Cells mutate and become cancerous all the time, but the immune system is usually able to recognize this and shut down the errant cells before it becomes a problem. It's only when the cancer gets lucky with the right site of mutations that it's able to evade the immune system and grow unchecked.

Early diagnosis would obviously be a good thing if we can reliably tell which cancers will become a problem. My concern is that most patients may have a hard time understanding this distinction, and we may end up with people demanding screenings and treatments that they'd be better off without.

In case anyone thinks these fears are unfounded, keep in mind this is already a problem with thyroid cancer. Thyroid cancer is very common and frequently harmless, but increased screening is leading to more people being treated for it, and it's unclear whether this is a net benefit.