Observational study of cone beam computed tomography based interfractional urinary bladder filling variation during image guided radiation therapy in pelvic malignancies

Background: Organ motion is an important factor that limits the precision of radiation treatment. Bladder filling variation has significant impact on the position of target volumes in pelvic malignancies. Aims and Objective: This study was an effort to maintain a consistent urinary bladder volume after following a bladder protocol, which was then analyzed by in-room cone-beam computed tomography (CBCT) imaging. Material and Methods: A total number of 26 patients/300 scans, i.e. 26 planning scan and 274 CBCT were analyzed. The bladder volumes and bladder wall dimension were analyzed comprehensively, thus adding considerable understanding to the bladder wall motions. Result: The mean bladder volume for all 26 patients was 183.07 cc with standard deviation of 90.43 cc. The mean +/- standard deviation of transverse, anteroposterior and longitudinal diameter was 8.35+/- 1.03, 6.69+/-1.05 and 5.59+/-1.79 cm. Conclusion: This study showed considerable reduction of margin could be done on the lateral side, as there is less displacement on transverse diameter and more liberal margins should be considered in anteroposterior dimension and longitudinal dimensions. This study has reached the conclusion that bladder-filling protocol is necessary to maintain the consistent bladder volume, but it is too preliminary to state that it will translate into reduction of margin.