Author: Anthony J. Doemer, Yimei Huang, Benjamin Movsas, Ellen Park, Mira Shah, Salim Siddiqui, Karen C. Snyder, Kundan S Thind, Bo Zhao 👨🔬
Affiliation: Henry Ford Health 🌍
Purpose: The 6 degrees-of-freedom (6-DoF) robotic couch is considered essential for linac-based stereotactic radiosurgery (SRS), particularly for irregularly shaped targets adjacent to critical organs or multiple targets in a single isocenter. While the 6-DoF couch allows for more precise patient position, these adjustments may inadvertently induce unintended patient shifts which might exceed the action level required for SRS localization. Current SRS guidelines may not include re-imaging after the initial matching shifts. This study investigates residual shifts following 6-DoF corrections in intracranial SRS and the need of re-imaging.
Methods: A total of 38 fractions of intracranial SRS deliveries were reviewed. All patients were immobilized using QFix Encompass mask and treated on a Varian TrueBeam linac equipped with PerfectPitch 6-DoF couch and BrainLab ExacTrac Dynamic (ETD). For each treatment, 6-DoF corrections were performed using ETD, followed by iterative ETD verification imaging until clinical tolerance (0.5mm, 0.5o) was achieved. Shift values from all fractions were then analyzed.
Results: Initial translational and yaw corrections showed no correlation with the shifts from subsequent imaging. However, initial pitch and roll corrections were strongly correlated with longitudinal (R²=0.80) and lateral (R²=0.84) shifts in verification imaging, respectively. Notably, combined shifts of around 1.0mm were observed in some cases when initial pitch and/or roll corrections exceeded 2o. The mean residual shifts (± standard deviation) after re-imaging in final verification were Lateral 0.04±0.08mm, Longitudinal -0.05±0.11mm, Vertical -0.01±0.16mm, Pitch -0.05±0.05o, Roll 0.01±0.06o, and Yaw 0±0.07o.
Conclusion: Large pitch and roll corrections on the 6-DoF couch can induce additional longitudinal and lateral shifts, likely due to patient settling on a tilted surface from gravitational effects. The magnitude of the observed shifts can approach or exceed 1mm, which would exceed the repositioning tolerance limit set by AAPM-TG-142 for SRS. Therefore, post-correction verification imaging is strongly recommended to ensure positional accuracy during SRS treatments.