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. 2023 Feb;24(2):e13816.
doi: 10.1002/acm2.13816. Epub 2022 Nov 24.

A simplified and effective off-axis Winston-Lutz for single-isocenter multi-target SRS

Anton Eagle  1 , Mike Tallhamer  2 , Justin Keener  3 , Sarah Geneser  4
Affiliations

A simplified and effective off-axis Winston-Lutz for single-isocenter multi-target SRS

Anton Eagle et al. J Appl Clin Med Phys. 2023 Feb.

Abstract

Purpose: To safely perform single-iso multi-target (SIMT) stereotactic radiosurgery (SRS), clinics must demonstrate SRS delivery accuracy for off-axis targets. The traditional Winston-Lutz (W-L) was widely adopted because it provides a simple and accurate solution for testing radiation-isocenter coincidence that uses a static target, enables testing arbitrary treatment angles, and does not require expensive commercial phantoms. The current noncommercial tests are cumbersome and insufficiently accurate. For an off-axis Winston-Lutz (OAWL) test, one must design MLC fields centered on off-axis targets. Unfortunately, because MLC leaf-interfaces are often misaligned with the target center, accomplishing this presents a nontrivial geometry problem that has not been previously solved in the literature. We present a solution for evaluating SIMT SRS accuracy that provides a straightforward method for creating OAWL test fields and offers all the benefits of the standard W-L test.

Methods: We have developed a method to use any gantry, table, and initial collimator angles to create OAWL fields. This method calculates a series of nested coordinate transformations that produce a small collimator angle adjustment to align the MLC and create a symmetric field around an off-axis target.

Results: For an 8 cm off-axis target, the described method yields OAWL results within 0.07 mm of standard isocentric W-L results. Our six most recent isocentric W-L tests show max and mean errors of 0.59 and 0.37 mm, respectively. For six runs of our proposed OAWL test, the average max and mean errors are 0.66 and 0.40 mm, respectively.

Conclusion: This method accurately evaluates SRS delivery accuracy for off-axis distances that span the majority of a typical human brain for a centered SIMT arc. We have made this method publicly available, so that physicists can employ it within their clinics, foregoing the need for expensive phantoms and improving access to the state-of-the-art SIMT SRS technique.

Keywords: SIMT; SRS; Winston-Lutz.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

FIGURE 1
FIGURE 1
These images depict the beam's eye views (BEVs) for test fields with arbitrarily selected gantry, collimator, and couch angles of 335°, 335°, and 35°, respectively. The phantom is aligned with target 1 (depicted in blue) at isocenter. Note that no leaf‐interfaces are centered on the off‐axis target (red), making it infeasible to create a symmetric field about this target using the MLCs. Parts (a) and (b) depict attempts to create a test field using the closest MLC leaves, which highlights the inability to center a field on the off‐axis target for these treatment angles.
FIGURE 2
FIGURE 2
Part (a) depicts our treatment planning system (TPS) coordinate system (X*, Y*, Z*) and the sign of TPS axis rotations. Part (b) depicts the beam's eye view (BEV) coordinate system (X, Y, Z) and its rotation configuration. Part (c) shows the apparent object rotations (in the BEV) due to axis rotations and also shows the resultant sign for these rotations used in the coordinate transformations.
FIGURE 3
FIGURE 3
Target position, XBEV , target and YBEV , target , with the Y‐coordinate (Yint ) of the two nearest leaf‐interfaces
FIGURE 4
FIGURE 4
The beam's eye view (BEV) of symmetric off‐axis Winston–Lutz (OAWL) fields for the off‐axis target with collimator angles of 342.1° (a) aligning leaf‐interface 44–45, and 325.8° (b) aligning leaf‐interface 45–46, respectively
See this image and copyright information in PMC

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