I have drank seven to nine cans of Mountain Dew a day for the last 15 years. Internally what health risks are there?

Bad idea. There is nothing specific that any soda can cause, but it certainly contains a great deal of caffeine, sugar, and citric acids. You should severely cut back, or even stop. Likewise, there are a lot of calories in drinks like that.
Possibly a few. It depends on what this rest of your diet, lifestyle and health is like, but any soda can cause weight gain which leads to obesity, diabetes, high blood pressure. Its also bad for your tooth enamel. In childhood and adolescence, it had been shown to decrease bone density, possibly because if the phosphate but also possibly because kids drink it instead if milk.
OH MY GOSH! Regularly drinking soda has been linked to Non-Alcoholic Fatty Liver Disease, Diabetes, heart disease & metabolic syndrome. Other possible risks include: increased weight, decreased bone mineral density (BMD) obesity, erosion of tooth enamel, mineral depletion & dehydration. If it contains artificial sweeteners & is used daily, this is linked to a 3 fold ^ in dementia & stroke.