Hypothesis:

Mutations occurr in the spontaneous parasite containment organ in order to stymy the parasite's adaptation to it's human/mammalian host.

Radiation and smoking "cause" cancer indirectly by retarding the body's natural defenses against parasitical infection.

Cancer grows without limit because the infection has already advanced to the point where containment is the only option, and even that can fail to save the host given enough time.