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

## TL;DR

This study evaluates the geometric accuracy of three linear accelerators in radiotherapy to determine optimal tumor margin sizes for treatment planning.

## Contribution

The study introduces a method to calculate machine-specific PTV margins using MultiMet WL testing for off-axis targets in SBRT.

## Key findings

- TrueBeam STx showed the best geometric accuracy with ≤0.5 mm isocenter precision and ≤0.59 mm off-axis targeting.
- Trilogy exceeded tolerances and required 4 mm uniform margins, while TrueBeam (MMLC) required 2–3 mm anisotropic margins.
- Isotropic margins caused up to 11% delineation errors due to machine-specific geometric variations.

## Abstract

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.

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.

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.

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.