Treatment plan quality during online adaptive re-planning

Background Online adaptive radiotherapy is intended to prevent plan degradation caused by inter-fractional tumor volume and shape changes, but time limitations make online re-planning challenging. The aim of this study was to compare the quality of online-adapted plans to their respective reference treatment plans. Methods Fifty-two patients treated on a ViewRay MRIdian Linac were included in this retrospective study. In total 238 online-adapted plans were analyzed, which were optimized with either changing of the segment weights (n = 85) or full re-optimization (n = 153). Five different treatment sites were evaluated: prostate, abdomen, liver, lung and pelvis. Dosimetric parameters of gross tumor volume (GTV), planning target volume (PTV), 2 cm ring around the PTV and organs at risk (OARs) were considered. The Wilcoxon signed-rank test was used to assess differences between online-adapted and reference treatment plans,p < 0.05 was considered significant. Results The average duration of the online adaptation, consisting of contour editing, plan optimization and quality assurance (QA), was 24 +/- 6 min. The GTV was slightly larger (average +/- SD: 1.9% +/- 9.0%) in the adapted plans than in the reference plans (p < 0.001). GTV-D(95%)exhibited no significant changes when considering all plans, but GTV-D(2%)increased by 0.40% +/- 1.5% on average (p < 0.001). There was a very small yet significant decrease in GTV-coverage for the abdomen plans. The ring D(mean)increased on average by 1.0% +/- 3.6% considering all plans (p < 0.001). There was a significant reduction of the dose to the rectum of 4.7% +/- 16% on average (p < 0.001) for prostate plans. Conclusions Dosimetric quality of online-adapted plans was comparable to reference treatment plans and OAR dose was either comparable or decreased, depending on treatment site. However, dose spillage was slightly increased.