Optimizing the Collimator Angles in Radiation Isocenter Localization Using Winston-Lutz Tests 📝

Author: Weiliang Du 👨‍🔬

Affiliation: The University of Texas MD Anderson Cancer Center 🌍

Abstract:

Purpose: Winston-Lutz (WL) tests are often used to localize the radiation isocenter of a linac. The selection of gantry and collimator angles can affect the accuracy of the resulting isocenter. The effects of mechanical imperfections and gravity are more prominent at certain gantry and collimator angles. This study investigates the impact of collimator angles on radiation isocenter localization.
Methods: WL tests were performed on four Varian TrueBeam linacs. In each test, 56 images of a tungsten sphere were obtained at eight gantry angles multiplied by seven collimator angles (225,270,315,0,45,90,135). Isocenters were computed using 31 sampling schemes with collimator angles being C=0, C=90|270, C=90&270, or C=Mixed. The differences in the calculated isocenters were compared among the four groups. The reference isocenter was estimated by averaging the isocenters in C=Mixed group.
Results: Systematic differences were observed in the resulting isocenters. Isocenters in C=0 group were 0.15-0.23 mm lower (towards the floor) than those in C=90|270 and C=90&270 groups. The lateral offsets of isocenters were <0.1 mm because opposing gantry angles were employed. The longitudinal offsets were <0.1 mm if opposing or mixed collimator angles were used. If a single collimator angle was used, the longitudinal offsets reached as much as 0.4 mm. As the number of gantry and collimator angles increased, the calculated isocenters converged to the reference isocenter. All isocenters in C=Mixed group were within 0.1 mm from the reference isocenter.
Conclusion: Opposing collimator angles were essential to cancel the lateral and longitudinal collimator misalignments. In vertical direction, the commonly used pair of collimator angles, 90&270, were not adequate to achieve an accuracy of 0.1 mm or less. The resulting isocenters were systematically higher than those with C=0. Other pairs of collimator angles such as 45&225 and 135&315 are effective alternatives to yield <0.1 mm accuracy in radiation isocenter localization.

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