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(Tuesday, 7/31/2018) 1:15 PM - 1:45 PM
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: Varian’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 “small” (1cm) or “large” (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 Levene’s test compared variability between small and large shifts.
Results: Reproducibility measurements had a mean offset of 0.25±0.09mm. Small and large shifts had significantly different (p=0.001) offsets, with a mean of 0.30±0.12mm and 0.55±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.
Keywords
Patient Positioning, Image Guidance, Setup Errors
Taxonomy
TH- External beam- photons: General (most aspects)
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