# Clinical Utility of 18F-FDG PET/CT in Rheumatology: Diagnostic and Therapeutic Insights from a Ten-Year Real-World Cohort

## TL;DR

This study shows how 18F-FDG PET/CT helps diagnose and manage complex rheumatologic conditions by providing insights into inflammation and malignancy.

## Contribution

The study demonstrates the clinical utility of PET/CT in rheumatology through a ten-year real-world cohort analysis.

## Key findings

- PET/CT led to new diagnoses in 29.6% of patients, especially in those with unexplained inflammation.
- Therapeutic modifications occurred in 27.1% of cases based on PET/CT findings.
- PET/CT showed higher diagnostic value in vasculitis, IgG4-related disease, and sarcoidosis.

## Abstract

Objective: To evaluate the diagnostic yield and clinical impact of 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) in patients with unexplained inflammation, fever, or suspected malignancy, and to assess its role across different rheumatologic subgroups. Methods: We retrospectively analyzed 280 patients who underwent PET/CT between 2010 and 2020 in a tertiary rheumatology center. Demographic, clinical, and laboratory data including erythrocyte sedimentation rate, C-reactive protein, and PET/CT indications were collected. PET/CT findings were categorized as inflammatory, neoplastic, or normal based on visual assessment and SUVmax. Final diagnoses were confirmed using clinical, histopathological, or follow-up data. Statistical analysis compared PET/CT results, inflammatory markers, and diagnostic outcomes among disease subgroups. Results: Of 280 patients (mean age 58 ± 15 years, 63.9% female), 72% had an established rheumatologic diagnosis prior to PET/CT. A new diagnosis, confirmed by predefined clinical, histopathological, or follow-up criteria, was established in 29.6% of patients, predominantly among those undergoing diagnostic evaluation for unexplained inflammation, including 40 rheumatologic and 43 non-rheumatologic conditions (22 malignancies). PET/CT led to therapeutic modification in 27.1% of all cases, based on multidisciplinary clinical decision-making. PET/CT demonstrated the highest diagnostic contribution in vasculitis, IgG4-related disease, and sarcoidosis. Median SUVmax was higher in malignancies than in inflammatory diseases [8.0 vs. 4.6, p < 0.05]. Lymphadenopathy was more frequent in non-rheumatologic and malignant conditions (p = 0.002). PET/CT findings showed variable but clinically relevant concordance with other imaging modalities. Conclusions: PET/CT provides supportive diagnostic and management insights in complex or atypical rheumatologic presentations. It demonstrated high yield in systemic inflammatory disorders and providing supportive information for malignancy exclusion in connective tissue diseases when interpreted alongside clinical and laboratory follow-up. Integration of PET/CT with clinical and laboratory data enhances diagnostic accuracy and supports patient-centered management in rheumatology.