# [18F]FDG PET/CT for treatment monitoring and prediction of progression in retroperitoneal fibrosis

## TL;DR

This study shows that [18F]FDG PET is better than CT for monitoring treatment response and predicting disease progression in retroperitoneal fibrosis.

## Contribution

The study demonstrates that PET metrics, particularly metabolic active volume, are superior to CT for predicting progression in retroperitoneal fibrosis.

## Key findings

- PET parameters like SUVmax, SUVmean, and metabolic active volume (MAV) showed significant correlations with morphological changes in retroperitoneal fibrosis.
- Higher baseline MAV was a strong predictor of disease progression at follow-up.
- PET outperformed CT in detecting treatment response and progression in retroperitoneal fibrosis patients.

## Abstract

Retroperitoneal fibrosis (RPF) is a rare inflammatory disease, that, if left untreated, can lead to ureteral obstruction and subsequent renal impairment. First-line treatment is prednisolone, with rituximab, often used for refractory cases. This study evaluates treatment response in both [18F]FDG PET and CT, and potential baseline parameters for early prediction of progression.

50 patients with RPF underwent at least two [18F]FDG PET/CT scans (baseline, BL, and first follow-up, FU1), 36 patients a second (FU2), 18 patients a third follow-up (FU3). PET parameters SUVmax, SUVmean, SUVpeak, metabolic active volume (MAV), thickness (CTrim) and cranio-caudal extension (CTcc) of the retroperitoneal mass were measured. Therapy groups were divided into prednisolone, rituximab and the combination of both.

All PET parameters showed significant correlations with CTrim at all four timepoints. After therapy all PET parameters and CTrim decreased significantly (p ≤ 0.021). Highly significant metabolic and morphologic response was seen in the prednisolone (p ≤ 0.003) and the combination therapy group (p ≤ 0.001). At FU2, eight patients showed progression, with MAV as a good predictor of progression in BL (p = 0.041; 217.33 versus 100.86 ml). At FU2, SUVmax, SUVpeak and MAV differed significantly between progression and non-progression group (p ≤ 0.009), while CT showed no significant differences.

Our findings underscore the superiority of PET against CT in therapy monitoring of RPF, especially in the detection of progression at FU2. Higher BL MAV correlated with progression at FU2, indicating its potential as a predictive marker. Still, especially when PET is not available, CT can be considered for initial therapy monitoring.