Pelvic dose accumulation accuracy in a CBCT based online adaptive radiotherapy system.

INTRODUCTION

Accurate dose accumulation is essential for understanding the delivered dose in online adaptive radiotherapy (OART). The aim of this study is to quantify the dose accumulation accuracy and clinical significance of dose accumulation errors in pelvis treatments using the Ethos v1.1 and v2.0 systems.

METHOD

Three pelvic CTs had anatomically realistic deformation vector fields (DVF) applied to create new modified images. These modified images were then used as treatment images in simulated OART treatment sessions, and the dose was accumulated by the Ethos system (D(fx)). The inverse of the applied (true) DVF was also used to accumulate the delivered dose (D(fx)). The maximum applied DVF magnitude was compared to the maximum DVF magnitude calculated by the Ethos v1.1 and v2.0 systems. A 3D gamma analysis was performed between D(fx) and D(fx), and the clinical goals for each scenario were compared.

RESULTS

Large discrepancies in deformable registration magnitude were noted between the applied DVF and the Ethos DVF from both Ethos versions. All Ethos v1.1 results had 90% of points passing a 3%/2 mm gamma analysis, whilst Ethos v2.0 results did not reach this benchmark in all of the simulated scenarios. Results were particularly poor for the v2.0 prostate case. Inaccuracies in the dose accumulation process caused 6% of goals to change sufficiently such that the goal was reported as met or exceeded when it was not.

CONCLUSION

The Ethos v1.1 dose accumulation system has been found to perform acceptably for the pelvic CTs tested in this study. For the Ethos v2.0 system substantial discrepancies in the dose accumulation were noted.