Post-treatment residual tissue in idiopathic retroperitoneal fibrosis: active residual disease or silent "scar" ? A study using 18F-fluorodeoxyglucose positron emission tomography.

OBJECTIVE: Medical treatment is often effective in idiopathic retroperitoneal fibrosis (IRF) but frequently leads to residual retroperitoneal masses that may represent active disease or simply consist of inactive fibrotic tissue. 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) is a functional imaging modality that reliably assesses disease activity in a number of inflammatory diseases including IRF. We used 18F-FDG PET to evaluate the metabolic activity of residual masses in a series of IRF patients.

METHODS: We studied 7 consecutive IRF patients, all of whom presented constitutional symptoms and/or pain, and had high acute-phase reactant levels; 6 had ureteral involvement. IRF was diagnosed by means of computed tomography (CT), which revealed a peri-aortoiliac mass in all cases. Three patients underwent surgical ureterolysis and 2 received ureteral stents. Subsequently, 5 patients received prednisone, one sequential treatment with prednisone and tamoxifen, and one prednisolone plus methotrexate. All of the patients underwent 18F-FDG PET at varying times after the end of treatment.

RESULTS: The presenting signs/symptoms improved in all patients and the levels of acute-phase reactants significantly decreased or normalised. Ureteral obstructive disease resolved in all cases. Post-treatment CT revealed a considerable reduction in the amount of IRF but all of the patients had a residual retroperitoneal mass. PET revealed slight aorto-iliac 18F-FDG uptake in only one patient; all of the others were negative. No patient relapsed during the follow-up.

CONCLUSIONS: Post-treatment residual masses are frequent in IRF patients but, in most cases, probably represent metabolically inactive tissue.