Chemotherapy can cause changes in brain metabolism and blood flow that last 10 years, a new study suggests, which could explain why some patients say they're more forgetful and confused after treatment.

Researchers found 16 breast cancer survivors who had been treated with chemotherapy and surgery showed lower metabolism in a key region of the brain's frontal cortex when compared with 18 women who had had no chemotherapy.

Five of the other women were breast cancer survivors who had been treated with surgery alone, and the rest had had no cancer.

"People with 'chemo brain' often can't focus, remember things or multitask the way they did before chemotherapy," said study author Prof. Daniel Silverman, head of neuronuclear imaging at University of California, Los Angeles.

"Our study demonstrates for the first time that patients suffering from these cognitive symptoms have specific alterations in brain metabolism," he added in a release.

Participants' brain metabolism was monitored using positron emission tomography (PET), which uses a special camera to follow the progress of an injected radioactive tracer.

The women who had been treated with chemotherapy five to 10 years before the study showed a lower resting brain metabolism and had more trouble performing short-term memory tests than the women had in the other groups.

The group who had had chemotherapy also showed more blood flow to the frontal cortex and cerebellum. The increased activity suggests the brains of chemotherapy patients were working harder than the control subjects' to recall information, Silverman said.

Chemotherapy used to be prescribed mainly to treat metastatic disease, but now doctors often give it to patients near surgery to prevent metastasis. As more people become long-term survivors, doctors are recognizing the lasting side-effects of chemotherapy.

At least 25 per cent of chemotherapy patients are thought to be affected by chemo brain. A recent study at the University of Minnesota reported the rate to be 82 per cent.

The findings suggest PET scans, which are already used to monitor how a patient's tumour responds to therapy, could also be added to monitor the effects of chemotherapy on brain metabolism, Silverman said.

In 2005, there were an estimated 21,800 new cases of breast cancer in Canada and 5,300 deaths, according to Canadian Cancer Statistics 2005.