Among the many unpleasant side effects of chemotherapy treatment, researchers have just confirmed another: chemo brain. The term refers to the mental fog that chemotherapy patients report feeling during and after treatment. According to Jame Abraham, a professor at West Virginia University, about a quarter of patients undergoing chemotherapy have trouble focusing, processing numbers, and using short-term memory. A recent study points to the cause. The study relied on PET (positron emission tomography) brain scanning to examine brain blood flow, a marker for brain activity. Abraham and colleagues scanned the brains of 128 breast cancer patients before chemotherapy began and then 6 months later. The results showed a significant decrease in activity in regions responsible for memory, attention, planning and prioritizing. The findings aren't immediately useful for treating or preventing the condition of chemo brain, but the hard and fast evidence may comfort those experiencing chemo-related forgetfulness. And luckily chemo brain is almost always temporary: patients' mental processing generally returns to normal within a year or two after chemotherapy treatment ends. Image courtesy of Radiological Society of North America