Without nutrients how will the body be able to produce antibodies? White blood cells to defend you against viruses?
Anyway, about cancer cells. The body will not consume it but slow down the growth of those cells, since there are no nutrients to support it. That way you can fight it easier.
But nevertheless it is dangerous to fast for such a long time and can actually have long lasting aftereffects.
And you said it yourself, if you want to exceed 10 days you have to keep in touch with a doctor. Would you call that healthy?