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. 2023 Oct;24(10):e14057.
doi: 10.1002/acm2.14057. Epub 2023 Jun 5.

Influence of air mapping errors on the dosimetric accuracy of prostate CBCT-guided online adaptive radiation therapy

Olga M Dona Lemus  1 , Sean Tanny  1 , Michael Cummings  1 , Matthew Webster  1 , Joshua Wancura  1 , Hyunuk Jung  1 , Yuwei Zhou  1 , Jihyung Yoon  1 , Matthew Pacella  1 , Dandan Zheng  1
Affiliations

Influence of air mapping errors on the dosimetric accuracy of prostate CBCT-guided online adaptive radiation therapy

Olga M Dona Lemus et al. J Appl Clin Med Phys. 2023 Oct.

Abstract

Purpose: CBCT-guided online adaptive radiotherapy (oART) plans presently utilize daily synthetic CTs (sCT) that are automatically generated using deformable registration algorithms. These algorithms may have poor performance at reproducing variable volumes of gas present during treatment. Therefore, we have analyzed the air mapping error between the daily CBCTs and the corresponding sCT and explored its dosimetric effect on oART plan calculation.

Methods: Abdominopelvic air volume was contoured on both the daily CBCT images and the corresponding synthetic images for 207 online adaptive pelvic treatments. Air mapping errors were tracked over all fractions. For two case studies representing worst case scenarios, dosimetric effects of air mapping errors were corrected in the sCT images using the daily CBCT air contours, then recalculating dose. Dose volume histogram statistics and 3D gamma passing rates were used to compare the original and air-corrected sCT-based dose calculations.

Results: All analyzed patients showed observable air pocket contour differences between the sCT and the CBCT images. The largest air volume difference observed in daily CBCT images for a given patient was 276.3 cc, a difference of more than 386% compared to the sCT. For the two case studies, the largest observed change in DVH metrics was a 2.6% reduction in minimum PTV dose, with all other metrics varying by less than 1.5%. 3D gamma passing rates using 1%/1 mm criteria were above 90% when comparing the uncorrected and corrected dose distributions.

Conclusion: Current CBCT-based oART workflow can lead to inaccuracies in the mapping of abdominopelvic air pockets from daily CBCT to the sCT images used for the optimization and calculation of the adaptive plan. Despite the large observed mapping errors, the dosimetric effects of such differences on the accuracy of the adapted plan dose calculation are unlikely to cause differences greater than 3% for prostate treatments.

Keywords: adaptive therapy; air mapping errors; prostate cancer; synthetic CT.

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Conflict of interest statement

The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent‐licensing arrangements), or non‐financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Figures

FIGURE 1
FIGURE 1
(a) CBCT image. (b) Registered sCT image. (c) Corrected sCT. The sCT air volume (purple) would be overridden with HU = 0 and the CBCT air (magenta) would be overridden with HU = −700 in the corrected sCT. Sagittal images from P3‐FX21.
FIGURE 2
FIGURE 2
Air volume tracking during treatment of the studied patients P1‐P6. Dark Grey plots show the air volume in the daily CBCT image while light grey plots show the air volume of the daily sCT image. The dotted blue line indicates the ending of phase 1.
FIGURE 3
FIGURE 3
Air volume differences [cc] (air mapping error) between the daily CBCT and respective sCT for each patient (P1 ‐ P6). In the plots, the black horizontal line inside the box plot represents the median and the red dot represents the mean. The box in the center is the interquartile range (−25% to 25%), the black vertical line depicts the rest of the distribution, and the black dots represent the outliers. The shape of the violin illustrates the distribution probability.
FIGURE 4
FIGURE 4
CBCT and respective sCT images for P3‐FX21 showing the differences in abdominopelvic air cavities. (a) sCT axial. (b) sCT sagittal. (c) CBCT axial. (d) CBCT sagittal. PTV4860 (magenta), PTV5400 (cyan), and PTV7800 (red) were overlaid on both images to highlight the proximity and overlap with the uncorrected air volume.
FIGURE 5
FIGURE 5
CBCT and respective sCT images for P1‐FX37 showing the differences in abdominopelvic air cavities. (a) sCT axial. (b) sCT sagittal. (c) CBCT axial. (d) CBCT sagittal. PTV6840 (magenta) and PTV4500 (red) were overlaid on both images to show the proximity and overlap with the uncorrected air volume.
FIGURE 6
FIGURE 6
DVH and gamma overlay between uncorrected and corrected air volume calculation for a single fraction rescaled to full treatment dose. (a) DVH differences for PTV4860 (magenta), PTV5400 (cyan), and PTV7800 (red) for P3‐FX21. (b) DVH differences for PTV6840 (magenta) and PTV4500 (red) for P1‐FX37. (c) and (d) show respective gamma index overlay on the sCT. Gamma upper bound was set at 2 to simplify the analysis and a threshold was applied to the plot to show gamma 0.5.
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