Exactrac Cranial Verification Phantom (Radiology Support Devices Inc., Long Beach, CA) with three 5mm /0.2 inch diameter tungsten carbide spheres was used in the investigation. It was made tissue equivalent material embedded with natural human skeleton, which provided adequate bony definitions for matching. The cranial phantom was scanned by the computed tomography-simulator unit (Brilliance CT, Philips, Cleveland, USA) in supine position following the local departmental scanning protocols of the 2 regions, respectively. A dummy plan was created for each phantom by the Eclipse treatment planning system (Version 15.6; Varian Medical Systems, Palo Alto, USA), and DRRs of the required projections centered at the isocenter were generated. The CT images of the phantom were transferred from Varian Eclipse (Version 15.6; Varian Medical Systems, Palo Alto, USA) work station to ExacTrac system and which were used as reference images in the image registrations. In the treatment unit, the phantom was immobilized with MedTech frame on the treatment couch. Marks at the reference position, which included the anterior midline, the reference principal plane, and 2 lateral horizontal levels, were drawn on the phantoms with the help of the laser system. The cranial phantom was immobilized in the same state for both the verification image using 6D ExacTrac online and offline. In addition, after radiation treatment, registration between the computed tomography simulation images and the ExacTrac images was performed with offline 6D fusion in an offline review. The phantoms were then moved to the assigned deviations with known magnitudes from the reference marks in 3 directions, namely, the lateral translation (x-direction: ± 1, ± 2, ± 3, ± 5 and ± 10 mm), the longitudinal translation (y-direction: ± 1, ± 2, ± 3, ± 5 and ± 10 mm) and the vertical translation (z-direction: ± 1, ± 2, ± 3, ± 5 and ± 10 mm) and marks were made at each deviation accordingly (Figs. 2). After the phantom was moved to isocenter with the ExacTrac 6D couch, 2 orthogonal ExacTrac X-ray images were taken. The phantom was then shifted using the 6D couch according to the image registration results. The study was conducted for each of the 40 isocenters.
A total of 2320 daily ExacTracs of 225 patients were analyzed. Of the total 225 patients, 75 were head and neck cancer patients, 75 were only neck cancer patients and the other 75 were the cranial cancer patients. All patients were fixed in treatment position with MedTech frame and individualized thermoplastic facial masks. All underwent virtual CT simulation with 3.0 mm slices following the local departmental scanning protocols of the 2 regions, respectively. Magnetic resonance imaging (MRI) scans were also obtained accordingly. The CT and MRI fusion images were transferred to Varian Eclipse (Version 15.6; Varian Medical Systems, Palo Alto, USA) work station to create IMRT or ARC plans. Written informed consent was obtained from all patients and the study was approved by the Medical Ethics Committee of Xinqiao Hospital, Army Military Medical University. In addition, the reporting in the research follows the recommendations in the ARRIVE guidelines. For 75 cranial cancer patients, the VOI was designed for the whole CT range or head only. For 75 neck cancer patients, the VOI was designed for the whole CT range or neck only. For 75 head&neck cancer patients, the VOI was designed for the whole CT range or head only.
In both phantom and patient studies, 6D offline image registrations with different VOIs were performed and residual errors in the 3 translational directions (vertical, longitudinal, and lateral) and in the 3 rotational directions (rotation, pitch, and roll) were evaluated.
Calibration of IGRT System
A routine Winston-Lutz test is performed to verify radiation-laser isocenter coincidence at first. Before daily use, the ExacTrac IR system is calibrated using the room lasers to define the IR isocenter. The infrared camera calibration is accomplished according to ExacTrac Clinical Users Guide version 5.0 using the infrared calibration grid, which is a frame comprised of 25 infrared markers at known relative locations from one another. Then, the position of isocenter in the ExacTrac IR system is registered using the isocenter phantom. Finally, the X-ray calibration phantom is aligned to IR isocenter based on the calibrated IR coordinate system. Daily calibration removes the system’s sensitivity to small shifts in position of the ceiling mounted, flat panel detectors or IR cameras.
In this study, all shifts indicated in the imageguided ExacTrac systems were considered as displacements between the planned treatment isocenter in the phantom or patients and the radiation isocenter of the linear accelerator. The setup errors of the 40 sets of phantom and 225 patients were analyzed using SPSS 19.0 (SPSS Inc., Chicago, IL, USA) and Origin 7.0 (Origin Lab Corparation, Northampton, MA01060 USA) software. The root mean square (RMS) and standard deviation (SD) of the residual setup errors in the LR, SI, and AP translational directions, and rotational variations: pitch, roll, and yaw were calculated.
Images acquisition and registration
At our institution, all treatments of patients are performed using the Trilogy linear accelerator system (Varian Medical Systems, Inc., Palo Alto, USA) used in the study, which was equipped with the ExacTrac system from Brainlab (BrainLABAG, Feldkirchen, Germany). ExacTrac system mainly consists of (1) an infrared (IR)based optical positioning system which is used for initial patient setup and precise control of couch movement with a robotic couch, (2) two floormounted kV Xray tubes which projects medial, anterior, and inferior obliquely into two corresponding flat panel detectors mounted on the ceiling, (3) a radiographic kV Xray imaging system (Xray 6D) for position verification and readjustment based on the internal anatomy or implanted fiducial markers. The X-ray tubes have variable energy (40 kV-150 kV), current (10 mA-320 mA), and time (2 ms-6300 ms) settings for a range of contrast and brightness. The flat panel detectors are 512 x 512 pixels with an active area of 20 cm x 20 cm, which provides a field of view of view of ≈ 13 cm x 13 cm at isocenter with an image pixel size of 0.4 mm◊0.4 mm.