Evaluating Inter-fractional Changes in Volume and Position during Bladder Radiotherapy and the Effect of Volume Limitation as a Method of Reducing the Internal Margin of the Planning Target Volume

Aims: To quantify the inter-fractional variation in bladder volume and position during a course of bladder radiotherapy, and to assess the feasibility of reducing the planning target volume (PTV) internal margin using an empty bladder protocol. Materials and methods: Weekly computed tomography scans were taken immediately after micturition on 15 patients undergoing radical radiotherapy for bladder cancer. Bladder volume and positional variation were compared by co-registration of the serial computed tomography scans with the initial planning scan and a single 'full' scan at the onset of treatment for each patient. A PTV was generated on the initial planning scan using both our departmental standard of 1.5 cm and a reduced 1 cm isotropic internal margin around the target (whole bladder) and the relative proportion of the bladder breaching the PTV using both margins compared. Results: The mean post void residual volume from the planning scan was 112 cm(3) (standard deviation 42 cm(3)). The mean weekly variation in bladder volume relative to the planning volume was 0-12% (standard deviation 20-34%) with no observable trends over time. No statistically significant differences were seen in the proportion of bladder breaching the 1.5 and 1 cm internal margin (P = 0.18). Regression analysis showed that it is possible to ensure complete coverage of the bladder with a 1 cm margin, providing the volume did not exceed over 50% of the initial planning scan volume. Conclusion: Using an empty bladder protocol and where on-line imaging is available it is feasible to reduce the internal margin of the PTV from 1.5 to 1cm, providing the volumes do not exceed > 50% of the planning scan volume. Mangar, S. A. et al. (2008). Clinical Oncology 20, 698-704 (c) 2008 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.