 There's a new report out from the American Cancer Society showing... As many as 40% of cancer cases and up to half of cancer-related deaths could have been prevented. The study identifying modifiable risk factors for 30 types of cancers in adults. The biggest factor, cigarette smoking, which accounted for 30% of cancer deaths and 20% of cancer cases. Excess body weight came in second, followed by UV radiation and physical inactivity.