Simple calculation using anatomical features on pre-treatment verification CT for bladder volume estimation during radiation therapy for rectal cancer

Abstract

Background: Despite detailed instruction for full bladder, patients are unable to maintain consistent bladder filling during a 5-week pelvic radiation therapy (RT) course. We investigated the best bladder volume estimation procedure for verifying consistent bladder volume. Methods: We reviewed 462 patients who underwent pelvic RT. Biofeedback using a bladder scanner was conducted before simulation and during treatment Exact bladder volume was calculated by bladder inner wall contour based on CT images (V-ctsim). Bladder volume was estimated either by bladder scanner (V-scan) or anatomical features from the presacral promontory to the bladder base and dome in the sagittal plane of CT (V-ratio). The feasibility of V-ratio was validated using daily megavoltage or kV cone-beam CT before treatment. Results: Mean V-ctsim, was 335.6 +/- 147.5 cc. Despite a positive correlation between V-ctsim and V-scan (R-2 = 0.278) and between V-ctsim and V-ratio (R-2 = 0.424), V-ratio yielded more consistent results than V-scan, with a mean percentage error of 26.3 (SD 19.6, p < 0.001). The correlation between V-ratio and V-ctsim was stronger than that between Van and V-ctsim (Z-score: - 7.782, p < 0.001). An accuracy of V(ratio )was consistent in megavoltage or kV cone-beam CT during treatment. In a representative case, we can dichotomize for clinical scenarios with or without bowel displacement, using a ratio of 0.8 resulting in significant changes in bowel volume exposed to low radiation doses. Conclusions: Bladder volume estimation using personalized anatomical features based on pre-treatment verification CT images was useful and more accurate than physician-dependent bladder scanners.