The discovery could explain the mental fog and confusion that affect many cancer survivors.
Researchers from the University of California, Los Angeles (UCLA) found that women who had undergone chemotherapy five to 10 years earlier had lower metabolism in part of the frontal cortex in the brain.
Women treated with chemotherapy also showed a spike in blood flow to the frontal cortex and cerebellum while performing memory tests, indicating a rapid jump in activity level, the researchers said in a statement about their study.
"In effect, these women's brains were working harder than the control subjects' to recall the same information," said Daniel Silverman, the UCLA associate professor who led the study.
Experts estimate that at least 25 per cent of chemotherapy patients are affected by symptoms of confusion, so-called "chemo brain", and a recent study by the University of Minnesota reported an 82 per cent rate, the statement said.
"People with 'chemo brain' often can't focus, remember things or multi-task the way they did before chemotherapy," Silverman said.
"Our study demonstrates for the first time that patients suffering from these cognitive symptoms have specific alterations in brain metabolism."
The study, published on Thursday in the online edition of Breast Cancer Research and Treatment, tested 21 women who had surgery to remove breast tumours, 16 of whom had received chemotherapy and five who had not.
The researchers used positron emission tomography (PET)scans to compare the brain function of the women.
The scans create an image of sections of the body using a camera that follows the progress of an injected radioactive tracer.
They also compared the scans with those of 13 women who had not had breast cancer or chemotherapy.
Researchers used the scans to examine the women's resting brain metabolism as well as the blood flow to their brains as they did a short-term memory exercise.
Silverman said the findings suggested that PET scans could be used to monitor the effects of chemotherapy on brain metabolism.
Since the scans are already used to monitor patients for tumour response to therapy, the additional tests would be easy to add, he said.
Breast cancer is the most common cancer among women, with about 211,000 new cases diagnosed each year, the statement said.