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

Abstract

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.