Volumetric imaging in image guided radiotherapy of the post prostatectomy surgical bed

Full text: Determine whether, patients scanned with ultrasound prior to treatment, reduces the number of times bladder length (determined on eBeT) is outside the tolerance level of 2 cm (compared to simulation bladder length) when compared with the number of violations in our previous study. We report on phase I of the project, the establishment of patient specific bladder volume limits. The bladder volume in 26 patients was measured with an ultrasound scanner prior to simulation. This volume was compared with the simulation CT cranio-caudal bladder length. Using this relationship, patient specific, pre-treatment bladder volume limits can be defined and the effectiveness of the protocol determined through measurement of bladder length from CBCT images obtained prior to treatment. Results Prior to introducing the bladder scanner, CBCT determined bladder lengths were within pre-defined limits in 22 and 77% of patients by treatment fraction 5 and end of treatment, respectively. The estimated Pearson's correlation between CT bladder length at simulation and ultrasound volume was 0.73, and the corresponding 95% confidence interval is [0.45, 0.88]. Using this relationship, it was expected that pre-treatment patient specific bladder volume limits could be defined. In practice, the model required some refinement and a pragmatic approach to set practical limits. Offline volumetric imaging can be used to minimise systematic errors in treatment delivery in the post-prostatectomy patient. A simple hand-held ultrasound bladder scanner has the potential to provide a practical, on-line solution to confirming bladder volumes are within acceptable, patient-specific limits prior to treatment.