Chemotherapy produces long-term changes in the part of the brain dealing with memory, US research suggests.
Patients treated with chemotherapy can suffer from memory problems
Scans of women treated for breast cancer five to 10 years earlier suggested the brain was having to work harder during memory tests.
Researchers said the findings may explain why some cancer survivors suffer from problems with memory, confusion, and concentration.
The study of 21 women is published in Breast Cancer Research and Treatment.
But a Cancer Research UK research fellow said psychologists had looked at British women having the most common breast cancer treatments and found few had experienced memory or attention problems.
Researchers from the University of California, Los Angeles (UCLA), used a technique called positron emission tomography to take images of key regions of the brain while participants performed memory-related tasks.
They compared the results from 16 women who had received chemotherapy with five women who had been treated with surgery alone, and 13 healthy controls.
Images taken during a short-term recall test showed a significant increase in blood flow in a region called the frontal cortex in women who had been treated with chemotherapy, indicating a rapid jump in activity.
The women who had undergone chemotherapy also had lower levels of another measure of brain activity that looked at how the brain uses sugar to make energy, called brain metabolism.
Study leader Dr Daniel Silverman, who is associate professor of molecular and medical pharmacology at UCLA, said: "In effect, these women's brains were working harder than the control subjects' to recall the same information.
"We also found that the lower the patient's resting brain metabolism was, the more difficulty she had performing the memory test."
He said experts estimated at least a quarter of patients treated with chemotherapy were affected by symptoms of confusion, had difficulty in focusing and could not multi-task as they used to.
The study also looked at drug-specific effects in patients who had received tamoxifen. There was a significant drop in metabolism in a part of the brain that links thoughts and actions.
"Our study demonstrates for the first time that patients suffering from these cognitive symptoms have specific alterations in brain metabolism," he added.
Dr Valerie Jenkins, senior research fellow at Cancer Research UK's Psychosocial Oncology Group at Brighton and Sussex Medical School, said: "There are many reasons why brain activity found on PET scanning might differ, apart from the effects of cancer treatments.
"The type of problems women say they experience are poor attention and concentration but other things also affect cognition including age, level of intelligence, menopausal status, and mood state, particularly anxiety.
"Cancer Research UK psychologists have looked at women who were having the most common form of breast cancer treatments in the UK and found few women experienced any problems with their memory or attention.
"Women in the UK should not panic as the benefits of current breast cancer treatment are likely to far outweigh any small impairments in attention."