Introduction Symptoms of occult brain cancer may mimic stroke. Misdiagnosis may lead to improper treatment and delayed diagnosis. We characterised strokes associated with occult primary brain cancer and determined risk that ischaemic and haemorrhagic strokes are associated with occult primary brain cancer. Patients and methods All patients 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 stroke history (n = 858,740). This cohort was linked to the Danish Cancer Registry and prevalence of occult primary brain cancer defined as the event of previously unknown primary brain cancer during a one-year follow-up was estimated. We used Cox regression models to study risk of occult primary brain cancer in comparison to the background population. Results Of 77,484 patients with ischaemic strokes, 39 (1 in 2000) were associated with primary brain cancer; of 8409 with haemorrhagic strokes, it was 126 (1 in 66). In the background cohort, 205 (1 in 4000) had occult primary brain cancer. The multivariate stroke risk factor analysis showed that patients with occult primary brain cancer differed significantly from those without occult primary brain cancer indicating they might have stroke mimics rather than true strokes. Discussion and conclusions: Strokes associated with occult primary brain cancer tend to be stroke mimics rather than true strokes. Primary brain cancer is rare in patients with ischaemic stroke (1 in 2000); risk that misdiagnosis results in maltreatment is, therefore, very low. Occult primary brain cancers are mainly found among patients with haemorrhagic stroke; they are not uncommon (1 in 66) and should always be kept in mind.

Background: 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. Methods: 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. Results: 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. Conclusions: 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.

Objectives Stroke is associated with a higher risk of occult cancer. We studied the types of occult cancer most often associated with stroke. Materials and Methods All patients 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 history of stroke (n = 858 740). Linking data to the Danish Cancer Registry, we determined the prevalence of occult cancer in stroke defined as the event of previously unknown cancer during a one-year follow-up in the stroke and the background population. We stratified cancers into the 15 most common cancers and into cancers related to and not related to smoking. Results Prevalence (per 1000 person-years; stroke/background population) of smoking-related occult cancers (lung, colon, bladder rectum, pancreas, kidney, stomach, and head and neck) was 13.3/8.1 (P < .01) and of cancers not related to smoking (prostate, breast, melanoma, non-Hodgkin lymphoma, ovary, endometrial) it was 6.6/6.2 (P > .05). Among men with stroke prostate (5.8), lung (5.7), and colon cancers (2.9) were most frequent; among women, it was lung (4.7), breast(3.5), and colon cancer(2.8). Among men in the background population, prostate(5.4), lung (3.0), and colon cancers (2.1) were most frequent. Among women, it was breast (3.7), lung(2.1), and colon cancers (1.9). Conclusion Stroke patients' increased risk of occult cancer was predominantly for smoking-related cancers while there was no higher risk for the most frequent cancers unrelated to smoking, that is, prostate and breast cancers. Lung cancer is the dominant type of occult cancer in stroke.