For an interesting discussion of cancer as metaphor (not only war, but also roller coaster and others) by a panel of oncologists, see
theoncologist.alphamedpress.org/content/9/6/708.full.
War metaphors for the fight against deadly disease of all sorts, not only cancer, are age old. On YouTube you can view a newsreel from 1961 titled "War on Polio!":
/www.youtube.com/watch?v=W88DcouLijY. Here in Micronesia, where genetics and poor diet contribute to unusually high rates of diabetes, I often hear talk of a "war on diabetes." The campaign to fluoridate water has often used the metaphor "war on cavities."
The war metaphor suggests that there's something each of us could do to prevent getting the disease. The upside is it prods people to make healthful changes in diet, exercise, etc. The downside is it shames those who contract the disease.
What makes the war metaphor especially destructive with respect to cancer is the sense that it is a disease where your own body "attacks" itself. Why can't I stop myself from killing myself? Why did my prostate go turncoat? Also, metastasis invites the invasion metaphor: what are my prostate cells doing in my rib cage? Why can't I tell them to just stop it? It's instinctive to feel that since the enemy is, in some sense, oneself, one should be able to fight it off. Or to feel "betrayed" by one's own body. I think a lot of the psychological appeal of immunotherapy is that it would allow us to heal ourselves.