Comparison of the verification performance and radiation dose between ExacTrac x-ray system and On-Board Imager-A phantom study

Recently, On-Board Imager (OBI) and ExacTrac x-ray 6 degree-of-freedom system (ExacTrac) are increasingly used verification systems in local radiotherapy centers. This study aimed to compare the differences between these two systems in terms of verification accuracy, organ doses, and verification time for head-and-neck (H&N) and pelvic cases. Rando anthropomorphic phantoms of H&N and pelvic regions were positioned with known set-up deviations from the reference position in the linear accelerator. x-Ray verification images were then acquired using both systems. Verification accuracy was evaluated based on the residual positioning error (δD) after image registration. Thermoluminescence dose meters (TLD-100s) were placed in specific locations of the phantoms for the measurement of imaging doses at the organs of interest. Besides, the verification time was also recorded for comparison. Most average detection errors for both systems were within 1 mm. The detection error of ExacTrac was significantly larger than OBI in the H&N region in all directions (p < 0.05), but was significantly lower in the pelvis (p < 0.05). The mean imaging doses to all organs of interest from ExacTrac were significantly lower than OBI (p < 0.05). The mean verification time for ExacTrac was about 10 seconds, which was significantly shorter than the 100 seconds in OBI (p < 0.001). Both verification systems achieved satisfactory performance in the H&N and pelvic regions despite ExacTrac being better in terms of verification time and organ dose. The verification accuracy of Exactrac was better in pelvic region than the H&N region when compared with OBI.