Comparison of DWI to T2 Bladder Target Volume Analysis for Treatment Adaptation in Muscle Invasive Bladder Cancer

Purpose We hypothesised changes in target volume (drawn on weekly Diffusion weighted imaging (DWI) and T2W MRI) and apparent diffusion coefficients (ADC) associates with cumulative delivered dose. Methodology Images and treatment planning information from 9 patients with muscle invasive bladder cancer, recruited from a single centre to an ethics approved study (MOMENTUM, NCT04075305) were included in this retrospective evaluation. Patients received 55Gy in 20 fractions over 4 weeks on a 1.5 Tesla (T) MR Linac (Elekta, Stockholm) between March 2022 and December 2023. All patients were imaged in the treatment position immediately following treatment using a T2 3D 2min axial scan (TR= 1400ms TE= 183ms TA= 147s) and an axial DWI (b=0,150, 500 s/mm2 TR=3521 ms TE= 66ms TA = 222s). ADC maps were calculated using b = 150 and b = 500 s/mm2 images. Images were transferred to offline Monaco planning system for contouring by a single observer. Target volume parameters were extracted from the T2W, B150 and B500 images at baseline, and treatment weeks one through four. Mean ADC was extracted from a fixed 10 mm2 ROI placed within gross tumour aimed at sampling the lowest ADC. Changes in ADC biomarker and absolute volume were used to quantify treatment response. Absolute target volume and mean ADC value were associated to cumulative dose using Spearman's rank test. Changes in ADC biomarker and absolute volume were used to quantify treatment response. Results The population mean (standard deviation) percentage changes from baseline are presented below: Changes in T2W target volume were small ranging from 4% (SD = 0.20) in week one to 11% (SD = 0.28) in week four. B150 and B500 volumes showed the largest decrease from baseline at weeks 3 and 4. B150 decreased by 25% and 36% (SD = 0.25 and 0.31), at weeks 3 and 4 respectively. B500 decreased by 25% and 39% (SD = 0.17 and 0.37), at weeks 3 and 4 respectively. Mean ADC value increased the most at weeks 3 and 4, by 22% for both (SD = 0.13 and 0.07 respectively). DWI/ADC parameters may be most sensitive to therapy-induced changes following 27.5 Gy, suggesting an optimal window for treatment plan review. There was a varied association between T2W volume and cumulative dose (range -0.9 to 0.9). Five patients showed a positive association (range 0.3 to 0.9), three showed a negative association (range -0.9 to -0.2) and one showed no association. A negative association was observed between cumulative dose and DWI volume, ranging from -1.0 to -0.5 for B150 and -1.0 to – 0.1 for B500. Mean ADC values expressed a positive association (range 0.3 – 1.0) with cumulative dose. Indicating as cumulative dose increased, ADC increased, and DWI target volume decreased. Conclusions DWI provides better edge contrast and may provide more reliable imaging parameters for treatment plan adaptations. Therapy induced changes in DWI volume and Mean ADC value associated negatively and positively with cumulative dose. Inter-observer evaluations and multi-centre studies are required to assess feasibility of using DWI imaging for MRI-guided adaptive radiotherapy.