Association of Tumor [¹⁸F]FDG Activity and Diffusion Restriction with Clinical Outcomes of Rhabdomyosarcomas

Purpose: To evaluate whether the extent of restricted diffusion and 2-deoxy-2-[¹⁸F] fluoro-d-glucose ([¹⁸F]FDG) uptake of pediatric rhabdomyosarcomas (RMS) on positron emission tomography (PET)/magnetic resonance (MR) images provides prognostic information. Procedure: In a retrospective, IRB-approved study, we evaluated [¹⁸F]FDG PET/CT and diffusion-weighted (DW) MR imaging studies of 21 children and adolescents (age 1–20 years) with RMS of the head and neck. [¹⁸F]FDG PET and DW MR scans at the time of the initial tumor diagnosis were fused using MIM software. Quantitative measures of the tumor mass with restricted diffusion, [¹⁸F]FDG hypermetabolism, or both were dichotomized at the median and tested for significance using Gray’s test. Data were analyzed using a survival analysis and competing risk model with death as the competing risk. Results: [¹⁸F]FDG PET/MR images demonstrated a mismatch between tumor areas with increased [¹⁸F]FDG uptake and restricted diffusion. The DWI, PET, and DWI + PET tumor volumes were dichotomized at their median values, 23.7, 16.4, and 9.5 cm³, respectively, and were used to estimate survival. DWI, PET, and DWI + PET overlap tumor volumes above the cutoff values were associated with tumor recurrence, regardless of post therapy COG stage (p = 0.007, p = 0.04, and p = 0.07, respectively). Conclusion: The extent of restricted diffusion within RMS and overlap of hypermetabolism plus restricted diffusion predict unfavorable clinical outcomes.