Single-Isocenter SBRT for Five or More Liver Metastases: Impact of Rotational Setup Uncertainty on Target Coverage
Abstract
Purpose
Single-isocenter SBRT enables efficient treatment of multiple liver metastases but limits mitigation of rotational error. This study evaluates rotational uncertainty effects on target coverage in single-isocenter liver SBRT for patients with 5+ metastases.
Methods
Liver SBRT plans were generated in Ethos 2.0 using a single isocenter to treat 5+ liver metastases per plan (mean 12 targets/patient), yielding 36 analyzed targets. Plans were optimized to the planning target volume (PTV = ITV+3mm) such that all targets met the prescription coverage goal (V5000cGy ≥ 95%) in the unrotated case. Target contours were generated in VelocityAI. To evaluate sensitivity to rotational setup uncertainty, target contours were rigidly rotated by 1°, 2°, and 3° about the patient superior–inferior axis through image center as a geometric approximation of rotational error. Treatment plans were not reoptimized; MLC patterns and monitor units were held fixed. Clinical coverage goals were reevaluated in Eclipse. Coverage was assessed for ITV+1mm, ITV+2mm, and PTV target expansions.
Results
Target coverage decreased with increasing rotational error across the 36 targets analyzed. When evaluating PTV coverage, 5 targets failed to meet the clinical coverage criterion at a 1° rotation; this increased to 23 targets at 2° and 29 targets at 3°. Coverage failures were reduced for ITV-based evaluations but were not eliminated. At 2°, 5 targets failed for ITV+2mm and 2 targets for ITV+1mm, while at 3° failures increased to 21 targets for ITV+2mm and 9 targets for ITV+1mm. Coverage loss was more frequent for smaller targets and those located farther from isocenter, with substantial variability across targets.
Conclusion
In single-isocenter SBRT for patients with 5+ liver metastases, rotational setup uncertainty can meaningfully impact target coverage when translational corrections are constrained. These findings suggest that additional attention to rotational uncertainty may be warranted in this treatment paradigm.