Women treated for breast cancer with radiation with or without chemotherapy had more thinking and memory problems a few years after their treatment ended than women who'd never had cancer, in a new study.

Research has suggested some women experience mental haziness, dubbed "chemo brain," during and soon after chemotherapy treatment. And one recent study found evidence of changes in the activity of certain brain regions in women who'd undergone chemotherapy.

But some researchers have questioned whether those problems are due to the specific drug treatments, or possibly to the cancer itself. In the new report, breast cancer survivors showed certain small mental deficits, regardless of whether or not they'd had chemotherapy.

"It's a very, very subtle thing. We're not talking about patients becoming delirious, demented, amnesic," said Barbara Collins, a neuropsychologist who has studied chemotherapy-related cognitive changes at Ottawa Hospital in Ontario, Canada, but wasn't involved in the new study.

"We're talking about a group of people that are saying, 'I'm pretty much still able to function, but I find it harder...it doesn't come as easily, and I can't do as many things at the same time.'"

The current study involved 129 breast cancer survivors in their fifties, on average. About half of them had been treated with radiation and chemotherapy, while the other women only had radiation.

Six months after finishing treatment, and another three years later, women took a range of thinking and memory tests. Their scores were compared against the performance of 184 women who'd never had cancer, but were a similar age and from the same areas.

On three out of five types of memory tests, women who'd had either course of treatment performed similarly to the non-cancer group. But on two, their scores were noticeably lower.

At both six months and a few years after treatment, cancer survivors scored worse on tests of "executive functioning," which included naming words beginning with a particular letter.

And on tests of processing speed, which included marking specific numbers on lists of random numbers and letters -- a measure of speed and concentration -- women who'd received radiation only or chemo and radiation had lower scores than women with no cancer history at the later time point.

Those scores differed by about one to three points on a scale where 50 is considered average.

One limitation of using tests to measure cognition is that it's not clear how exactly they apply to functioning in everyday life, Paul Jacobsen, from the Moffitt Cancer Center in Tampa, Florida, and his colleagues wrote Monday in the journal Cancer.

The researchers also didn't have information on women's thinking and memory skills before they were diagnosed with cancer or treated.

Cancer survivors who'd had radiation without chemotherapy scored similarly to those who were treated with radiation and chemo on all measures of mental ability.

That challenges the notion that chemotherapy is the driving force behind mental changes in breast cancer survivors, researchers said.

"People talk about 'chemo brain,' and there's sort of a general view that if people have cognitive problems after the cancer treatment, it must be due to the fact that they had chemotherapy," Jacobsen told Reuters Health.

"We provided the most definitive evidence to date to suspect it's not just chemotherapy that is contributing to cognitive problems after breast cancer."

What exactly might be the cause, or causes, is still up for debate.

"There is very likely something to do with having cancer that already affects your cognitive function," Collins said. "What is it? Could it be stress? Could it be anxiety? Could it be depression? That's a possibility."

It could also be that the immune system's response to cancer affects the brain, she added.

Collins said that most of the data still points to some mental effect of chemotherapy in certain patients -- but that small differences between treatment groups might have been missed in this analysis.

Still, she said, "We can't be too quick to conclude, even if we find some subtle things, that they're all due to the chemotherapy. We have to step very carefully here in terms of understanding what the real factors are."

Collins told Reuters Health that women should know foggy thinking and memory after cancer treatment tends to improve over time. "Nobody's suggesting they don't get their chemotherapy, not at all," she said.

Many women won't notice any mental fuzziness after treatment at all, Jacobsen added, but he said those that do should talk to their doctors to rule out other causes and to consider strategies to compensate for those problems.