Actually, radiation oncology isn't all that new (depends upon your definition of "new," I guess). Radiation oncologists use radiation to treat patients with cancer (and with a few other, non-cancerous diagnoses). Sometimes this is done with curative intent, other times with only palliative intent. The Radiation Oncologist works closely with the medical oncologist and the surgeon, as 2-3 of these modalities are used on many cancer patients.
The concept of therapeautic radiation is certainly not new (~100 years old). For years, therapeautic radiation was considered a sub-category of radiology. Radiology and Radiation Oncology split sometime in the 1970s or so, and since then Radiation Oncology has been a separate residency. There are some very well known centers (MD Anderson, Memorial Sloan Kettering, and Stanford, to name a few), but there are radiation oncology facilities available at more hospitals than you would think (my home town of 18,000 has facilities available at their hospital). Hope this answers your question.