ABSTRACT
AIM: To analyze intrafraction movement in patients undergoing frameless robotic radiosurgery and evaluate the influence of image acquisition frequency on global accuracy. BACKGROUND: Stereotactic radiosurgery requires high spatial accuracy in dose delivery. In conventional radiosurgery, a rigid frame is used to guarantee a correct target alignment and no subsequent movement. Frameless radiosurgery with thermoplastic mask for immobilization cannot completely eliminate intrafraction patient movement. In such cases, it is necessary to evaluate its influence on global treatment accuracy. MATERIALS AND METHODS: We analyzed the intrafraction motion of the first 15 patients undergoing intracranial radiosurgery (39 fractions) with the CyberKnife VSI system at our institution. Patient position was measured at a 15-90-s interval and was used to estimate intrafraction patient movement. RESULTS: With our acquisition image protocol and immobilization device, the 99% displacement error was lower than 0.85 mm. The systematic movement components were lower than 0.05 mm and the random component was lower than 0.3 mm in the 3 translational axes. Clear linear time dependence was found in the random component. CONCLUSIONS: Selection of the X-ray image acquisition time is necessary to meet the accuracy required for radiosurgery procedures with the CyberKnife VSI system. We verified that our image acquisition protocol met the 1-mm criterion.
ABSTRACT
In December 2016 the clinical operation has started at the particle therapy centre MedAustron, Wiener Neustadt, Austria. Different commercial immobilisation devices are used for head patients. These immobilisation devices are a combination of table tops (Qfix BoS™ Headframe, Elekta HeadStep™), pillows (BoS™ Standard pillow, Moldcare®, HeadStep™ pillow) and thermoplastic masks (Klarity Green™, Qfix Fibreplast™, HeadStep™ iCAST double). For each patient image-guided radiotherapy (IGRT) is performed by acquiring orthogonal X-ray imaging and 2D3D registration and the application of the resulting 6-degree of freedom (DOF) position correction on the robotic couch. The inter- and intrafraction displacement of 101 adult head patients and 27 paediatric sedated head patients were evaluated and compared among each other regarding reproducibility during the entire treatment and stability during each fraction. For the comparison, statistical methods (Shapiro-Wilk test, Mann-Whitney U-test) were applied on the position corrections as well as on the position verifications. The actual planning target volume margins of 3mm (adults) and 2mm (children) were evaluated by applying the van Herk formula on the intrafraction displacement results and performing treatment plan robustness simulations of twelve different translational offset scenarios including a HU uncertainty of 3.5%. Statistically significant differences between the immobilisation devices were found, but they turned out to be clinically irrelevant. The margin calculation for adult head patients resulted in 0.8mm (lateral), 1.2mm (cranio-caudal) and 0.6mm (anterior-posterior), and for paediatric head patients under anaesthesia in 0.8mm (lateral), 0.5mm (cranio-caudal) and 0.9mm (anterior-posterior). Based on these values, robustness evaluations of selected adult head patients and sedated children showed the validity of the currently used PTV margins.