INTRODUCTION: The Trans-Tasman Radiation Oncology Group 03.04 Randomised Androgen Deprivation and Radiotherapy multicentre prostate cancer trial examined the optimal duration of androgen deprivation in combination with dose-escalated radiotherapy. Rigorous quality assurance (QA) processes were undertaken to ensure the validity and reliability of the radiation therapy treatment plan data. METHOD: QA processes included a planning benchmarking exercise and a periodic audit of target and normal tissue delineation. Centralised electronic review of digital plan data for external-beam radiotherapy was undertaken to detect protocol variations. The impact of clinical factors and feedback to submitting centres during the trial on variation rates was investigated. RESULTS: Twenty-three centres across Australia and New Zealand recruited 1071 participants to the trial. Treatment plans for 754 participants receiving external-beam treatment alone were reviewed. From these, 1185 minor and 86 major variations were identified, leading to feedback to treating centres to reduce variations for subsequent patients treatment and plans, suggesting improvement in treatment quality through these QA programs. Participant anatomical factors (delineated clinical target volume and rectal volume) and treatment planning factors (beam energy, beam definition and patient position orientation) were found to significantly impact variation rates. The dummy run demonstrated disagreement in identification of the base of the prostate and the superior extent of the rectum. Feedback from the periodic audit led to a change of practice at five contributing centres. CONCLUSION: The application of a suite of complementary QA activities allows the quality of trial data to be optimised and quantified, and can provide a catalyst for reforming treatment practices.