WASHINGTON — Cancer patients often wonder “why me?” Does their tumor run in the family? Did they try hard enough to avoid risks such as smoking, too much sun or a bad diet?

Lifestyle and heredity get the most blame, but new research suggests random chance plays a bigger role than people realize: Healthy cells naturally make mistakes when they multiply, unavoidable typos in DNA that can leave new cells carrying cancer-prone genetic mutations.

How big? About two-thirds of the mutations that occur in various forms of cancer stem from those random copying errors, researchers at Johns Hopkins University reported Thursday in the journal Science.

That doesn’t mean most cases of cancer are caused solely by “bad luck.” It takes multiple mutations to turn cells into tumors – and a lot of cancer is preventable, the Hopkins team stressed, if people take proven protective steps.

Thursday’s report is an estimate, based on a math model, that is sure to be hotly debated by scientists who say those unavoidable mistakes of nature play a much smaller role.

But whatever the ultimate number, the research offers a peek at how cancer may begin.

And it should help with the “why me” question from people who have “done everything we know can be done to prevent cancer but they still get it,” said Hopkins’ Dr. Bert Vogelstein, a pioneer in cancer genetics who co-authored the study. “They need to understand that these cancers would have occurred no matter what they did.”