Background and Purpose: A quality assurance (QA) program in conjunction with the EORTC trial, investigating the role of adjuvant internal mammary and medial supraclavicular irradiation in stage I-III breast cancer is presented. The results of a dummy run procedure and of an individual case review are compared to each other. The effects of recommendations based on QA procedures on the protocol compliance are evaluated. Material and Methods: Prior to protocol activation all participating institutes were asked to produce treatment plans according to the guidelines of the protocol based on manual outlines of an average patient. Thereafter, they were asked to provide data on each of their first six randomized patients. Results: The dummy run provided a lot of information on specific treatment techniques. In the individual case review, additional patient- and tumor-related data were collected, showing the use of anatomic information for treatment planning. A comparison between both procedures revealed that the individual case reports concurred more accurately with protocol guidelines than the dummy run. Conclusion: It was observed that the number of systematic protocol deviations was substantially decreased in trial patients compared to the dummy run case. Therefore, it is concluded that this extensive QA program had a positive effect on the consistency of all institutes participating in the trial. © Urban & Vogel.