The radiation is meant to stop the cell division completely. In order for this to be successful, the tumor's cells must be dividing at the given time. The benefit is that radiation can often destroy dividing cancer cells without harming normal cells of the body. However, this is not always the case. Sometimes body cells are affected directly from the radiation, like cells of the ovaries or testes which can cause sterility.
Chemotherapy is similar to radiation.
Drugs are given to the patient in order to disrupt cell division. Side affects often occur, as well. However, it has been found that radiation therapy, along with chemotherapy, is the most effective way to treat a cancer patient.