Impact of changes in bladder and rectal filling volume on organ motion and dose distribution of the bladder in radiotherapy for urinary bladder cancer

Purpose: To determine the impact of filling volume changes of the urinary bladder and rectum on organ motion and dose distribution of the bladder and rectum during radical radiotherapy for bladder cancer and to calculate the internal margins to secure target coverage. Methods and Materials: In 15 patients with muscle-invasive bladder cancer, a planning CT scan was performed with a bladder and rectal catheter, followed by three immediate CT scans with various filling of the urinary bladder and rectum. After 20 fractions, a fifth CT scan, without catherization, was performed. In each CT study, the bladder and rectum volumes were delineated and matched to the planning CT scan to measure the organ motion and calculate internal margins. These margins were compared with an isotropic standard margin of 2 cm. Dose-volume histograms were analyzed to describe the dose distribution in the bladder and rectum corresponding to various filling volumes. Results: Bladder movement was most pronounced in the anterior and cranial directions. The internal margins required to cover the bladder movements due to filling of the bladder and rectum in 87% of the patients were 2.4 cm in the anterior, 1.1 cm in the posterior, 3.5 cm in the cranial, 0.5 cm in the caudal, and 1.3 cm in the lateral direction. Conclusion: The filling volumes of the bladder and rectum have a large impact on bladder movements, especially in the anterior and cranial directions. This should be included in the internal target volume with the introduction of anisotropic margins in conformal radiotherapy for bladder cancer. (C) 2004 Elsevier Inc.