Of some relevance is an Isaac Asimov essay titled The Enemy Within, which describes, in summary
[ul][li]DNA always contains certain elements; carbon, phosphorus, oxygen, etc.[/li][li]Some small but measurable percentage of these elements in nature consists of unstable isotopes[/li][li]Given the number of DNA molecules in a human body, some percentage contain unstable elements[/li][li]Over a human lifetime, there will be a small but steady number of breakdowns where an isotopic atom within a DNA molecule collapses, breaking the molecule and creating a mutation[/li][li]The vast majority of these fragmented mutations are nonviable and quickly removed, but some could be precancerous[/ul][/li]
Hence, an external agent isn’t necessary to cause mutations/cancer. Each of us is carrying billions of tiny time-bombs and while the vast majority explode unnoticed, some… don’t.