I found an article in today's Wall St. Journal titled "Chemo Brain" May Be Caused By Cancer Itself. I found it interesting as I've noticed some cognitive changes in both of my parent's since being treated for cancer.

Researchers studying the phemomenon of "chemo brain" have learned that a small number of patients suffer memory and cognitive problems that may be caused by cancer itself.
The study from University of Texas - M.D. Anderson Cancer Center in Houston is the latest evidence showing that complaints by cancer patients of memory and thinking problems are a real concern. Many doctors have dismisssed such complaints as being due to anxiety, age or even menopause.
But while there has been growing evidence that chemotherapy treatment may trigger cognirtive declines or "chemo brain" in both male and female patients, the latest study, released today by the journal of cancerf, shows that the cancer itself may play a role for some patients. Among 84 breast cancer patients studied, 35% showed cognitive impairment prior to treatment, far higher that what would be expected in an average population. Whether the impairment was directly related to the cancer of how cancer might affect cognitive function isn't known.
It's important to note that the study doesn't debunk the notion oc chemo brain. The same researchers found that, regardless of a patient's cognitive scores prior to treatment, 61% declined from that baseline after chemotherapy. And 46% of women were shown to be "false negatives" for chemo brain.. That means they showed a decline in cognitive function but their final test scores still fell into the "normal" range.
As a result, the satest study shows that to truly understand the cognitive impact of both cancer and chemotherapy in an individual patient, it's not enough to teat a patient after treatment. Instead, for patients concerned about such problems, a full neuro-psychological evaluation should be performed before treatment starts.
"Chemo brain" is real, says Christian A. Meyers, professor of neuropsychology at M.D. Anderson and study co-author. "But this study shows you might misattribute poor performance to chemo when it didn't occur, and you might miss a true chemotherapy-related decline when it does occur. That's why looking at people before they start treatment is important."
For most patients, the risk of cognitive problems after chemo isn't significaant enough to turn down treatment. But cognitive testing both before and after treatment can help calm patient fears about dementia or Alzheimer's disease.