be found in cranial and caudal direction. Plan_2 fulfils the dose-volume parameters of the normal-tissue-tolerance-limits on the rectum of V50 < 50%, V60 < 35%, V65 < 25%, V70 < 20% and of the bladder of V65 < 50%, V70 < 30%. Therefore, Plan_2 in this empirical research proves to be the best treatment plan to prevent the rectum from damage.