Author: Brian D. Casto, Mathew Y Kang, Lichung Ku, Samuel J Wang, Joseph B. Ying, Walker Ray Zimmerman 👨🔬
Affiliation: Salem Health Cancer Center, Salem Health 🌍
Purpose: Varian HyperArc is an advanced radiotherapy module designed to efficiently deliver high-quality multi-target stereotactic radiosurgery (SRS) treatment plans on Varian linacs. However, the interplay between multileaf collimator (MLC) dynamics and intra-fractional motion may render these highly modulated HyperArc plans more vulnerable to motion-induced dosimetric deviations. This study aims to investigate the dosimetric impact of intra-fractional motion on HyperArc treatment plans.
Methods: In this study, intra-fractional motion was simulated by introducing isocenter shifts during treatment. The original treatment plan was divided into two segments: the first half remained unaltered, while the second half was recalculated with a shifted isocenter. Each segment of the simulated plan retained the same number of MLC control points and calculation parameters as the original plan. Based on observations from real patient treatments, isocenter shifts were simulated as 1 mm in the superior-inferior and 1 mm in the posterior-anterior directions. To assess the dosimetric impact, the simulated plans were compared against the original plans using appropriate evaluation metrics.
Results: A total of 13 cases were analyzed. The GTV maximum dose remained nearly unchanged (<1%). However, the GTV minimum dose decreased by an average of 4.60%, with a standard deviation of 2.64%. Moreover, the most significant dose differences were observed in the regions surrounding the GTV, where the average maximum dose deviation was 12.8% of the prescribed dose, with a standard deviation of 2.40%.
Conclusion: Using simulated plans, we have demonstrated that small (~1 mm) intra-fractional movements during HyperArc SRS treatment can result in considerable dosimetric deviations. When using this radiosurgery delivery system, it is important to be aware of the potential for these dosimetric variations, and consider measures to mitigate these effects, such as incorporating a suitable PTV margin and/or using real-time tracking systems for detecting intra-factional motion.