Cancer and Dementia: It's Complicated.

The relationship between dementia and cancer is complex. A wealth of observational data suggest (1) reduced risk of certain cancers in Alzheimer and Parkinson diseases; and (2) increased risk of other cancers in Parkinson disease. These relationships persist despite correcting for reporting artifacts and survival bias. Several potential mechanisms have been proposed and warrant further investigation. Aging is a risk factor for both. Common environmental exposures, such as smoking, may play roles. Common mechanisms such as chronic inflammation and immunosenescence, and common risk factors such as diabetes and obesity, have been implicated. Shared genetic pathways are a major focus, particularly those favoring apoptosis and cell proliferation at opposite ends of the spectrum. To complicate the picture further, certain cancer chemotherapy and adjuvant therapy agents have neurotoxic effects, whereas animal studies show other cancer drugs reducing neurodegeneration, raising the possibility of repurposing those agents for use in Alzheimer disease. These multiple potential lines of evidence must be disentangled to investigate underlying mechanisms, the end-game being to develop and to test potential prevention and treatment strategies.