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Evaluation of a New Winston Lutz Tool for the Treatment of Multiple Brain Metastases for Mono-Isocentric Beams

A Perez-Andujar*, Y Mutaf, G Klein, J Chen, Kaiser Permanente, Dublin, CA

Presentations

PO-GePV-T-354 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To evaluate the use of a new Winston Lutz tool for the treatment of multiple brain metastases for mono-isocentric beams.

Methods: The Winston Lutz phantom has 5 built in BBs positioned at different distances from isocenter including off axis positions. The phantom was imaged before delivering the plan to ensure setup position. In total, twenty-one Winston Lutz tests were performed. Each test included eight different gantry, couch, and collimator configurations. The actual position of the phantom and each BB was also calculated for all the measurements.

Results: The average position of the phantom ranged from 0.08-0.19 mm in the x,y, z direction and 0.1 degrees for the pitch, roll and yaw positions. Target 4 (5.8 cm from isocenter) and Target 5 (7 cm from isocenter) had the largest number of Winston Lutz test with a maximum 2D deviation between the BBs and the field center of 0.75 mm and 1 mm for a configuration with the gantry, couch and collimator at 0 degrees. The same targets showed a larger percent of times with a Winston Lutz > 1 mm. Overall target 5 showed 3% of the time a value ≥ 1.0.

Conclusion: Winston Lutz tests were performed for a total of 168 beam arrangements for a phantom aligned within our clinical tolerances < 0.5 mm and < 0.5°. Overall targets 4 and 5, the targets further from isocenter, had the largest number of Winston Lutz > 1 mm among all shifts and all gantry combinations. Longitudinal shifts seemed to affect the overall results the most. Nevertheless, a Winston Lutz < 0.75 mm seems to be achievable for most circumstances. Our result might be an indication that careful consideration should be taken for those targets further away from isocenter even when tight setup constrains are used during treatment.

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    Keywords

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    Taxonomy

    TH- External Beam- Photons: intracranial stereotactic/SBRT

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