Cancer survivors may be at risk of problems with mental abilities such as memory and learning, research suggests.
Chemotherapy is toxic
It found those who had undergone cancer treatment were twice as likely to develop cognitive problems than people who had never been treated for cancer.
The University of Southern California team say it is possible chemotherapy damage may be to blame, but stress more research is needed.
The study features in the Journal of the National Cancer Institute.
The possibility that cancer, and its treatment, may be linked to cognitive dysfunction has been raised by previous studies.
But these concentrated on the short term impact of cancer - the latest study examined the longer-term effect.
It focused on 702 people who had survived following cancer treatment, and their cancer-free twins.
About 15% of the cancer survivors showed signs of cognitive dysfunction.
Worse over time
The researchers said the findings raised the possibility that cognitive problems among cancer survivors got worse over time.
They argued that comparing cancer survivors with their cancer-free twins means the increased dysfunction cannot be attributed to the normal aging process.
The study does not suggest a cause for the cognitive problems in cancer survivors.
The researchers say it is possible chemotherapy or other cancer treatments may cause long-term damage.
However, it is also possible that the cognitive impairment was present before treatment began.
The researchers also accept that factors such as alcohol consumption and a sedentary lifestyle, which increase the risk of cancer, also increase the risk of cognitive decline.
They plan a follow-up study comparing survivors who received different treatments.
Researcher Professor Margaret Gatz said: "Those with cancer might also be advised to have their cognition monitored as part of long-term follow-up."
In an editorial in the same journal, experts from the University of Texas say there is some evidence that cancer patients experience a short-term effect on cognitive function.
But they say there is little firm evidence that patients are at long-term risk of developing problems.
Professor Lesley Fallowfield, of Cancer Research UK, said: "Although this is an interesting study, the accompanying editorial does urge a cautious interpretation of the results.
"It's worth remembering that we are all at risk of dementia as we age, and the actual increase in risk is fairly small.
"Around 15 per 100 cancer survivors suffered dementia compared with nine per 100 non-cancer twins.
"This is certainly not a reason for cancer patients to panic and refuse treatment.
"Research into the way in which some treatments might impair intellectual activity is still at an early stage."