I'll assume you're asking in good faith. You don't have to prevent mutations or scan every cell in the human body to "solve" cancer. You can:

- make detection easier and cheaper (e.g. with less invasive biopsies)

- make treatment easier and cheaper (e.g. more precise targeted treatments, making chemo a last resort instead of the only option)

Among other things. There's even emerging research that certain types of cancers could be "vaccinated" against: https://www.nature.com/articles/d41586-026-02612-3

It means being able to stop any uncontrolled cell replication.