The value of ¹⁸F-FDG PET/CT in the assessment of active idiopathic retroperitoneal fibrosis

The different stages in idiopathic retroperitoneal fibrosis (IRF) are generally assessed by assay of inflammatory markers and analysis of contrast-enhanced CT images of the retroperitoneal mass. We investigated the potential role of ¹⁸F-FDG PET/CT in this clinical setting.

¹⁸F-FDG uptake was assessed visually and semiquantitatively (using maximum standardized uptake values, SUVmax) in images of the abdominal mass in 22 patients prospectively enrolled from June 2008 to December 2010 who underwent a total of 33 PET/CT studies. The accuracy in discriminating active from inactive disease was calculated assuming as reference a biochemical instrumental evaluation of patients with IRF mostly based on the level of inflammatory indices and contrast enhancement (CE) of the mass at the time of each PET study. In particular, the relationship between SUVmax and CE, the latter calculated from the change in radiodensity (Hounsfield units) between the basal and postcontrast venous portal phases, was evaluated on a three-point scale (0 <20 HU, 1 20–30 HU, 2 ≥30 HU). SUVmax and CE scores were correlated with the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) levels. The value of PET/CT in assessing the variation of disease activity over time was also investigated by analysing the changes in metabolic volume (MV) of the retroperitoneal lesion between repeat patient studies.

PET/CT accurately discriminated (93.9 %) active from inactive disease. Significant agreement (p < 0.01) was observed between visual and semiquantitative analysis of ¹⁸F-FDG uptake, and CE score. A significant correlation (p < 0.01) was found among SUVmax, CRP levels (rho = 0.54) and ESR (rho = 0.55). Corresponding variations in MV and CE score were observed in patients with multiple studies (p < 0.01; rho = 0.68).

¹⁸F-FDG PET/CT may be considered an alternative imaging method for the assessment of different stages of IRF.