What question did this study set out to answer?

The aim is to quantify geometric uncertainties in off-axis targets during SBRT to optimize treatment margins.

February 14, 2026Journal of Applied Clinical Medical Physics0 citationsOpen Access

The geometric accuracy of off‐axis targets in stereotactic body radiotherapy treatments across three linear accelerators

Key Points

The aim is to quantify geometric uncertainties in off-axis targets during SBRT to optimize treatment margins.
Utilized MultiMet Winston–Lutz tests for measurement.
Analyzed images from Trilogy, TrueBeam, and TrueBeam STx linear accelerators.
Measured 3D displacements at four gantry angles using a cube with tungsten carbide markers.
Calculated GTV-to-PTV margins using a modified van Herk formula.
TrueBeam STx achieved superior accuracy with ≤0.5 mm isocenter precision.
Trilogy exceeded tolerances, requiring 4 mm uniform margins across all targets.
TrueBeam showed optimal variations, with isocenter accuracy at 0.68 ± 0.34 mm.
Isotropic margins led to up to 11% delineation errors for off-axis targets.

Abstract

Abstract Purpose This study quantifies isocenter and off‐axis geometric uncertainties using MultiMet Winston–Lutz (WL) tests to optimize gross tumour volume (GTV)‐to‐PTV (planning target volume) margin for single isocenter multiple target (SIMT) stereotactic body radiotherapy (SBRT) across three linear accelerators. Methods and Materials Geometric inaccuracies were quantified for Trilogy (HD‐MLC, 6 MV SRS), TrueBeam (Millennium MLC), and TrueBeam STx (HD‐MLC, 6‐degrees‐of‐freedom (6DoF) couch) using a Sun Nuclear MultiMet‐WL cube containing six tungsten carbide markers arranged along the superior–inferior axis. The electronic portal imaging device (EPID) images acquired at four cardinal gantry angles with varied collimator/couch rotations were analyzed using MultiMet‐WL software (v2.1) to measure 3D (Δ) displacements for all the LINACs. The required GTV‐to‐PTV margins were calculated using a modified van Herk formula (2.5Σ+1.64σ), incorporating measured 3D displacements for isocenter and off‐axis targets. Results The TrueBeam STx (HD‐MLC/6DoF) demonstrated superior geometric accuracy, maintaining ≤0.5 mm isocenter precision and ≤0.59 mm off‐axis targeting (3–7 cm). The Trilogy exceeded TG‐142 tolerances (1.06 ± 0.59 to 1.09 ± 0.57 mm) at all targets, requiring 4 mm uniform margins, while the TrueBeam (MMLC) showed optimal variations (isocenter: 0.68 ± 0.34 mm; superior off‐axis: 0.74 ± 0.36 mm). Both TrueBeam platforms achieved sub‐millimeter accuracy but demonstrated direction dependency for off‐axis targets, requiring 2–3 mm anisotropic margins. Notably, isotropic margins introduced up to 11% delineation errors for off‐axis targets due to these machine‐specific geometric variations, highlighting the imperative for platform‐specific margin protocols in SIMT SBRT. Conclusion This study demonstrates that routine analysis of MultiMet WL testing is an essential tool for the spatial accuracy of the LINAC to establish machine‐specific PTV margin expansion in SIMT‐SBRT, particularly for targets where rotational errors dominate.

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