Exhibit Hall | Forum 2
Purpose: Online CBCT-Guided adaptive radiotherapy (ART) plans on the Varian Ethos platform use daily synthetic CTs automatically generated using a B-spline deformable registration algorithm. This class of algorithms cannot add or remove voxels, and therefore may have poor performance at reproducing variable volumes of gas present on a daily basis. We have analyzed the air pocket volume discrepancy between the daily CBCTs and synthetic images and explored its dosimetric effect on ART plan calculation.
Methods: Abdominopelvic air volume was contoured on both the daily CBCT images and the corresponding synthetic images for online adaptive pelvic treatments with Varian Ethos. Total air, bowel air, and rectal air volume differences were tracked over all fractions. To assess the dosimetric impact of air volume difference, the fraction with the largest variation was recalculated using the CBCT-corrected contours and appropriate density overrides. Gamma criteria of 1%/2mm restricted to the PTV were used to compare synthetic CT-based dose calculation to that with CBCT-corrected air pockets.
Results: Mean daily abdominopelvic air volume difference between CBCTs and corresponding synthetic images was 28.80% ± 72.98% (Min 3.63%, Max 181.02%). Air in the bowel had the largest volume variation (>200cc difference). Rectal air had large relative variations (mean difference -71.0% ± 32.8%), but the absolute volume of air was small (mean difference 18.8cc). For the fraction showing 181% difference in air volume, the gamma passing rate (1%/2mm) between synthetic CT-based dose calculation and that with CBCT-corrected air pockets was 93.22%. The maximum calculated dose difference was -1.5% of the prescription dose.
Conclusion: Current online CBCT-based ART workflow can lead to large differences in the volume of air present during planning. Despite the large observed air volume differences, the dosimetric effect of such differences on the adaptive plan dose calculation accuracy was limited.