Chemotherapy

Chemotherapy is used to treat cancer because cancer cells are more susceptible to radiation. This is because radiation damages DNA and cancer cells are not as capable of repairing DNA as well as normal, healthy cells. It is still a weakening procedure and can possibly lead to side effects like leukemia, but there are drugs to take that lessen the blow of the treatment.

Source: Naked Scientists Novermber 7th, 2010