Treatment plan quality during online adaptive re-planning

Abstract

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.

Keywords: Adaptive; MR-guided; MR-linac; MRgRT; Online; Online-adaptive radiation therapy; Planning; Radiotherapy; SBRT.

Fig. 1

Workflow of treatment using MR-guided online-adaptive radiotherapy on the MRIdian linac

Fig. 2

Boxplots indicating the relative volume difference in GTV of the adapted plans compared to the respective reference treatment plans. Diamonds indicate the mean. Dashed lines indicate 0% and dotted lines indicate ±2% differences. Asterisks indicate significance: p-value< 0.05 (*), p-value< 0.01 (**) and p-value< 0.001 (***)

Fig. 3

Boxplot indicating relative change of OAR D_1.0cc of adapted plans with respect to reference treatment plans. For abdomen, liver and pelvis, either stomach, bowel or duodenum was evaluated. For prostate, both rectum and bladder were evaluated. Diamonds indicate the mean. Dashed lines indicate 0% and dotted lines indicate ±2% differences. Asterisks indicate significance: p-value< 0.05 (*), p-value< 0.01 (**) and p-value< 0.001 (***)