A new horizon' The inverse link between cancer and dementia

Scientists have long been aware of a curious connection between these common and feared diseases. At last, a clearer picture is emerging

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With age comes disease. Cancer and Alzheimer's dementia are among the commonest and most feared health conditions - particularly in countries with ageing populations. Several decades ago, researchers at a psychiatric centre in New York observed a curious relationship between these two diseases. At autopsy, they found an inverse relation between cancer and Alzheimer's disease.

In one of the first epidemiological studies on the topic, Jane Driver of Brigham and Women's Hospital in Boston, Massachusetts, tracked 1,278 participants aged 65 and older for a mean of 10 years. Published in 2012, the results showed cancer survivors had a 33% decreased risk of then developing Alzheimer's compared with people without a history of cancer.

As intriguing as the finding was, the scientific community urged caution and pointed out potential pitfalls in dealing with age-related diseases.

One of them concerned a so-called survival-bias: perhaps people with a history of cancer simply do not live long enough to develop Alzheimer's.

Since then, scientists have analysed the relationship between cancer and Alzheimer's in more detail and built a compelling case. In the largest study to date, published in July this year, researchers at Imperial College London provide convincing evidence of a lower incidence of dementia following a cancer diagnosis.

They looked at the NHS health data of more than 3 million people aged 60 and over and followed them for a mean period of 9.3 years, taking extra care to correct for potential biases. Their results show that cancer survivors have a 25% lower risk of developing age-related dementia compared with people without a history of cancer.

The inverse association was observed for the most common types of cancers such as prostate, colon, lung and breast.

"The relationship between cancer and Alzheimer's disease is intriguing and persistent," says Erin Abner, a professor at the University of Kentucky.