@turquoise-- I think all cancer treatments are called cytotoxic treatment. It's basically drugs that help kill cancer cells.
I don't think that these would cause typhlitis directly, but more indirectly. I agree that it must be because of the drugs' affects on the immune system. Even though these drugs are meant to kill cancer cells, inevitably they affect normal cells too. Every medical treatment comes with consequences in my opinion.
I don't know if antibiotics could prevent infections from cytotoxic treatment. I guess it wouldn't hurt but patients can't keep taking antibiotics forever. Treatments like chemotherapy can take months and years.