Author: Shifeng Chen, Yannick P. Poirier, Huijun Xu, Byong Yong Yi, Baoshe Zhang, Hong Zhang, Jinghao Zhou 👨🔬
Affiliation: University of Maryland School of Medicine, University of Maryland Shore Regional Cancer Center, Vanderbilt University Medical Center, Department of Radiation Oncology, University of Maryland School of Medicine, Capital Region Medical Center 🌍
Purpose: To thoroughly assess the resilience of HyperArc VMAT plans for multiple brain metastases (MBT) in relation to (i) high-definition MLC leaf positional inaccuracies and (ii) patient rotational and translational setup discrepancies.
Methods:
A retrospective analysis was conducted on 25 MBT patients with 99 lesions treated with HyperArc VMAT plans. Prescription doses (21–24 Gy) were directed at planning target volumes (PTVs) (gross tumor volumes (GTVs) + 1mm), with each plan accommodating 2–5 brain metastases (less than 2 cm). D100 for GTVs and V12Gy for normal brain were reassessed under simulated conditions involving (i) 20 MLC positional inaccuracies (0.2–1mm) and (ii) 52 patients' rotational (Yaw/Pitch/Roll by 1°) and translational discrepancies (SI/LR/AP by 1mm). The percentage changes in GTVs D100 were modeled using multiple linear regression, using GTVs distance to isocenter (DTI) and GTVs volume.
Results:
GTVs D100 are highly sensitive to both MLC and patient errors. Their significant dosimetric effects follow multiple linear regression: 1mm MLC shift reduces D100 by 8.6%; symmetric MLC openings increase D100 by >23% per 1mm MLC opening while D100 decreases when volume increases; symmetric closings decrease D100 by >25% per 1mm MLC closing while D100 increases when volume increases. Patient rotations decrease D100 by 0.06-0.33% per 1mm DTI increase. Patient translations decrease D100 by 8.2-26.4%. V12Gy is mainly sensitive to MLC symmetric openings and closings. Symmetric openings increase V12Gy 4.9CC per 1mm MLC opening and symmetric closings decrease V12Gy 2.7CC per 1mm MLC closing.
Conclusion: Overall, MLC errors and patients’ translations have a greater impact than patient rotational setup errors on D100. The increase of degree of freedom translation increases the impact. Only MLC symmetric openings and closings can change V12Gy significantly. The pronounced effects of these errors highlight the importance of rigorous plan robustness verification, accounting for target size, margins, and DTI.