Cone Beam CT Imaging Analysis of Interfractional Variations in Bladder Volume and Position During Radiotherapy for Bladder Cancer

Abstract

To quantify daily bladder size and position variations during bladder cancer radiotherapy. Ten bladder cancer patients underwent daily cone beam CT (CBCT) imaging of the bladder during radiotherapy. Bladder and planning target volumes (bladder/PTV) from CBCT and planning CT scans were compared with respect to bladder center-of-mass shifts in the x (lateral), y (anterior-posterior), and z (superior-inferior) coordinates, bladder/PTV size, bladder/PTV margin positions, overlapping areas, and mutually exclusive regions. A total of 262 CBCT images were obtained from 10 bladder cancer patients. Bladder center of mass shifted most in the y coordinate (mean, -0.32 cm). The anterior bladder wall shifted the most (mean, -0.58 cm). Mean ratios of CBCT-derived bladder and PTV volumes to planning CT-derived counterparts were 0.83 and 0.88. The mean CBCT-derived bladder volume (+/- standard deviation [SD]) outside the planning CT counterpart was 29.24 cm(3) (SD, 29.71 cm(3)). The mean planning CT-derived bladder volume outside the CBCT counterpart was 47.74 cm(3) (SD, 21.64 cm(3)). The mean CBCT PTV outside the planning CT-derived PTV was 47.35 cm(3) (SD, 36.51 cm(3)). The mean planning CT-derived PTV outside the CBCT-derived PTV was 93.16 cm(3) (SD, 50.21). The mean CBCT-derived bladder volume outside the planning PTV was 2.41 cm(3) (SD, 3.97 cm(3)). CBCT bladder/ PTV volumes significantly differed from planning CT counterparts (p = 0.047). Significant variations in bladder and PTV volume and position occurred in patients in this trial.