Have you ever heard of any cases that a doctor ordered radiation on the brain just as a preventative measure if there's no cancer in the brain?

Sometimes: Low dose preventative brain radiation is used in small cell lung cancer and some kinds of leukemia which often spread to the brain. The blood-brain barrier keeps chemo out of the brain, so it is a place where the cancer can hide from it and grow. Brain radiation can kill these cells and has been shown to minimize the risk of brain tumors developing in these patients and extend life.

Never.: Radiation therapy is never done as a preventive measure.

Yes in certain cases: Prophylactic cranial irradiation is recommended in small cell lung cancer as preventative measure- in limited stage as well as in the extended stage providing that the rest of the cancer is stable/improved with chemotherapy.