Comment by jrpt

Because cancer isn't a parasite. They are your own cells, just with various mutations. A parasite is a foreign living organism that lives off of another, for example, a parasitic worm. Anyone can easily confirm cancer cells are genetic mutations to your own by doing a genetic test - and in fact this is done regularly, it's called a germline and somatic genetic test, which can help identify if certain drugs would work well on the cancer. This is a form of personalized medicine.