The role of functional MRI to characterise tumour boost delineation in patients undergoing radical radiotherapy for muscle invasive bladder cancer (MIBC)
Session type: Poster / e-Poster / Silent Theatre session
MIBC exhibits dose response relationship to radiotherapy with improved local control and overall survival seen at higher doses. Dose escalation to the tumour alone is currently under investigation with the aim of minimising normal tissue irradiation and improving outcome. This approach is dependent on a number of factors including accurately defining volumes for radiotherapy planning. Diffusion-weighted MRI (DW-MRI) is a functional imaging technique that quantifies water molecule motion within tissue. Tumours restrict diffusion and demonstrate low apparent diffusion coefficient (ADC). We report on the potential use of DW-MRI to aid radiotherapy planning.
18 patients with confirmed MIBC suitable for radical radiotherapy within a dose escalation protocol were recruited prospectively. A radiotherapy planning CT was acquired for treatment. The whole bladder (CTV) and tumour boost volume (GTV) was delineated using anatomical information (CT/MRI, cystoscopy and bladder map were available). Prior to radiotherapy, DW-MRI was performed on a 1.5T system using b values 0, 50,100, 250, 500 and 750s/mm2. Images were retrospectively analysed off-line using in-house IDL-based software. GTV was drawn by a second clinician on the 750 s/mm2 images and transferred onto the corresponding ADC map to record the mean values. Volumes were compared using Wilcoxon signed rank test.
All tumours seen on T2 weighted images were identified on DW-images (b 750s/mm2). In 3 patients no tumour was seen on T2 images or DW-image. The GTV median ADC value was 1.4 x10-3mm2/s (range 0.9-2.3 mm2/s). Mean conventional GTV was 31.0 cm3 (range 6.7-78.7cm3). Mean DW-MRI GTV was 16.1cm3 (range 0-35.3cm3). There was significant reduction in GTV using DW-MRI (p=0.002).
Acquiring DW-MRI for radiotherapy planning may provide additional biological information, complementing anatomical imaging to aid target volume delineation and in the future define a biological target volume for future dose escalation trials.