External beam radiotherapy of bladder carcinoma: considerations in determining the irradiation field margins

Purpose: Motions of the bladder and rectum during pelvic irradiation are considered as major causes of geometrical uncertainties. As a result, the volume status of these organs is changed and the definition of the treatment margins is imperative. The aim of this study was, firstly, to determine these margins, comparing series of CT scans, performed at simulation time, with empty (EB) and full bladder (17B) and, secondly, to evaluate the dose volume histograms (DVHs) of tumor and rectum using standard treatment margins. Methods: Fifteen patients with muscle-invading urinary bladder carcinoma underwent two scan series with EB and FB bladder during radiotherapy (R T) simulation. Gross tumor volume (GTV) clinical target volume (CTV), planning treatment volume (PTV) and organs at risk (OAR) were contoured. Displacements of the bladder wall were determined at all directions. Cumulative DVHs were generated for the volumes of interest. Using the same beam arrangements for both the EB and FB CT series, DVHs were also produced. Results: The mean bladder volume was 119.3+/-55.9 cm(3) and 264.3+/-145.7 cm3 for EB and FB CT series, respectively (p <0.001). The maximum bladder wall displacement was observed at cranial direction (2.2+/-0.6 cm for the EB vs. 3.4+/-1.0 cm for the FB series; p <0.001) and at caudal direction (2.3+/-0.6 cm for the EB vs. 3.6+/-1.0 cm for the FB series; p <0.001). Standard anisotropic margins of 2 cm in cranio-caudal and posterior-anterior directions and 1.2 cm in lateral direction gave coverage to 75% of all bladder movements caused by FB. Analysis of DVHs and tumor control probability (TCP) calculations gave same results (74%), while normal tissue complication probability (NTCP) of the rectum showed no significant changes. Conclusion: CT scans series with empty and full bladder, performed at simulation time, could offer a potential advantage to evaluate the target expansion necessary to cover the bladder wall for each patient, giving more information about safe margining.