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

## TL;DR

An older smoker with arthritis was later found to have lung cancer, showing how cancer can mimic rheumatoid arthritis and delay diagnosis.

## Contribution

This case highlights the diagnostic challenge of paraneoplastic arthritis mimicking rheumatoid arthritis in older smokers.

## Key findings

- An older male with arthritis and low-titer RF was initially diagnosed with RA.
- Persistent respiratory symptoms led to the discovery of lung cancer.
- The arthritis was reclassified as paraneoplastic, emphasizing the need for cancer screening in similar cases.

## Abstract

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.