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 1.5cm and a reduced 1cm 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 112cm(3) (standard deviation 42cm(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 1cm internal margin (P=0.18). Regression analysis showed that it is possible to ensure complete coverage of the bladder with a 1cm 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.
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 1.5cm and a reduced 1cm 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 112cm(3) (standard deviation 42cm(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 1cm internal margin (P=0.18). Regression analysis showed that it is possible to ensure complete coverage of the bladder with a 1cm 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.