18F-FDG PET/CT in idiopathic retroperitoneal fibrosis with predominant presacral involvement

Idiopathic retroperitoneal fibrosis (IRF) is a rare fibro-inflammatory disease that is often diagnosed only after the development of complications, making early recognition and long-term management challenging. 18 F-FDG PET/CT provides both anatomical and functional information, enabling accurate assessment of disease extent and monitoring of treatment response in patients with IRF. We report a 64-year-old man who presented with abdominal pain and difficulty defecating for 8 months. Laboratory tests showed elevated inflammatory markers, whereas tumor markers and immunological parameters were within normal limits. 18 F-FDG PET/CT revealed a hypermetabolic soft-tissue mass with predominant presacral involvement. The lesion encased the bilateral internal and external iliac arteries and pelvic ureters, with limited cranial extension to the distal common iliac arteries, resulting in bilateral hydroureteronephrosis. Histopathological examination confirmed IRF. Following glucocorticoid therapy, follow-up 18 F-FDG PET/CT demonstrated a marked reduction in lesion size and metabolic activity, indicating a favorable treatment response. This case represents one of the few reported cases of IRF with predominant presacral involvement evaluated with serial 18 F-FDG PET/CT. The findings highlight the value of 18 F-FDG PET/CT in assessing disease activity, supporting differential diagnosis, and monitoring response to corticosteroid therapy in atypical presentations of IRF.