Paul Jacobsen of the Moffitt Cancer Center and Research Institute in Tampa, Fla., and colleagues examined 62 breast cancer patients treated with chemotherapy plus radiation, 67 patients treated with radiation only, and 184 women with no history of cancer.
Study participants completed neuropsychological assessments six months after completing treatment and again 36 months later, which is further out from the end of treatment than most previous studies of this type.
The study, published online ahead of print in the journal Cancer, confirmed chemotherapy can cause cognitive problems in breast cancer survivors that persist for three years after they finish treatment.
However, the study also found that breast cancer survivors who had been treated with radiation, and not chemotherapy, often experienced problems similar to those in breast cancer survivors treated with both chemotherapy and radiation.
The study did not find that hormonal therapy such as tamoxifen caused cognitive difficulties.
"These findings suggest that the problems some breast cancer survivors have with their mental abilities are not due just to the administration of chemotherapy," Jacobsen said in a statement.
"Our findings also provide a more complete picture of the impact of cancer treatment on mental abilities than studies that did not follow patients as long or look at mental abilities in breast cancer survivors who had not been treated with chemotherapy."