There are several reasons why cancer is so difficult to treat, but the main one is simply that cancer cells is the patients own cells that have a couple of muta

There are several reasons why cancer is so difficult to treat, but the main one is simply that cancer cells is the patients own cells that have a couple of mutations, so most things that kill cancer cells also kill healthy cells. Thus successful cancer treatments are those who kills the cancer cells, but only almost kill the patient. The other main reason why cancer treatment is difficult is that there are many different combinations of genes that can mutate and cause cancer, so that even the same cell type can get cancer several different ways. There are at least six different kinds of breast cancer for example, where a drug effective against one can be totally ineffective against another, and this is the case for a lot of cancer types. Thus cancer is not one decease, but hundreds of different deceases, each requiring different treatment. It is quite amazing that more than half of those getting cancer treatment actually get cured today.