Effect of bladder filling protocols on bladder volume variation in the age of adaptive radiotherapy

Abstract

Purpose or Objective Drinking protocols are increasingly used in pelvic radiotherapy to optimise bladder volume reproducibility. However, there is no data to show their efficacy. This study assesses the effect of a standardised bladder filling protocol on bladder volumes during each fraction and across the radiotherapy schedule. Materials and Methods Twenty patients undergoing radiotherapy for cancers of the cervix or bladder at a tertiary centre were included. All patients were instructed to void prior to drinking 300mls of water. They were imaged using 1.5T Siemens Aera MRI scanner after 30min. Two T2 sequences were taken 20min apart. This process was repeated weekly throughout treatment. Bladders were contoured at both time points on Raystation 7R by a single oncologist. Two patients were excluded due to poor image quality. Factors affecting bladder filling were investigated in models using multiple measures analysis of variation and mixed linear regression. Results Complete data was available in 8 bladder and 10 cervical cancer patients. Mean bladder volume was 143ml (95% CI, 137-150ml), and the volume range was large (35-645ml). There was no significant difference in bladder volume between bladder cancer and cervical cancer patients (median volume 120ml vs 90ml; p=0.2). When adjusting for intra-patient variability using the mixed linear model, there was no trend with tumour site or at any measured time point (Figure 1). However, bladder volume in the whole cohort was significantly higher in the secConclusion Given that the bladder collects urine inflow, the significant increase in bladder volume between scan 1 and 2 each week was expected. Our data demonstrates significant variation between patients and across time points despite a standardised bladder filling protocol. Whilst it seems that for some patients the filling protocol produced consistent bladder volumes, we could not identify any variables allowing us to predict for this. We acknowledge that the sample size was low, but we believe this study adds weight to the adoption of adaptive radiotherapy with a “plan of the day” rather than relying on a drinking protocol to achieve an optimal target coverage throughout treatment.ond scan each week (122ml vs 108ml, p<0.001). The coefficient of variability (CV) for intra-patient variability was 0.47 (95% CI, 0.32-0.62) while that of inter-patient variability was 0.41 (95% CI, 0.28- 0.54).