Evaluation of standardized uptake value (SUV) and apparent diffusion coefficient (ADC) as prognostic imaging...

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. .

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.

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).

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.