The study looked at 21 breast cancer patients who underwent surgery five to 10 years ago. Of the group, 16 women were in chemotherapy following the operation to prevent the cancer from returning.
The University of California, Los Angeles (UCLA) researchers scanned the women's brains while they took focus and memory exercises.
"People with 'chemo brain' often can't focus, remember things or multitask the way they did before chemotherapy," said Daniel Silverman, head of neuro-nuclear imaging and associate professor of molecular and medical pharmacology at UCLA.
"Our study demonstrates for the first time that patients suffering from these cognitive symptoms have specific alterations in brain metabolism and blood flow”, he said.
At least one in four breast cancer patients who undergo chemotherapy have symptoms of mental confusion, the researchers said.
A recent University of Minnesota study puts the rate at 82 percent, the UCLA researchers said in their study published in the online journal Breast Cancer Research and Treatment.