Alzheimer’s disease and cancer rarely strike together concluded researchers in the US who found no such association between vascular dementia and cancer, adding weight to the idea that cancers and neurological disorders are less likely to occur together, a trend first noticed in people with Parkinson’s, and latterly in people with multiple sclerosis and Down syndrome.

The discovery was the work of Dr Catherine M. Roe, research instructor in neurology at Washington University School of Medicine in St Louis, Missouri, USA, and colleagues, and a study about it was published online on 23 December in the journal Neurology, with Roe as the lead author.

Roe told the media that they still found people who had both Alzheimer’s and cancer but to a lesser degree than they expected:

“If there truly is an inverse association, it gives us one more way of finding out what’s going wrong in both cancer and Alzheimer’s, and that could lead us to new ways to treat either condition,” she added.

Suggestions of a link between cancer and neurological disorders have been mounting for years. Researchers first noticed it in patients with Parkinson’s disease, who tend to get cancer less often, and then more recently it was suggested that people with multiple sclerosis have fewer tumors, and people with Down syndrome who live to middle age or older also show fewer incidences of cancer.

The trail that led up to this latest study began when co-author Dr Maria Behrens, now with the University of Pittsburgh School of Medicine in the US and the Hospital Clínico at the University of Chile in Santiago, noticed that fewer people in nursing homes in her native Chile seemed to have cancer.

This prompted an initial investigation six years ago in the Washington University’s Alzheimer’s Disease Research Center (ADRC), when Behrens was a postdoctoral fellow there. She, Roe and colleagues looked at data on volunteers at the ADRC and found that people with Alzheimer’s were slower to be diagnosed with cancer in the future.

For this study, Roe, Behrens and colleagues looked at data on 3,020 people aged 65 and over enrolled in the Cognition Substudy of the Cardiovascular Health Study run by the National Heart, Lung, and Blood Institute (NHLBI) of the US. For this prospective cohort study, participants were monitored for a mean of 5.4 years for dementia and 8.3 years for cancer.

Roe, Behrens and colleagues examined this data with statistical tools called Cox proportional hazards models to look for significant links in two directions. First they looked for a significant link between dementia prevalence and the risk of future hospitalization for cancer, and then they looked for a significant link between cancer prevalence and subsequent dementia diagnosis.

Their results showed that:

  • When the study began, 164 participants had already been diagnosed with Alzheimer’s and 522 with cancer.
  • During the course of the study, 478 developed dementia and 376 developed invasive cancer.
  • Compared with those who did not have Alzheimer’s at the start of the study, those who did were 69 per cent less likely to be hospitalized for cancer.
  • Caucasian people who had cancer when the study began were 43 per cent less likely to develop Alzheimer’s later on.
  • However, this was not the case in minority groups.
  • No significant links were found between vascular dementia and cancer.

The authors concluded that:

“In white older adults, prevalent Alzheimer disease (AD) was longitudinally associated with a reduced risk of cancer, and a history of cancer was associated with a reduced risk of AD.”

“Together with other work showing associations between cancer and Parkinson disease, these findings suggest the possibility that cancer is linked to neurodegeneration,” they added.

Some might say that the results were just reflecting the fact that Alzheimer’s patients were dying before they could be diagnosed with cancer: but Roe said their statistical tools adjusted for this potential confounder. She also commented that patients with vascular dementia died sooner than patients with Alzheimer’s, yet they still showed a higher risk of developing cancer than patients with Alzheimer’s.

However, the researchers were careful to point out that this study only counted cancer cases that resulted in hospitalization, so inoperable, benign, or otherwise untreatable cancers may have slipped through unnoticed.

Some experts have commented in the media that more research was needed to confirm the findings of this study.

Professor Clive Ballard, of the UK’s Alzheimer’s Society, told the BBC it was possible that Alzheimer’s was masking the symptoms of cancer, and vice versa, and thus affect diagnosis.

However, Roe said:

“If the decreased chance of cancer diagnosis was simply due to the fact that physicians don’t notice cancer in people with dementia, the decrease should have shown up for both the Alzheimer’s patients and those with vascular dementia.”

“But the drop in cancer risk was only seen in those with Alzheimer’s,” she added.

Scientists believe vascular dementia is caused by lack of blood to the brain.

Roe and colleagues are now planning to do a larger study to investigate if Alzheimer’s disease is more strongly linked to particular cancers, and if particular cancers have a large or small effect on Alzheimer’s risk.

“Cancer linked to Alzheimer disease but not vascular dementia.”
C. M. Roe, A. L. Fitzpatrick, C. Xiong, W. Sieh, L. Kuller, J. P. Miller, M. M. Williams, R. Kopan, M. I. Behrens, and J. C. Morris
Neurology, first published online on December 23, 2009.

Source: Washington University in St Louis, BBC.

Written by: Catharine Paddock, PhD