Contour and dosimetric evaluation of an in-room mobile CBCT scanner for cylinder based brachytherapy

In-room cone-beam CT (CBCT) is an emerging imaging solution for brachytherapy. This study evaluates organ-at-risk (OAR) contour and dosimetric differences between CT- and CBCT-based treatment plans for cylinder-based brachytherapy.

Ten patients who received high-dose-rate (HDR) cylinder-based brachytherapy were retrospectively analyzed. Standard pelvic CT images were used for treatment planning, and in-room CBCT images were acquired prior to the treatment to verify cylinder placement. The CT-based plans were replanned on their corresponding CBCT images. OARs were independently contoured on both CT and CBCT and compared using Dice similarity coefficient (DSC), and Hausdorff distance (HD). Dosimetrically, D₂cc were compared between CT and CBCT.

Among the ten patients, CBCT provided reliable images for HDR cylinder treatment planning in terms of applicator digitalization and OAR delineation. Considerable variabilities between CT and CBCT for OARs were observed. Median DSC values were 0.73, 0.78, 0.74, and 0.48 and median HD values were 15.5 mm, 12.2 mm, 16.0 mm, and 22.5 mm for the bladder, rectum, small bowel, and sigmoid, respectively. Median values of D₂cc differences between the CT-based plans and the CBCT-based plans were 3.4 cGy, 25.1 cGy, 26.2 cGy, and 58.3 cGy for the bladder, rectum, small bowel, and sigmoid, respectively.

In-room mobile CBCT provides a practical and efficient solution for vaginal cylinder HDR brachytherapy. The image quality was sufficient for accurate delineation of the applicator and OARs, helping streamline the workflow. Although inter-fractional variations in OAR position and volume were observed, these changes remained within clinically acceptable limits.