Since radiation exposure causes cancer, why do cancer patients get treated with radiation?

Simply stated, radiation kills cells. In specific types of cancer therapy, radiation is used to kill cancerous cells. It also may damage normal cells, but they are usually able to recover more fully than cancerous cells.

Specific types of ionizing radioactivity are used in therapy, and generally they are tailored not to induce further cancer in a patient.

Check out: http://www.cancer.gov/CANCERTOPICS/FACTSHEET/THERAPY/RADIATION