Hearing the news that you have a tumor that could shorten your life is something we all fear. But there may be an unexpected bonus for those lucky enough to survive cancer and make a full recovery.
This additional benefit applies to all types of cancer, but particularly to men after prostate cancer.
Scientists have found, through a series of large-scale studies, that having cancer can significantly reduce your chances of developing dementia in the future.
In other words, one of the world’s biggest killers seems to protect us in part against one of the world’s other big killers.
The most recent evidence of this extraordinary effect comes from what is believed to be the largest study yet looking at the relationship between cancer and dementia.
An international team of researchers, including experts from Imperial College London, analyzed data from the UK’s Clinical Practice Research Datalink, a registry of more than 30 years of health data covering more than 60 million people, including 18 million currently registered patients.
The lower chances of developing dementia especially apply to prostate cancer survivors.
They compared rates of dementia diagnosis in cancer survivors over 60 with people who had never had a tumor. Cancer survivors were followed for an average of eight years after their diagnosis.
The results, published in the Journal of Prevention of Alzheimer’s Disease, showed that cancer survivors were about 25 per cent less likely to develop the disease, which kills around 74,000 people a year in the UK.
And this surprising benefit appears to occur with most major types of cancer.
In the case of breast cancer, survivors were up to 20 percent less likely to develop dementia later in life than women who had not had the disease.
And in men with prostate cancer, the protective effect was even greater: They were 32 percent less likely to have severe cognitive impairment.
The result is similar with other major cancers, such as lung, bowel and malignant melanoma: in each case the risk of dementia is between 20 and 30 percent lower.
These findings echo the results of a 2022 study conducted by Fudan University in China.
Using data from the UK Biobank (another British database with details of half a million patients), researchers tracked dementia rates and found that cancer survivors had, on average, an 11 percent lower risk, reported the journal Alzheimer’s Research and Therapy.
Other studies have found that the protective effect may be even greater, with the risk of dementia reduced by up to 40 percent in those who had some type of cancer.
Crucially, some research suggests that even those who carry the APOE4 gene (which doubles the risk of Alzheimer’s disease) are also, at least partly, protected from its effect if they had cancer.
These findings come at a time when cases of people living with dementia in the UK are expected to rise from more than 900,000 currently to 1.5 million in 2040.
Evidence pointing to a link between cancer and reduced chances of developing dementia in old age comes from a study analyzing more than 30 years of data covering 60 million patients.
Doctors recommend regular physical exercise, a healthy diet and limited alcohol intake to protect against the disease.
An active social life and learning new skills, such as a language or a musical instrument, are also thought to help.
But what could explain why the development of one life-threatening disease could mitigate another?
One obvious explanation is that some cancer survivors, and especially those who develop cancer in childhood, tend to have a shorter life expectancy (often due to the toxic effects of treatments such as chemotherapy).
In simple terms, this means that some people may not live long enough to develop dementia.
However, it could also be that people who survive cancer are generally healthier (eat sensibly, exercise regularly, and limit their alcohol consumption) and therefore less susceptible to dementia.
In fact, most studies took these confounders into account in their calculations and still found that cancer reduced the dangers.
One possible explanation being investigated involves a protein called PIN1, which is released by cancer cells to stimulate further tumor development and growth.
Studies have found that the protein also appears to reduce the buildup of harmful deposits, called amyloid plaques, in the brain that are linked to dementia.
This could mean that the more active the PIN1 protein is in causing cancer, the more protected the brain is against cognitive failure.
There are other possible factors that are being investigated by Pat Kehoe, professor of translational dementia research at the University of Bristol.
One of them is an enzyme called P13K, which helps regulate cell growth.
Pat Kehoe, from the University of Bristol, is investigating a possible link between reduced chances of dementia and a protein the body produces in response to cancer cells.
In cancer, this enzyme is very active, helping the proliferation of malignant cells and helping the disease spread.
But in dementia patients who have not had cancer, their activity decreases.
The theory is that cancer stimulates the activity of the P13K enzyme and this then protects the brain against dementia in the future.
Professor Kehoe adds: “It would make sense that systems that drive cell growth (as in cancer) protect against a disease in which cells are destroyed by deposits of amyloid plaque (dementia).”
Some experts say proof of the link between the two can be seen in studies showing that having dementia also reduces the risk of developing cancer later.
This is possibly because the destruction of brain cells as part of the disease process means that those same enzymes that also promote cancer growth are being suppressed.
A 2012 study in the BMJ, by Boston University in the US, followed 1,278 people over the age of 64 for at least a decade and found that those who developed Alzheimer’s were almost 60 percent less likely to be diagnosed with cancer.
But it is also possible that some drugs used to treat cancer reduce the risk of dementia.
For example, drug treatment called molecular targeted therapy (which acts on particular molecules on cancer cells), which includes drugs such as atezolizumab for lung cancer and capivasertib for advanced breast cancer, reduced patients’ chances of developing Alzheimer’s. by 40 percent, according to a recent study by Hallym University Sacred Heart Hospital in South Korea, published in Nature.
Again, it’s not clear why, but scientists believe the drugs may reduce brain inflammation that allows plaque buildup that leads to the disease.
But not everyone is convinced of the apparent protective effects of cancer, or that drugs for the disease could represent a breakthrough in the treatment of dementia.
“The research is a bit messy,” says Professor Paul Morgan, acting director of the UK Dementia Research Institute and a dementia specialist at Cardiff University.
‘Some studies suggest it reduces risk, but others have very different results.
“In the case of breast cancer, for example, some research suggests an increased risk of dementia among survivors.”
One theory is that both conditions are associated with tissue inflammation that can promote the disease.
But Professor Kehoe insists the links are worth investigating in the hope they could lead to new, highly effective treatments for dementia.
“I believe there is no smoke without fire,” he says. “I’m pretty convinced something is going on here.”