For decades, doctors suspected that dementia patients were less likely to get cancer than other people and, likewise, that cancer patients and survivors were less likely to develop dementia.
In the last few years, a handful of observational studies seemed to support this kind of reverse association. For example, one study of more than 1,100 older volunteers suggested that dementia was a third less common among people with non-melanoma skin cancer. Researchers wondered whether something about a predisposition to one disease might reduce the risk of the other.
Now, a large retrospective study of patient data from Denmark casts doubt on that idea.
This new, larger study found that people with non-melanoma skin cancer were only 2 to 10 percent less likely to be diagnosed with dementia than patients without cancer. More specifically, at the end of 34 years, the cumulative risk of Alzheimer's disease was 4.6 percent among patients with non-melanoma skin cancer versus 4.7 percent among those without cancer. Senior author Victor Henderson, MD, professor of health research and policy at Stanford, said the assumption that there is a true association between cancer and dementia needs to be reexamined.
"We were surprised that the association was so weak," he told me.
The study -- a collaboration between Henderson and a team of Danish researchers that includes first author Sigrún Schmidt, MD -- looked at medical records collected between 1980 and 2013 from more than 200,000 Danish patients who had non-melanoma skin cancer. They also looked at more than a million age- and sex- matched controls, five individuals for each skin cancer patient.
Henderson says they found that in the first 5 to 10 years after a patient was diagnosed, there seemed to be a small association between having non-melanoma skin cancer and not developing dementia. But as the years ticked by, the effect size dwindled.
"If there were a direct mechanism between cancer and dementia," he said, "the effect should persist."
It's quite possible the larger effects measured in earlier studies resulted from bias, not biology. Problems that can bias the results, said Henderson, could include much smaller sample sizes, volunteers who are self-selected and different from the general population: "For example, volunteers are better educated and healthier."
Though the results for the current study throw cold water on the cancer-dementia hypothesis, Henderson emphasized the importance of resolving the issue. If there really is a meaningful, negative association between cancer and Alzheimer's disease and other dementias, researchers should look for a common molecular lever that could serve, far down the road, as a drug target.
The study appears online today in PLOS ONE. Henderson's co-senior author, Henrik Sørensen, MD, is professor and chair in the department of clinical epidemiology at Aarhus University and a consulting professor of health research and policy here at Stanford.
Previously: Patient data shows that common prostate cancer treatment likely doubles risk of dementia and When dementia hits home: The global impact of dementia on women
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