ESTRO 2023 - Abstract Book

S1614

Digital Posters

ESTRO 2023

vertebra 4 and lower, positioned using only a simple head rest with arm/knee supports. Prior to treatment, for each gantry angle, a digitally reconstructed radiograph, the template, is generated of the vertebra within the PTV. During 10MV FFF VMAT delivery, kV fluoroscopic images are continuously acquired (7/second) and online registered with the nearest template (on a separate computer with no control over the LINAC), resulting in 2D positions of the tumor. Each 2D registration is triangulated with multiple 2D positions from at least 14° earlier, resulting in a 3D target position. If RTR showed a deviation of >1mm in lateral, longitudinal or vertical direction, treatment was interrupted and a new CBCT-based couch correction was performed. Results We analyzed 22 patients. Median age 62 years, 55% were male. Average time between start 1st imaging and end of 2nd (final) arc was 17.3 minutes (8.1-50.2). Figure 1 shows agreement in 3 translational directions between the mean RTR position derived from CBCT kV projections and the actual clinical shift performed using the same CBCT after treatment interruption. 97% of the data points agree to within 0.5mm and 90% within 0.3mm. Out of 220 arcs, MV delivery was interrupted 44 times (36/8 in 1st/2nd arcs respectively; average 35s after start of the arc; 25/44 for lateral deviation). 9 patients had ≥ 3 interventions and 8 no interventions. In 12/44 cases the CBCT after interruption showed translational shifts <1mm (mean 0.7mm, SD 0.2mm). 22 interruptions were during L-spine treatments (n=10) and 21 during T-spine (n=12). Absolute mean (SD) lateral/longitudinal/vertical CBCT-based shifts were 1.2(0.6)/0.5(0.5)/0.4(0.4)mm. Correlation coefficient between lateral CBCT shift and magnitude of earlier roll correction was 0.5.

Conclusion RTR and CBCT determined positions agree well. RTR spine tracking can be used to trigger CBCT acquisition and patient re positioning during spine SBRT. Most patients are very stable (within 1mm of intended position) for most of the time, despite no immobilization. Lateral movement is the most prominent and vigilance is required especially after prior roll correction and at the start of the arc.

PO-1871 Surface-guidance effectiveness in intracranial treatments with a closed immobilization mask

A. Gutierrez 1 , K. Van Santvoort 1 , T. Gevaert 1 , M. Boussaer 1 , T. Everaert 1 , M. De Ridder 2

1 UZ Brussel, Radiotherapy, Brussels, Belgium; 2 UZ Brussel, Radiotherapy, Brussel, Belgium

Purpose or Objective Surface guidance (SG) is used in combination with IGRT for patient positioning and monitoring during radiotherapy. In the case of intracranial treatments, especially stereotactic radiosurgery, the use of a closed immobilization mask is a common practice. These devices prevent patient motion but also hinder the task of tracking their external anatomy. The purpose of this study was to evaluate the effectiveness and the added value of surface tracking in detecting intra-fraction errors when immobilization devices are in place. Materials and Methods Patient positioning information was obtained with stereoscopic x-rays (ExacTrac Dynamic, Brainlab, Munchen, Germany) taken when the gantry angle allowed image acquisition, providing 6DOF position information by comparing the x-ray with a set of DRR generated from the planning CT. The SG system obtains 6DOF information by continuous comparison of a ROI on the patient’s surface with a reference acquired after performing IGRT. In total, 173 snapshot verifications using stereoscopic X-rays and ST corresponding to 50 fractions of treatments including brain FSRT and SRS were analyzed. Discrepancies between errors measured by both systems were computed for all directions and all rotations. Results Mean absolute differences between x-rays and SG across all verifications were 0.13 mm (SD 0.24), -0.18 mm (SD 0.22) and -0.12 mm (SD 0.16) for the lateral, longitudinal and vertical directions, and -0.10° (SD 0.16), -0.07° (SD 0.13) and 0.09° (SD 0.15) for pitch, roll and yaw respectively. Absolute differences stayed below 0.4mm for all directions in 90% of the snapshots. Paired t-tests showed some correlation between the variables for lateral and vertical directions but no statistical significance for paired t-test or Wilcoxon tests was found for the data analyzed.