ESTRO 2025 - Abstract Book

S3592

Physics - Quality assurance and auditing

ESTRO 2025

errors [2], however studies examining long term trends do not exist. This study aims to quantify the long-term trends of the OAWLT on 3 Varian TrueBeam linacs.

Material/Methods: A StereoPHAN MultiMet-WLT Cube (Sun Nuclear, Melbourne, Florida, USA) was utilised for the off-axis WLT. The cube contains 6 tungsten spheres in multiple locations (Figure 1). The phantom was positioned using a cone-beam computed tomography scan with six degree of freedom manual image match, after which eight 2x2 cm 2 multi-leaf collimator defined fields were used to irradiate each sphere at different combinations of gantry, collimator, and couch angles. The MultiMet software (v2.1.0.30, Sun Nuclear, Melbourne, Florida, USA) was used to analyse the difference between centre of the tungsten spheres and the fields with a tolerance of 1. The test was repeated weekly for a 6-month period. Timing studies were undertaken to determine the feasibility of performing the test on a routine basis. Varian Machine Performance Check (MPC) data was collected on a daily basis throughout the same 6-month period to allow for correlation analysis with off-axis WLT results.

Results: The mean displacement between the tungsten spheres and MLC defined fields across all measurements was 0.48 mm (range 0.01 – 1.48 mm). The lowest displacement was found for the tungsten sphere placed at isocentre and the largest for the sphere furthest off-axis. The displacement was found to depend not only on the sphere location, but also on the gantry/collimator/couch angle, and the linac used. A correlation with off-axis WLT result and Varian TrueBeam MPC results (kV isocentre offset, couch radiation induced shift) was observed. The mean time taken to perform the off-axis WLT was 15.5 minutes (range 13 - 17 minutes). Conclusion: Long-term trends of off-axis WLT results across 3 Varian TrueBeam linacs were collected. Based on the stability of the result over a 6 month period, the dependence of the result on the off-axis position of the tungsten sphere, and the time taken to perform the check, it is recommended that an off-axis WLT be performed weekly for departments offering a MTSI SRS technique. Keywords: winston-lutz, stereotactic radiosurgery, linac References: [1] Pudsey L, Haworth A, White P, Moutrie Z, Jonker B, Foote M, et al. Current status of intra-cranial stereotactic radiotherapy and stereotactic radiosurgery in Australia and New Zealand: key considerations from a workshop and surveys. Physical and Engineering Sciences in Medicine. 2022;45:251-9. [2] Pudsey LM, Biasi G, Ralston A, Rosenfeld A, Poder J. Detection of rotational errors in single ‐ isocenter multiple ‐ target radiosurgery: Is a routine off ‐ axis Winston–Lutz test necessary? Journal of Applied Clinical Medical Physics. 2022;23:e13665.