High-precision localization of radiation isocenter using Winston-Lutz test: Impact of collimator angle, phantom position, and field size

Abstract

Purpose: This study aimed to evaluate the impact of collimator angle, ball bearing (BB) phantom position, and field size on the accuracy of Winston-Lutz (WL) test-derived radiation isocenters.

Methods: WL tests were performed on four TrueBeam linear accelerators. Fifty-six images (eight gantry angles multiplied by seven collimator angles) were acquired for each WL test. Images with different sets of collimator angles were used to compute the radiation isocenters. The resulting radiation isocenters were correlated with the collimator angles. Then, the BB position and radiation field size were varied for the subsequent WL tests. The calculated BB shifts were compared with the known shifts, and the radiation isocenters were compared between different field sizes.

Results: The use of a single collimator angle led to errors of as much as 0.4 mm in the calculated radiation isocenters. Systematic differences were observed between the radiation isocenters derived with collimator angle 0° and those derived with 90° and/or 270°. A commonly used opposing collimator angle pair, 90° and 270°, resulted in a vertical 0.1 mm offset of the radiation isocenters toward the ceiling. Oblique opposite or mixed collimator angles yielded radiation isocenter errors less than 0.1 mm. The BB shifts derived from WL tests were less than 0.1 mm from the known shifts. The radiation isocenters varied by less than 0.1 mm between field sizes ranging from 2 × 2 cm² to 20 × 20 cm².

Conclusions: Oblique opposing collimator angle pairs should be considered to minimize errors in localizing radiation isocenters. Uncertainty in BB positioning could be eliminated if the BB is used as a static reference point in space. The field size had no significant effect on the radiation isocenters. With careful design of WL test parameters and image processing, it is possible to achieve a precision of 0.1 mm in localizing radiation isocenters using WL tests.

Keywords: Winston‐Lutz test; quality assurance; radiation isocenter.

FIGURE 1

The ball bearing (BB) phantom for Winston‐Lutz (WL) tests and the x‐y‐z coordinate system. The red arrows indicate the positive directions. The inset shows the tungsten BB glued on the tip of an acrylic rod. The gantry was set at 90° in this picture.

FIGURE 2

Radiation isocenters derived from different sampling schemes in Table 1. Data were obtained from one Winston‐Lutz (WL) test on Linac 1. The center of the plot was the reference radiation isocenter (i.e., an average of C = Mixed group). (a) Radiation isocenters in x‐y plane. (b) Radiation isocenters in z‐y plane. The dotted circle was centered at (0, 0) and had a radius of 0.1 mm.

FIGURE 3

Radiation isocenter deviations from the reference radiation isocenter on four TrueBeam linacs. For each linac, 31 radiation isocenters were calculated from the sampling schemes in Table 1. The reference radiation isocenter was the average of radiation isocenters in the C = Mixed group. The dotted lines are ± 0.1 mm from the reference radiation isocenter.

FIGURE 4

The ball bearing (BB)‐to‐isocenter shifts, or Δ(BB–Iso), derived from Winston‐Lutz (WL) tests versus the known couch shifts, ΔT. The solid diagonal lines are Δ(BB–Iso) = ΔT. The dotted lines are 0.1 mm above or below the solid lines.

FIGURE 5

Radiation isocenter‐to‐ball bearing (BB) positions determined with the Winston‐Lutz (WL) tests using different MLC field sizes. The solid lines are the average radiation isocenter coordinates from all seven field sizes. The dotted lines are ± 0.1 mm from the solid lines.