Cone beam CT imaging analysis of interfractional variations in bladder volume and position during radiotherapy for bladder cancer

Abstract

Purpose: To quantify daily bladder size and position variations during bladder cancer radiotherapy.

Methods and materials: 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.

Results: 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).

Conclusions: Significant variations in bladder and PTV volume and position occurred in patients in this trial.