Paraneoplastic Arthritis Mimicking Late-Onset Rheumatoid Arthritis in an Older Smoker: A Diagnostic Challenge.

Paraneoplastic rheumatic syndromes may present with inflammatory arthritis that closely mimics rheumatoid arthritis (RA), leading to diagnostic delay. We describe an older male with progressive inflammatory polyarthritis and a low-titer positive rheumatoid factor (RF), initially diagnosed with late-onset seropositive RA. Although his arthritis improved with glucocorticoids and hydroxychloroquine, persistent respiratory symptoms prompted further evaluation. High-resolution computed tomography (HRCT) of the chest revealed progressive fibrotic interstitial lung disease (ILD) with an indeterminate usual interstitial pneumonia (UIP) pattern and multiple pulmonary nodules suspicious for malignancy. Subsequent biopsy confirmed synchronous non-small cell lung carcinoma. The rheumatologic diagnosis was revised to paraneoplastic arthritis mimicking RA. This case highlights the importance of maintaining a high index of suspicion for occult malignancy in older patients presenting with new-onset inflammatory arthritis, particularly in smokers with pulmonary symptoms.