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Session: Imaging for Patient Setup and Target Alignment [Return to Session]

A Simplified and Accurate Off-Axis Winston-Lutz Method for Single-Isocenter Multiple-Target Stereotactic Radiosurgery

A Eagle1*, M Tallhamer2, J Keener3, S Geneser4, (1) Centura Health, Longmont, CO, (2) Centura Health, Parker, CO, (3) Centura Health, Littleton, CO, (4) University of Washington, Seattle, WA


TU-D1030-IePD-F1-5 (Tuesday, 7/12/2022) 10:30 AM - 11:00 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 1

Purpose: To safely perform Multi-Target Single-Iso (MTSI) SRS, clinics must demonstrate SRS accuracy for off-axis targets. The traditional Winston-Lutz was widely adopted because it provides a simple, accurate, and inexpensive 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 non-commercial MTSI tests are cumbersome and inaccurate. To create an all-purpose MTSI Winston-Lutz test, one must create MLC fields that center on off-axis targets. Unfortunately, accomplishing this presents a non-trivial geometry problem that has not been previously solved in the literature.We present a solution for evaluating MTSI SRS accuracy that provides a straightforward method for creating test fields and offers all the benefits of the standard W-L test.

Methods: We developed a method to calculate the required gantry, table, and collimator angles to create Off-Axis Winston-Lutz (OAWL) fields, that can be used by any clinic to test MTSI SRS accuracy. This method calculates a series of nested coordinate transformations that align the MLC to create a symmetric field around the off-axis target for any arbitrary combination of treatment angles.

Results: For a target 8 cm off-axis, the described method yields OAWL results that are within 0.10 mm of the standard isocentric WL results. Averaging six DoseLab isocentric WL tests, we have a max error of 0.59 mm, and mean error of 0.37 mm. For this new OAWL test, the average max error for six runs is 0.65 mm, with a mean error of 0.42 mm.

Conclusion: This method accurately evaluates SRS accuracy of off-axis targets anywhere within a 15 cm wide field. We intend to make 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 MTSI SRS technique.


Stereotactic Radiosurgery, Quality Assurance


TH- External Beam- Photons: intracranial stereotactic/SBRT

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