Purpose: To estimate bladder movements and changes in dose distribution in the bladder and surrounding tissues associated with changes in bladder filling and to estimate the internal treatment margins.
Methods and materials: A total of 16 patients with bladder cancer underwent planning computed tomography scans with 80- and 150-mL bladder volumes. The bladder displacements associated with the change in volume were measured. Each patient had treatment plans constructed for a "partially empty" (80 mL) and a "partially full" (150 mL) bladder. An additional plan was constructed for tumor irradiation alone. A subsequent 9 patients underwent sequential weekly computed tomography scanning during radiotherapy to verify the bladder movements and estimate the internal margins.
Results: Bladder movements were mainly observed cranially, and the estimated internal margins were nonuniform and largest (>2 cm) anteriorly and cranially. The dose distribution in the bladder worsened if the bladder increased in volume: 70% of patients (11 of 16) would have had bladder underdosed to <95% of the prescribed dose. The dose distribution in the rectum and intestines was better with a "partially empty" bladder (volume that received >70%, 80%, and 90% of the prescribed dose was 23%, 20%, and 15% for the rectum and 162, 144, 123 cm(3) for the intestines, respectively) than with a "partially full" bladder (volume that received >70%, 80%, and 90% of the prescribed dose was 28%, 24%, and 18% for the rectum and 180, 158, 136 cm(3) for the intestines, respectively). The change in bladder filling during RT was significant for the dose distribution in the intestines. Tumor irradiation alone was significantly better than whole bladder irradiation in terms of organ sparing.
Conclusion: The displacements of the bladder due to volume changes were mainly related to the upper wall. The internal margins should be nonuniform, with the largest margins cranially and anteriorly. The changes in bladder filling during RT could influence the dose distribution in the bladder and intestines. The dose distribution in the rectum and bowel was slightly better with a "partially empty" than with a "full" bladder.