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 extra-legal benefit applies to all forms of cancer, but especially to men after prostate cancer.
Scientists have discovered through a series of large-scale studies that having cancer can significantly reduce the risk of developing dementia.
In other words, one of the world’s greatest killers appears to be partially protecting us from one of the world’s other great killers.
The latest evidence of this extraordinary effect comes from what is believed to be the largest study to date on the relationship between cancer and dementia.
An international team of researchers, including experts from Imperial College London, analyzed data from the UK Clinical Practice Research Datalink – a record of more than 30 years of health data on more than 60 million people, including 18 million currently registered patients.
The reduced risk of developing dementia especially applies to prostate cancer survivors
They compared dementia diagnosis rates in cancer survivors over the age of 60 with people who had never had a tumor. The 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 around 25 percent less likely to develop the disease, which kills an estimated 74,000 people a year in Britain.
And this surprising benefit appears to occur in most of the major types of cancer.
In breast cancer, survivors were up to 20 percent less likely to develop dementia later 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 experience severe cognitive decline.
The result is comparable to other serious forms of cancer, such as lung cancer, colon cancer and malignant melanoma: in both cases the risk of dementia is 20 to 30 percent lower.
These findings mirror the results of a 2022 study by Fudan University in China.
Using data from UK Biobank – another British database containing the details of half a million patients – the researchers tracked dementia rates and found that cancer survivors had an average 11 percent lower risk, the journal Alzheimer’s Research and Therapy reported.
Other studies have found that the protective effect may be even greater, with the risk of dementia reduced by as much as 40 percent in those who have had any form of cancer.
Crucially, some studies show that even those who carry the APOE4 gene – which doubles the risk of Alzheimer’s disease – are also at least partially protected from its effects if they were to develop cancer.
These findings come at a time when the number of dementia cases in Britain is expected to rise from over 900,000 currently to 1.5 million by 2040.
Evidence pointing to a link between cancer and reduced chances of developing dementia later in life comes from a study of more than 30 years of data on 60 million patients
Doctors recommend regular 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 – would also help.
But what could possibly explain why developing one life-threatening disease can mitigate another?
One obvious explanation is that some cancer survivors, and especially those who develop cancer in childhood, tend to have a shorter lifespan (often due to the toxic effects of treatments such as chemotherapy).
Simply put, this means that some may not live long enough to develop dementia.
However, it may also be the case that people who survive cancer are generally healthier (eat sensibly, exercise regularly and limit their alcohol consumption) and are therefore less susceptible to dementia.
In fact, most studies took these confounding factors into account in their calculations – and still found that cancer reduced the dangers.
One possible explanation being explored involves a protein called PIN1, which is released by cancer cells to stimulate the further development and growth of tumors.
Studies have shown that the protein also appears to reduce the build-up of harmful deposits called amyloid plaques in the brain, which are linked to dementia.
This could mean that the more active the PIN1 protein is in causing cancer, the better the brain is protected against cognitive failure.
There are other possible factors being explored by Pat Kehoe, professor of translational dementia research at the University of Bristol.
One of these is an enzyme called P13K, which helps regulate cell growth.
Pat Kehoe from the University of Bristol is investigating a possible link between a reduced risk of dementia and a protein produced by the body in response to cancer cells
In cancer, this enzyme is very active: it promotes the proliferation of malignant cells and helps the disease to spread.
But in dementia patients who have not had cancer, its activity is reduced.
The theory is that cancer stimulates the activity of the P13K enzyme and this later protects the brain against dementia.
Professor Kehoe added: ‘It would make sense that systems that control cell growth (such as in cancer) would be protective against a disease in which cells are destroyed by deposits of amyloid plaques (dementia).’
Some experts say proof of the link between the two can be found in studies showing that having dementia also reduces the risk of developing cancer.
This may be because the destruction of brain cells as part of the disease process means that the same enzymes that also promote cancer growth are suppressed.
A 2012 study in the BMJ at 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 disease were almost 60 percent less likely to be diagnosed with cancer later.
But it’s also possible that some medications used to treat cancer can lower the risk of dementia.
Drug treatment called molecular-targeted therapy (which acts on certain molecules in cancer cells), including drugs such as atezolizumab for lung cancer and capivasertib for advanced breast cancer, reduced patients’ chances of developing Alzheimer’s disease by 40 percent, according to a recent study from the Hallym University Sacred Heart Hospital in South Korea, published in Nature.
Again, it’s not clear why, but scientists think the drugs may reduce brain inflammation, allowing the plaque buildup that leads to the disease.
But not everyone is convinced of the apparent protective effects of cancer – or that drugs against the disease could be a breakthrough in the treatment of dementia.
“The research is a bit of a mess,” says Professor Paul Morgan, interim director of the UK Dementia Research Institute and a specialist in the disease at Cardiff University.
‘Some studies suggest it reduces risk, but others have very different results.
‘In breast cancer, for example, some studies indicate an increase in the risk of dementia among survivors.’
One theory is that both conditions are related to tissue inflammation that can promote disease.
But Professor Kehoe is convinced the links are worth investigating, in the hope that it could lead to new and highly effective treatments against dementia.
“I believe there is no smoke without fire,” he says. “I firmly believe there is something going on here.”