Monoarthritis in the knee can be an early warning for incipient lung cancer, especially for middle-aged men who are heavy smokers.

Researchers in Tuscany, Italy, discovered the paraneoplastic association while reviewing the clinical records of new outpatients at a regional rheumatology clinic. Among 296 patients who presented with isolated asymmetric arthritis of the knee, they found five (1.7%) whose condition showed no rheumatic features at onset. Subsequent chest x-rays detected non-small cell lung cancer (NSCLC) at stages IA or IB.

The five patients were between 53 and 66 years of age and had smoked more than a pack a day for more than 30 years. Their tumors were resected, and all five men were still in full remission after a median follow-up of 41 months, the researchers reported. Their arthritis remitted, too, “confirming its paraneoplastic nature” (Ann Rheum Dis. 2007: published online ahead of print).

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Approximately 85% of all lung cancers are NSCLC, including squamous cell carcinoma, adenocarcinoma, and large-cell undifferentiated carcinoma. Because NSCLC does not respond well to chemotherapy, the prognosis is “strictly dependent on surgical removal,” the researchers noted. Early detection is therefore especially important.

“Paraneoplastic knee monoarthritis was not a remote event. Lung cancer should be suspected in all heavy smokers who present with mild-to-moderate isolated knee monoarthritis, and chest x-rays should be ordered,” they urged.