Do they use radiation to treat leukemia, even though radiation also causes leukemia?

Yes, in some cases. Radiation can be used to treat leukemia in certain situations. It can be used as part of the leukemia treatment in preparation for a stem cell transplant. It can also be used to treat leukemia that has spread to the spinal fluid or formed a mass of leukemia cells outside the bone marrow.
Not really. Pure leukemia's are not treated with radiation , but its complications are may be treated with radiation like when brain , spinal cord involved.Or reduce the size of spleen, for large lymph nodes , joint pains etc, primary treatments is chemotherapy.