Aspartame is an artificial sweetener consumed by hundreds of millions of people worldwide. It is used in over 6,000 diet products including soft drinks, chewing gum, candy, desserts, yogurt as well as in pharmaceuticals, in particular, syrups and antibiotics for children. In 2005, the European Ramazzini Foundation published important experimental data demonstrating the carcinogenicity of aspartame. These data demonstrated for the first time that aspartame is a carcinogenic agent, inducing various types of malignant tumors in rats, even at dose levels currently considered acceptable for humans. [Source]
On an aside, perhaps rats are simply predisposed to get cancer.