Evaluation of standardized uptake value (SUV) and apparent diffusion coefficient (ADC) as prognostic imaging parameters for primary Ewing sarcoma in hybrid 18F-FDG-PET/MRI

Purpose Ewing sarcoma (EwS) is the second most prevalent malignant bone tumour among juvenile patients. Due to its highly malignant character, itis crucial to identify patients with unfavourable courses as early as possible. In this context, integrated 18F-FDG-PET/MRI combines high-contrast morphological information with functional diffusion imaging and metabolic data and thereby provides ADC and SUV max /SUV mean as established parameters for tumour cellularity and metabolic activity in a single examination. Therefore, our study evaluated the use of ADC and SUV max /SUV mean as prognostic parameters for treatment response and overall survival (OS) in this rare entity. . Methods In a monocentric retrospective study, patients with histopathologically-proven EwS who underwent 18F-FDG-PET/MRI before therapy start were included .A polygonal ROI was drawn encompassing the entire tumour mass on fused PET/MR images to determine SUV max and SUV mean and copied onto the ADC maps to quantify ADC mean and ADC min . Dependencies between these parameters and Salzer-Kutschik-Scale(SKS) were analysed by Spearman's correlation coefficients and Hazard ratio for OS. Results 39 patients (14 female, median age 16yrs[IQR: 11–23]) were evaluated. Strong inverse correlations between SUV max and ADC mean (ρ = -0.709,p < 0.001) and SUV mean and ADC mean (ρ = -0.689,p < 0.001) were observed. Furthermore, moderate correlations between SKS and SUV max (ρ = 0.502,p = 0.009), SUV mean (ρ = 0.389,p = 0.049) and ADC mean (ρ = -0.425,p = 0.03) were seen as well as correlations between OS and SUV max (HR:5.324,95 %-CI 1.065–26.615,p = 0.042) and SUV mean (HR:5.558, 95 %-CI 1.114–27.733,p = 0.037). Conclusion Hence, our study demonstrates a strong correlation between metabolic activity(SUV max /SUV mean ) and tumour cellularity(ADC) in EwS and proposes SUV max and SUV mean as prognostic markers for neoadjuvant treatment response and OS. ADC mean also correlates inversely with treatment response but calculating ADC mean and SUV max /SUV mean ratios does not improve prognostic value.