[nonbel]

Since this is a screening test, I don't think that 15 million number is correct for prevalence.

You are using % of population diagnosed with cancer. The correct value would be the % of population with undiagnosed cancer.

[carlmr]

True. That is really difficult to obtain though. But assuming that most people are eventually diagnosed with cancer (even if just close to death), then a similar maybe a little lower population must have undiagnosed cancers.

In the end you'll need a properly conducted study with random undiagnosed people to get reliable numbers though.

[nonbel]

It seems like pretty basic info to me. There should be some estimate out there somewhere, especially given the billions of dollars spent each year on studying cancer. A quick search didn't lead to anything promising though.