Using Cone-Beam Computed Tomography to Evaluate the Impact of Bladder Filling Status on Target Position in Prostate Radiotherapy

Purpose: : To assess bladder filling status and its impact on target position during daily intensity-modulated radiation therapy (IMRT) using cone-beam computed tomography (CBCT) in prostate cancer patients. Patients and Methods: : 23 patients with prostate cancer undergoing image-guided IMRT (78 Gy in 39 fractions) were included. On-board CBCT images were acquired daily and an endorectal balloon was placed daily. All patients were instructed to have a full bladder. The interfraction changes in bladder dimensions in left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions were measured from CBCT images. Distances from the uppermost part of prostate to pubic bone (PP) and from the uppermost part of prostate to treatment isocenter (PI) were measured to determine changes in target position. Standard deviation (SD) in all fractions of each patient was used to compare the variations between patients. Bladder dimension change ratio and Z-score were used to normalize data between patients. Results: : A total of 867 CBCT images were evaluated. The average LR, AP, and SI bladder dimensions were 7.8 ± 1.5 cm, 6.7 ± 1.4 cm, and 5.6 ± 1.7 cm, respectively. The average LR, AP, and SI bladder dimension change ratios were 0.88 ± 0.17, 0.90 ± 0.15, and 0.86 ± 0.32, respectively. The SD was significantly greater in SI dimension than in LR (p < 0.001) and AP (p < 0.001) dimensions. The interfraction changes in the three bladder dimensions were significantly larger than those of target position, and did not correlate with target position changes. Conclusion: : Though they were not negligible, changes in bladder filling status did not have a significant impact on target position.