# Dosimetric evaluation of Ethos 2.0 high‐fidelity mode for single‐isocenter SRS

## TL;DR

This study shows that using Ethos 2.0's high-fidelity mode with control rings improves the quality of brain metastasis SRS treatments by reducing normal tissue exposure and plan complexity.

## Contribution

The study introduces a new planning strategy combining Ethos 2.0's high-fidelity mode and control rings for optimized SRS treatments.

## Key findings

- HFonRon template significantly improved normal tissue sparing, conformity index, and falloff index compared to other templates.
- All templates provided clinically acceptable target coverage, with the HFonRon achieving 98.3% target coverage.
- Enabling HF mode significantly reduced plan complexity compared to HF disabled templates.

## Abstract

This study evaluates the impact of the Ethos 2.0 high‐fidelity (HF) mode on single‐isocenter, stereotactic radiosurgery (SRS) planning workflows. By comparing different planning templates, we assess the effects of HF mode and control rings (R) on plan quality, aiming to optimize treatment for patients with brain metastases.

A cohort of 45 patients with brain metastases was divided into a tuning (n = 15) and a validation set (n = 30). Four planning templates were evaluated: HFonRon, HFonRoff, HFoffRon, and HFoffRoff. Plans were generated using the Ethos Intelligent Optimization Engine (IOE v02.00.10), and all patients were prescribed 30 Gy in 5 fractions using 2 mm PTV margins. Plan quality was evaluated using target coverage (PTV V100%), normal brain dose (Brain‐PTV V30Gy), Paddick conformity index (CI), and Falloff index. Plan complexity was evaluated using total MU's on a per‐plan basis across the four templates.

The HFonRon template produced better quality plans, achieving significant improvements in normal tissue sparing (p < 0.0001), CI (p < 0.0001), and Falloff index (p < 0.0001) compared to other templates. All templates provided clinically acceptable target coverage. In the HFoffRoff configuration, 97.5 % of targets met the coverage criterion (V100% > 95%). The other setups achieved coverage in 99.2% (HFoffRon), 96.6% (HFonRoff), and 98.3% (HFonRon) of targets, respectively. Plan complexity decreased significantly (p < 0.0001) when enabling HF mode compared to both HF disabled templates

His study shows that combining Ethos 2.0's high‐fidelity mode with user‐defined control rings yields superior single‐isocenter SRS plans compared with planning strategies that omit one or both features. This approach improves dose conformity, reduces normal tissue exposure, and decreases plan complexity through semi‐automated, template‐based planning. These findings suggest that HF mode can be a valuable tool in clinical practice for optimizing SRS treatments for patients with brain metastases.