Inter‐observer variability in library plan selection on iterative CBCT and synthetic CT images of cervical cancer patients

INTRODUCTION: In the Library‐of‐Plans (LoP) approach, correct plan selection is essential for delivering radiotherapy treatment accurately. However, poor image quality of the cone‐beam computed tomography (CBCT) may introduce inter‐observer variability and thereby hamper accurate plan selection. In this study, we investigated whether new techniques to improve the CBCT image quality and improve consistency in plan selection, affects the accuracy of LoP selection in cervical cancer patients. MATERIALS AND METHODS: CBCT images of 12 patients were used to investigate the inter‐observer variability of plan selection based on different CBCT image types. Six observers were asked to individually select a plan based on clinical X‐ray Volumetric Imaging (XVI) CBCT, iterative reconstructed CBCT (iCBCT) and synthetic CTs (sCT). Selections were performed before and after a consensus meeting with the entire group, in which guidelines were created. A scoring by all observers on the image quality and plan selection procedure was also included. For plan selection, Fleiss' kappa (κ) statistical test was used to determine the inter‐observer variability within one image type. RESULTS: The agreement between observers was significantly higher on sCT compared to CBCT. The consensus meeting improved the duration and inter‐observer variability. In this manuscript, the guidelines attributed the overall results in the plan selection. Before the meeting, the gold standard was selected in 76% of the cases on XVI CBCT, 74% on iCBCT, and 76% on sCT. After the meeting, the gold standard was selected in 83% of the cases on XVI CBCT, 81% on iCBCT, and 90% on sCT. CONCLUSION: The use of sCTs can increase the agreement of plan selection among observers and the gold standard was indicated to be selected more often. It is important that clear guidelines for plan selection are implemented in order to benefit from the increased image quality, accurate selection, and decrease inter‐observer variability.