Acquisition matters - how do scan parameters affect apparent diffusion coefficient estimates in pediatric rhabdomyosarcoma

Background: The apparent diffusion coefficient (ADC) derived from diffusion-weighted imaging (DWI) is a potential biomarker for treatment response in pediatric rhabdomyosarcoma. Due to its rarity, investigations into this marker require multicenter approaches, which can result in variability in acquisition parameters. Objective: To evaluate the impact of different acquisition parameters on ADC estimates in a multicenter dataset of rhabdomyosarcoma patients. Materials and methods: We included 114 pediatric and adolescent rhabdomyosarcoma patients from 22 treatment centers (195 scans). Median age: 6.0 years (0.3–21.8). We evaluated the impact of voxel size, (number of) b-values, and echo time on tumor ADC values. The effect of the highest b-value was separately investigated on a subset of scans with five or more b-values. Results: We observed a large variability in key acquisition parameters in the overall cohort, and for individual imaging centers. No parameter showed a significant effect on ADC estimates of the whole cohort when corrected for multiple-comparisons. Decreasing the highest b-value within the same acquisition caused ADC to decrease on average by 2.8% per 100 s mm^-2. Differing b-values between scans at diagnosis and treatment response yielded significant changes in the longitudinal ADC for each patient (P<0.05). Conclusion: While we observed wide variation of acquisition parameters within a multicenter cohort, this did not lead to significant cross-sectional differences of tumor ADC. However, we found that modifying the highest b-value between baseline and follow-up can impact longitudinal ADC estimates. As such, we recommend the highest b-value to remain constant. This retrospective study was reviewed and approved by the Internal Review Board (UMC Utrecht, reference ID: 18-412).