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Ultrasound Imaging to Assess Inter- and Intra-fraction Motion during Bladder Radiotherapy and its Potential as a Verification Tool.AIMS: Organ motion is the principle source of error in bladder cancer radiotherapy. The aim of this study was to evaluate ultrasound bladder volume measurement as a surrogate measure of organ motion during radiotherapy: (1) to assess inter- and intra-fraction bladder variation and (2) as a potential treatment verification tool. MATERIALS AND METHODS: Twenty patients receiving radical radiotherapy for bladder cancer underwent post-void ultrasound bladder volume measurement at the time of radiotherapy treatment planning (RTP), and immediately before (post-void) and after receiving daily fractions. RESULTS: Ultrasound bladder volume measurement was found to be a simple and acceptable method to estimate relative bladder volume changes. Six patients showed significant changes to post-void bladder volume over the treatment course (P<0.05). The mean inter-fraction post-void bladder volume of five patients exceeded their RTP ultrasound bladder volume by more than 50%. Intra-fraction bladder volume increased on 275/308 (89%) assessed fractions, with the mean intra-fraction volume increases of seven patients exceeding their RTP ultrasound bladder volume by more than 50%. CONCLUSIONS: Both day-to-day bladder volume variation and bladder filling during treatment should be considered in RTP and delivery. Ultrasound may provide a practical daily verification tool by: supporting volume limitation as a method of treatment margin reduction; allowing detection of patients who may require interventions to promote bladder reproducibility; and identifying patients with prominent volume changes for the selective application of more advanced adaptive/image-guided radiotherapy techniques.