Chemotherapy promotes a short-term, but apparently reversible, shrinkage of key brain areas, new research shows.

These changes could explain the impairment of thinking, memory, and focus that many cancer patients complain of after treatment, a Japanese research team has found.

The changes are marked by a temporary dimunition of certain brain areas that help people concentrate, plan, problem-solve, execute, and remember. This shrinkage can bring on a general cognitive malaise often called "chemo-brain."

However, these reductions in brain matter were no longer evident three and four years after chemotherapy, the Japanese team reported Monday in the online edition of Cancer.

"These findings can provide new insights for future research to improve the quality of life of cancer patients," concluded a team led by Dr. Masatoshi Inagaki of the Research Center for Innovative Oncology, part of the National Cancer Center Hospital East in Chiba, Japan.

The current study both supports and contradicts prior research into chemo-brain.

For example, a study released last month by researchers at the University of California, Los Angeles, suggested that chemo-brain is linked to brain blood-flow changes that can endure for a decade or more.

The UCLA findings also suggested that anywhere from 25 percent to 80 percent of breast cancer patients who undergo chemotherapy are subject to chemo-brain. The condition is poorly understood and is often accompanied by a range of other chemo side-effects, such as gastrointestinal disturbances and weakened immunity.

Chemotherapy has greatly improved cancer survival rates in recent years, however.

So, to better understand the treatment's negative implications, the Japanese team analyzed three years of MRI scans from breast cancer survivors who received follow-up care at the Chiba hospital.

The women were between 18 and 55 years of age. None had experienced recurrent breast cancer or had a history of any other type of cancer. As well, none of the patients was still undergoing chemo at the start of the study, and none had a family history of dementia.

Over 100 patients underwent an initial MRI brain scan one year after cancer surgery. About half of this group had also undergone chemotherapy.

According to the researchers, patients who had received chemotherapy had smaller brain volumes in areas that control cognitive function, compared to those who had not been exposed to chemo.

However, imaging taken at the 3-year mark from 130 patients showed no remaining brain size differences whatsoever.

The authors stressed that cancer, on its own, did not explain the reductions in brain volume. Cancer patients often displayed brain volumes that were similar to healthy controls, they said.

Instead, the observed short-term changes seemed linked to chemo and not to malignant disease, they said.

Inagaki's group cautioned that their finding is just an observed association and does not confirm a cause-and-effect relationship between chemotherapy and brain changes. They called for additional MRI imaging to further investigate the issue.

Dr. Claudine Isaacs, an associate professor of medicine and the director of the Clinical Breast Cancer Program at Georgetown University in Washington, D.C., described the findings as "encouraging."

"The problem with chemo-brain is that it is often hard to tell what it is related to, because there are so many factors involved -- chemotherapy, the medication that goes with it, the fatigue, and everything else that goes along with a diagnosis of cancer," she noted. "They all play in together."

"So, although this study is relatively quite small, it is a good attempt to look at ways -- with MRI, functional PET scans -- of trying to get a better handle on a real phenomenon in a structural kind of way," Isaacs said.

"But we need to be careful," she cautioned, "because we still don't have the perfect study yet. So we really can't tell patients exactly what the parameters are at this point."