"Chemo brain is part of the language now, and just to have it acknowledged makes a difference," said Anne Grant, 57, who owns a picture-framing business in New York City. Ms. Grant, who had high-dose chemotherapy and a bone marrow transplant in 1995, said she could not concentrate well enough to read, garbled her sentences and struggled with simple decisions like which socks to wear.
Virtually all cancer survivors who have had toxic treatments like chemotherapy experience short-term memory loss and difficulty concentrating during and shortly afterward, experts say. But a vast majority improve. About 15 percent, or roughly 360,000 of the nation's 2.4 million female breast cancer survivors, the group that has dominated research on cognitive side effects, remain distracted years later, according to some experts. And nobody knows what distinguishes this 15 percent.
Most oncologists agree that the culprits include very high doses of chemotherapy, like those in anticipation of a bone marrow transplant; the combination of chemotherapy and supplementary hormonal treatments, like tamoxifen or aromatase inhibitors that lower the amount of estrogen in women who have cancers fueled by female hormones; and early-onset cancer that catapults women in their 30s and 40s into menopause.
Other clues come from studies too small to be considered definitive. One such study found a gene linked to Alzheimer's disease in cancer survivors with cognitive deficits. Another, using PET scans, found unusual activity in the part of the brain that controls short-term recall.
The central puzzle of chemo brain is that many of the symptoms can occur for reasons other than chemotherapy.
Abrupt menopause, which often follows treatment, also leaves many women fuzzy-headed in a more extreme way than natural menopause, which unfolds slowly. Those cognitive issues are also features of depression and anxiety, which often accompany a cancer diagnosis. Similar effects are also caused by medications for nausea and pain.
Dr. Tim Ahles, one of the first American scientists to study cognitive side effects, acknowledges that studies have been too small and lacked adequate baseline data to isolate a cause.
"So many factors affect cognitive function, and the kinds of cognitive problems associated with cancer treatment can be caused by many other things than chemotherapy," said Dr. Ahles, the director of neurocognitive research at Memorial Sloan-Kettering Cancer Center in New York.
The new interest in chemo brain is, in effect, a testimony to enormous strides in the field. Patients who once would have died now live long enough to have cognitive side effects, just as survivors of childhood leukemia did many years ago, forcing new treatment protocols to avoid learning disabilities.
"A large number of people are living long and normal lives," said Dr. Patricia Ganz, an oncologist at U.C.L.A. who is one of the nation's first specialists in the late side effects of treatment. "It's no longer enough to cure them. We have to acknowledge the potential consequences and address them early on."