Dosimetric Robustness of VMAT Craniospinal Irradiation (CSI) to Rotational Setup Errors on a Ring-Based Linear Accelerator
Abstract
Purpose
To evaluate the dosimetric impact of rotational alignment errors on volumetric modulated arc therapy (VMAT) craniospinal irradiation (CSI), with emphasis on ring-based gantry platforms that lack rotational couch corrections. Given increasing interest in anterior vertebral body–sparing CSI (VBS-CSI), rotational robustness was assessed for both nominal CSI and VBS-CSI techniques.
Methods
Nominal CSI and VBS-CSI VMAT plans were generated for one representative 20-year-old adult and one representative 5-year-old pediatric patient. The adult plan employed a four-isocenter field arrangement with an 8 cm longitudinal overlap, while the pediatric plan utilized three isocenters with an extended 14 cm overlap to account for shorter patient length and junction robustness. Rotational errors in roll, pitch, and yaw of up to ±5° were introduced independently, and plans were recalculated on rotated datasets using isocenter-specific translation-only couch corrections. Changes in D90% for the brain clinical target volume (CTV) and thecal sac target were quantified relative to non-rotated plans. For VBS-CSI, changes in vertebral body dose were evaluated.
Results
Across all evaluated rotations, brain CTV D90% was maintained within 1% of nominal values for both patients and planning techniques. Thecal sac D90% was generally robust and remained within 2.5% for rotations up to ±5°. Yaw and pitch were the most sensitive axes with respect to both target coverage and vertebral body dose. VBS-CSI plans demonstrated increased sensitivity to rotational errors compared to nominal CSI. Relative to the adult, the pediatric patient exhibited greater sensitivity in vertebral body dose but reduced sensitivity in target coverage.
Conclusion
VMAT CSI is robust to moderate rotational alignment errors when using translation-only couch corrections; however, sensitivity depends on rotation axis, treatment technique, and field arrangement. These results highlight the importance of rotational setup management and junction design on ring-based treatment platforms without rotational correction capabilities.