Answers: Its the aspartame (sp) which is the artificial sweetener not only within diet soda but in a great deal of diet or 'sugar free' items. I believe that it was proven surrounded by lab rats at massive doses that the avg human would never come close to consuming in their duration time.
I encourage you however to google for the information and read reliable information. When you ask question like that around here sometimes general public tend to answer who dont really actually know (and I will be honest I am not completly positive). You swot much more and will know for sure if you look up the info yourself rather than taking peoples word stale the internet.
Look for articles from reliable medical journals and reliable word sources. Check their sources, if they are reliable they will list other sources. Also, check the date. With the medical advances the more recent the info the more accurate
I don't know if it can motive cancer, but I do know that aspartane is bad for you. Slowly poisons you.