Hearing the news that you have a tumour which could shorten your life is something we all dread. But there may be an unexpected bonus for those fortunate to survive cancer and make a full recovery.
This fringe benefit applies for all kinds of cancer, but particularly for men after prostate cancer.
Scientists have discovered, through a series of large-scale studies, that having cancer can significantly reduce your chances of later developing dementia.
In other words, one of the world’s biggest killers seems to partly protect us against one of the world’s other big killers.
The latest evidence of this extraordinary effect comes from what’s believed to be the largest study so far to look at the relationship between cancer and dementia.
An international team of researchers, including experts from Imperial College London, analysed data from the UK Clinical Practice Research Datalink – a record of more than 30 years of health data covering more than 60 million people, including 18 million currently registered patients.
The reduced chances of developing dementia apply particularly to prostate cancer survivors
They compared dementia diagnosis rates in cancer survivors aged over 60 with people who’d never had a tumour. The cancer survivors were followed up 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 causes the deaths of an estimated 74,000 people a year in the UK.
And this surprising benefit seems to occur with most of the major types of cancer.
For breast cancer, survivors were up to 20 per cent less likely to later develop dementia than women who hadn’t had the disease.
And in men with prostate cancer, the protective effect was even greater – they were 32 per cent less likely to suffer severe cognitive decline.
It’s a similar result with other major cancers, such as lung, bowel and malignant melanoma: in each case dementia risk is 20 to 30 per cent lower.
These findings echo the results of a 2022 study by Fudan University in China.
Using data from UK Biobank – another British database with the details of half a million patients – the researchers tracked dementia rates and found that cancer survivors were, on average, 11 per cent less at risk, reported the journal Alzheimer’s Research and Therapy.
Other studies have found the protective effect may be even greater, with dementia risk cut by up to 40 per cent in those who had any type of cancer.
Crucially, some research suggests even those carrying the APOE4 gene – which doubles the risk of Alzheimer’s disease – are also at least partly shielded against its effect if they had cancer.
These findings come at a time when cases of people living with dementia in the UK are forecast to rise from more than 900,000 currently to 1.5 million by 2040.
Evidence pointing to a link between cancer and reduced chances of developing dementia in later life comes from a study looking at 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 illness.
An active social life and learning new skills – such as a language or a musical instrument – are also thought to help.
But what could possibly explain why developing one life-threatening disease might mitigate against another?
One obvious explanation is that some cancer survivors, and especially those who develop cancer in childhood, tend to have shorter lifespans (often because of the toxic effects of treatments such as chemotherapy).
In simple terms, this means some might not live long enough to develop dementia.
However, it could also be that people who survive cancer go on to be generally healthier – eating sensibly, exercising regularly and limiting their alcohol intake – and therefore less susceptible to dementia.
In fact, most studies took account of these confounding factors in their calculations – and still found cancer reduced the dangers.
One possible explanation being investigated involves a protein called PIN1, which is released by cancer cells to stimulate further tumour development and growth.
Studies have found that the protein also appears to reduce the build-up of harmful deposits, called amyloid plaques, in the brain that is linked to dementia.
This could mean that the more active the PIN1 protein is in driving cancer, the more protected the brain is against cognitive failure.
There are other possible factors being investigated by Pat Kehoe, a professor of translational dementia research at Bristol University.
One of these is an enzyme called P13K, which helps to regulate cell growth.
Bristol University’s Pat Kehoe is investigating a possible link between reduced dementia chances and a protein which is produced by the body in response to cancer cells
In cancer, this enzyme is highly active – aiding the proliferation of malignant cells and helping the disease spread.
But in dementia patients who haven’t had cancer, its activity is dialled down.
The theory is cancer stimulates the activity of the P13K enzyme and this then protects the brain against dementia later on.
Professor Kehoe adds: ‘It would make sense that systems which drive cell growth [as in cancer] are protective against a disease where cells get destroyed by amyloid plaque deposits [dementia].’
Some experts say proof of the link between the two can be seen in studies that show having dementia also reduces the risk of subsequently developing cancer.
This is possibly because the destruction of brain cells as part of the disease process means those same enzymes that also promote cancer growth are being suppressed.
A 2012 study in the BMJ, by Boston University in the U.S., tracked 1,278 people aged over 64 for at least a decade and found that those who developed Alzheimer’s were almost 60 per cent less likely to later be diagnosed with cancer.
But it is also possible that some drugs used to treat cancer could be lowering the risk of dementia.
For instance, drug treatment called molecular targeted therapy (which acts on particular molecules in cancer cells), which includes medicines such as atezolizumab for lung cancer and capivasertib for advanced breast cancer, reduced patients’ chances of getting Alzheimer’s by 40 per cent, 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 think the drugs may reduce brain inflammation that allows the build-up of plaque that leads to the disease.
But not everyone is convinced of cancer’s apparent protective effects – nor that drugs for the disease could be a breakthrough in dementia treatment.
‘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 the risk, but others have very different results.
‘In breast cancer, for example, some research suggests an increase in dementia risk among survivors.’
One theory is that both conditions are associated with tissue inflammation that may promote disease.
But Professor Kehoe is adamant that the links are worth investigating in the hope that it might lead to new and highly effective dementia treatments.
‘I believe there’s no smoke without fire,’ he says. ‘I’m quite convinced that something is going on here.’
