Author: Colton Baley, Bhuvaneswari Narayanan, Niko Papanikolaou, Daniel L. Saenz 👨🔬
Affiliation: UT Health San Antonio, Texas Oncology, University of Texas HSC San Antonio 🌍
Purpose: To evaluate the accuracy of off-axis winston lutz (WLT) for linac based stereotactic radiosurgery (SRS) treatments using in-house phantom.
Methods: Traditional WLT that uses cone and small marker bead at isocenter is limited in evaluating the accuracy at off- axis positions. With the advent of advanced treatment techniques that uses MLC to define treatment aperture, introduces complexity with beam not directed along the central axis. To overcome this challenge, a phantom with metal beads embedded at various positions which simulates off- axis targets was fabricated in-house. SRS test plans with single isocenter and multiple targets were generated using BrainLab Element TPS to conform to these off-axis target positions. The distance from isocenter and the metal beads which simulated an off-axis target was varied for each trial plan to simulate various clinical scenarios of multi target single isocenter SRS treatment. The plans were delivered on Elekta Versa HD Linac equipped with Agility MLC and Exactrac Dynamic localization system. Images for these off- axis Winston lutz were acquired using Elekta iView electronic portal imaging device (EPID) and analyzed using MATLAB software to determine the displacement between the planned centroid and delivered centroid.
Results: The methodology employed in this study which uses an in-house phantom successfully provided a robust framework for evaluating the performance of off-axis winston- lutz test even for MLC define aperture. The demonstrated accuracy for targets within 3cm distance from isocenter was less than 1mm, and for targets beyond 3cm from the isocenter but within 5cm was found to be within <1.5mm.
Conclusion: WLT test performed at isocenter is limited in providing the accuracy at off-axis positions. C-arm Linac based SRS delivery that uses single isocenter multi target treatments being a common practice, this in-house phantom is a cost-effective tool to evaluate the isocenter accuracy at off-axis positions.