OA234] Establishing acceptance criteria for VMAT Quality assurance (QA) in a new radiotherapy centre

Abstract

Purpose In this poster we present the statistical analysis applied to establish the criteria of acceptance for Volumetric Arctherapy (VMAT) plans during the QA phase for a recently opened radiotherapy centre. Methods Using a sample of 200 plans, we have analysed the 2D and 3D gamma indices in the three axes (coronal, sagittal and transverse), varying from 1 mm, 1% to 5 mm, 5%, both local and global, and compared these indices with a 4D QA method. This analysis has been done for the whole sample of plans and separately for each pathology. Results Our results show no statistical significant difference between using 2D or 3D gamma indices nor for the different axes. However, the transverse axis shows a much larger dispersion in the gamma indices and is thus considered not optimal in our case. We find that the gamma indices for the global maximum of each plan are significant (2.5 sigma) larger than when using local maxima and this difference increases as we lower the threshold. We have compared the gamma indices against the 4D method threefold: i) correlation (quantified by the Pearson coefficient); ii) absolute difference; and iii) number of catastrophic events (defined as those that would be accepted in terms of the gamma index but would be rejected in terms of the 4D method). Conclusions We conclude that from our sample of VMAT plans, the optimal criterion is 3 mm, 3% local. These results are consistent over the whole sample and for each pathology. Furthermore, this analysis has allowed us to establish standard benchmarks for acceptance testing, with an overall standard of 95% for a gamma 3 mm, 3% local which can be relaxed or strengthened depending on the pathology (ranging from 92% for bone cancer to 97% for prostate cancer).