Most kinds of cancer are associated with a significantly decreased risk of Alzheimer's disease, according to a study of 3.5 million veterans reported at the 2013 Alzheimer's Association International Conference (AAIC® 2013) in Boston. In addition, the study suggested that chemotherapy treatment for almost all of those cancers conferred an additional decrease in Alzheimer's risk.

A growing body of evidence suggests a possible association of cancer with reduced risk for Alzheimer's disease; until now, whether the association differs between cancer types or is modified by cancer treatment is unknown.

Laura Frain, M.D., a geriatrician at VA Boston Healthcare System, and colleagues analyzed the health records of 3,499,378 veterans age 65 and older who were seen in the VA health care system between 1996 and 2011 and who were free of dementia at baseline. The objective was to evaluate the relationship between a history of 19 different cancers, cancer treatment and subsequent Alzheimer's disease.

Over a median follow-up of 5.65 years, 82,028 veterans were diagnosed with Alzheimer's. Twenty-four (24) percent of those veterans with Alzheimer's had a history of cancer, while 76 percent did not.

The researchers found that most types of cancer were associated with reduced Alzheimer's risk, ranging from 9 percent to 51 percent. Reduced risk was greatest among survivors of liver cancer (51 percent lower risk), cancer of the pancreas (44 percent), cancer of the esophagus (33 percent), myeloma (26 percent), lung cancer (25 percent) and leukemia (23 percent). Cancers that did not confer a reduced Alzheimer's risk, or were associated with an increased risk, included melanoma, prostate and colorectal cancers.

The researchers found no association between cancer history and reduced risk of any other typical age-related health outcome; in fact, cancer was associated with an increased risk of stroke, osteoarthritis, cataracts and macular degeneration. Most cancer survivors were also at increased risk for non-Alzheimer's dementia.

"Together, these findings indicate that the protective relationship between most cancers and Alzheimer's disease is not simply explained by increased mortality among cancer patients," said Frain. "More research is needed to determine if these results have therapeutic implications for Alzheimer's."

Among veterans with a cancer history, treatment with chemotherapy but not radiation reduced Alzheimer's risk by 20 to 45 percent, depending on cancer type, with the exception of prostate cancer.

"The potential protective effect of chemotherapy is supported by recent experimental studies," Frain observed. "The results of this study are interesting because they could help focus future research onto the specific pathways and treatment agents involved in the individual cancers that are associated with a reduced risk of Alzheimer's. This could potentially open new therapeutic strategies for Alzheimer's prevention and treatment."