There has been speculation diet pop with asparatame is an issue. Researchers are finding people who drank a lot of diet soda's are coming down with PC at higher levels.
Source?
One early study suggested that an increased rate of brain tumors in the US during the 1980s might have been related to aspartame use. However, according to the National Cancer Institute (NCI), the increase in brain tumor rates actually began back in the early 1970s, well before aspartame was in use. And most of the increase was seen in people age 70 and older, a group that was not exposed to the highest doses of aspartame, which might also make this link less likely. Other studies have not found an increase in brain tumors related to aspartame use.
In the largest study of this issue, researchers from the NCI looked at cancer rates in more than 500,000 older adults. The study found that, compared to people who did not drink aspartame-containing beverages, those who did drink them did not have an increased risk of lymphomas, leukemias, or brain tumors.
What expert agencies say
Expert agencies in the United States and elsewhere that have evaluated aspartame have found it safe for use.
http://www.cancer.org/Cancer/CancerCauses/OtherCarcinogens/AtHome/aspartame