Role of diffusion-weighted MRI in assessing Ewing sarcoma treatment response: Findings from a COG phase III trial

To correlate quantitative measures of tumor cell density, derived from diffusion-weighted magnetic resonance imaging (MRI), with quantitative measures of tumor glucose metabolism, derived from 18F-FDG positron emission tomography (PET), and post-induction event-free survival (EFS) in patients with metastatic Ewing sarcoma.

We performed a secondary review of 21 patients (mean age 14.1 years ±4.67, 12 females) with metastatic Ewing sarcoma who were enrolled in a prospective, multicenter phase III clinical trial by the Children's Oncology Group (AEWS1221) and underwent paired diffusion-weighted MRI and ¹⁸F-FDG PET scans at baseline and post induction chemotherapy (after 6 cycles). Tumor size, apparent diffusion coefficient (ADC)_mean and ADC_min (whole tumor, viable, and necrotic portions), and standardized uptake values (SUV_max) were measured. Pearson's correlation coefficient was used to assess relationships between continuous imaging biomarkers.

Baseline ADC_mean of the viable tumor inversely correlated with SUV_max (R = -0.51, p = 0.022). After induction, SUV_max inversely correlated with whole tumor ADC_min (R = -0.59, p = 0.028) but not with viable tumor ADC_mean (R = -0.15, p = 0.62). Both whole tumor ADC_min and viable tumor ADC_mean post-induction correlated with tumor volume changes (R = -0.56, p = 0.008; R = -0.5, p = 0.022). None of these biomarkers were associated with EFS risk.

Baseline ADC_mean (excluding necrosis) may serve as a surrogate for SUV_max, while post-induction ADC_min correlates with metabolic activity but not survival outcomes. Further studies are needed to validate these findings.