Clinical Features Distinguishing Tumors From Tumor-Like Lesions in Patients With Rheumatoid Arthritis: An Observational Study

BACKGROUND: Rheumatoid arthritis (RA)-related lesions, including synovitis and rheumatoid nodules, may mimic musculoskeletal tumors and cause diagnostic uncertainty. This study aimed to characterize clinical features that distinguish true tumors from tumor-like lesions in RA patients referred for suspected bone or soft tissue tumors. METHODS: We retrospectively reviewed 70 RA patients evaluated between September 2009 and February 2025 for suspected musculoskeletal tumors. Patients were categorized as (1) newly diagnosed RA with tumor-like lesions (n=27), (2) established RA with tumor-like lesions (n=20), and (3) established RA with true tumors (n=23). Clinical characteristics, MRI findings, and histologic results were assessed. Logistic regression was used to identify factors associated with tumor diagnosis. RESULTS: Patients with true tumors were older (median 69 vs. 55 y, p<0.01) and more likely to have extra-articular lesions (96% vs. 28%, p<0.01). Synovitis-like MRI patterns-low-to-intermediate T1 and heterogeneous high T2 signals-were seen in 83% of tumor-like lesions but in only 43% of true tumors. Histologic confirmation was performed in 56% of all cases (74% of tumors vs. 47% of tumor-like lesions, p=0.03). Multivariate analysis suggested that older age (OR: 1.13, 95% CI: 1.04-1.23) and extra-articular location (OR: 434.86, 95% CI: 9.44-20,035.22) were associated with tumor diagnosis. CONCLUSIONS: Among RA patients evaluated for suspected musculoskeletal tumors, older age and extra-articular lesion location were associated with true tumor diagnosis, although the estimates showed wide confidence intervals. MRI features alone were insufficient for reliable differentiation, underscoring the continued need for histologic confirmation to ensure accurate and timely diagnosis.