Obviously for something like cancer of the white blood cells, or leukemia, surgery won't work because that cancer can't be cut out. Radiation therapy is another treatment option. Radiation can work well on a local area of cancer that either can't be treated surgically, or was treated surgically and some of the cancer remains in the tissue. An example where radiation is a good treatment choice is in a tissue that had a tumor removed but the cancer had spread to the local lymph nodes. The radiation can be focused on the lymph nodes and spare the rest of the tissues and organs.
Radiation works by harming rapidly dividing cells.
When irradiated, cancer cells can't divide and spread, so they die. Radiation is often less curative than surgery and some cancers simply don't respond to radiation treatment. Melanomas are an example of a cancer that doesn't die from radiation because it is made of pigment containing cells whose job is to absorb the sun's radiation. It's best for cancers that involve multiple sites or sites that aren't susceptible to surgery or radiation. In veterinary medicine, chemotherapy is not curative.