Can doctors use radiation therapy on the whole body to kill small cancer metastases?

No: The only setting where total body radiation is used is in the context of preparing a patient for a bone marrow transplant. Radiation is really a local treatment that is most commonly used to treat a particular mass or region but not so much the whole body.

Yes: It has been more common in the past to use whole body radiation to perform preparation of patient for bone marrow transplant. But these days chemotherapy is more common and effective. Whole body skin radiation can be used in widespread t cell lymphomas but there are easier alternatives. For most cancers whole body radiation is not feasible or advisable.

Not used anymore: But it used to be tried as part of lymphoma therapy as far back as the 1930's in very low doses. It was never proven to be beneficial. Half-body radiotherapy was used to treat pain in prostate cancer in the 1980's. Both have fallen out of favor because there are alternative better ways with fewer side effects.