There's a good reason to believe that most cancers are mutated sufficiently from human cells, that things harmless to humans can target the cancer cells. Not to mention their higher metabolism rendering them more vulnerable to toxins and such.

It's why Ivermectin and anti-parasitic treatments are often claimed to fight cancer, likely because they're dissimilar enough to be effected. In this regard, cancer could be seen as sporadically mutated parasite.