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

## TL;DR

This study evaluates the accuracy of dose accumulation in a radiotherapy system for pelvic treatments, finding that version 1.1 performs well while version 2.0 has significant issues.

## Contribution

The study provides a quantitative comparison of dose accumulation accuracy between two versions of the Ethos system in pelvic radiotherapy.

## Key findings

- Ethos v1.1 achieved 90% gamma analysis pass rate, but v2.0 failed in some scenarios.
- Dose accumulation errors in v2.0 caused 6% of clinical goals to be incorrectly reported as met.
- Prostate case results were particularly poor for Ethos v2.0.

## Abstract

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.

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)Ethos
). The inverse of the applied (true) DVF was also used to accumulate the delivered dose (D(fx)True
). 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)Ethos
and D(fx)True
, and the clinical goals for each scenario were compared.

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.

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.