Occult lung cancer manifesting within the first year after stroke

Lung cancer and stroke share smoking as a major cause of disease. We investigated prevalence and risk of occult lung cancer with manifestation during the first year after stroke. All patients >40 years of age with incident stroke in Denmark 2003–2015 were identified through the Danish Stroke Registry (n=85,893) and matched 1:10 on age and sex to the Danish background population without a history of stroke (n=858,740). Linking data to the Danish Cancer Registry we determined prevalence of occult primary lung cancer defined as the event of previously unknown lung cancer during a one-year follow-up in the stroke and the background population. Cox regression models with adjustments for demographics, co-morbidities and stroke risk factors were used to study risk compared to the background population. Prevalence (per 1000 person-years) of occult lung cancer in the stroke cohort was 5.3; in the background cohort 2.6. Prevalence separately for current smokers (n=26,055) was 9.6; ex-smokers (n=20,035) 6.5; never-smokers (n=27,268) 1.4. Risk of occult lung cancer (adjusted) was increased HR 1.95 in the stroke population. In the stroke population adjusting for stroke risk factors age (HR 1.24 per 10 years) and smoking (HR 7.1 in current smokers; HR 1.6 in ex-smokers) were the only significant risk factors for occult lung cancer. Occult lung cancer is rarely found in stroke patients who have never smoked. It is not uncommon in smokers in whom 1% of current smokers had occult lung cancer that became manifest within the first year after stroke.