Room: Exhibit Hall | Forum 9
Purpose: To evaluate the influence of initial patient setup on image-guided setup accuracy on a six degrees of freedom (6D) couch as evaluated by the magnitude of the discrepancy between imaging and radiation isocenters in Winston-Lutz (WL) measurements.
Methods: Varianâ€™s End-to-end Accuracy Phantom with a tungsten-BB insert was aligned with in-room lasers and setup via CBCT. This initial couch position was then offset by â€œsmallâ€? (1cm) or â€œlargeâ€? (2cm) shifts in each (lateral, vertical, longitudinal) or all directions to represent clinically realistic patient setup errors. Correct positioning was determined by auto-registering the CBCT and planning-CT. After image alignment and shifting the couch accordingly, EPID images were acquired for WL-tests to determine the agreement between radiation and imaging isocenters based on radiation field and tungsten-BB centroids. Repeated realignment of the CBCT and planning-CT with the couch at initial position verified image-guided setup and EPID imaging reproducibility. Three-dimensional offsets were calculated from 8 image acquisitions for WL-analysis in PIPSpro for each of the 30 reproducibility and 30 shift measurements. Independent samples t-test compared offsets while Leveneâ€™s test compared variability between small and large shifts.
Results: Reproducibility measurements had a mean offset of 0.25Â±0.09mm. Small and large shifts had significantly different (p=0.001) offsets, with a mean of 0.30Â±0.12mm and 0.55Â±0.23mm. Not only did small shifts result in a smaller offset, but also showed significantly less variability (F=18.562, p<0.000). Comparison of initial and post-CBCT positioning found the couch to be misaligned by as much as 0.9mm in a single direction for large shifts but within 0.3mm for small shifts.
Conclusion: Discrepancies in imaging and radiation isocenter alignment increased with increasing 6D couch shifts. These findings highlight the importance of initial patient positioning accuracy prior to image-guidance due to potential direct influence on the congruence of imaging and radiation isocenters, critical for treatment.
Funding Support, Disclosures, and Conflict of Interest: This research was supported by a Varian Research Grant and the AAPM Graduate Fellowship.