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Initial Patient Setup Influences Image-Guided Setup Accuracy

R Culcasi1*, G Baran2 , M Snyder1,3 , (1) Wayne State University School of Medicine, Detroit, MI, (2) Karmanos Cancer Institute, Detroit, MI, (3) Beaumont Health System, Royal Oak, MI


(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.


Patient Positioning, Image Guidance, Setup Errors


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