Although the occurrence of both cancer and Alzheimer’s disease (AD) increases exponentially with age, researchers have discovered an inverse relationship between the two illnesses: Older persons with cancer have a reduced risk for developing AD, and older persons with AD have a reduced risk for developing cancer.

“As AD dementia and cancer are negative hallmarks of aging and senescence, we suggest that AD dementia, cancer, and senescence could be manifestations of a unique phenomenon related to human aging,” wrote Massimo Musicco, MD, of the National Research Council of Italy, in Milan, Italy, and fellow investigators in Neurology (2013;81[4]:322-328), the medical journal of the American Academy of Neurology (AAN).

In their cohort study, involving 204,468 residents of Northern Italy aged 60 years and older, Musicco and colleagues used various health registries to derive cancer incidence as well as AD incidence. They then calculated the expected cases of AD dementia in persons with a new diagnosis of cancer and the expected cases of cancer in persons with AD.

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During the six-year period covered, 21,451 persons developed cancer and 2,832 developed AD. Although 281 of those with cancer were expected to have AD and 246 with AD were expected to have cancer, only 161 subjects had both diseases. The team calculated that persons with AD had a 50% reduced risk for cancer, and people with cancer had a 35% reduced risk for AD. These results were maintained in almost all subgroup analyses.

In a statement issued by the AAN, Musicco noted that the reduced risk for cancer and AD in patients who already had one of these diseases has been seen in other studies, but that his group’s research was the largest such study to date. Musicco also pointed out that his research has several strengths over previous studies, such as looking for the presence of the second disease both before and after the diagnosis of the first disease.

“This controls for the possibility that the presence of one disease might obscure the diagnosis of other diseases, because any new symptoms might be interpreted as a consequence of the already-diagnosed disease, or in the case of cancer, people might assume that memory problems were a side effect of chemotherapy,” Musicco commented.