It depends on where you are trying to get on …
In Australia, it helps to know people who work in Radiation Oncology in the state you are trying to apply in, to know what radiation oncology is and how it interacts with other specialities, and I also believe it helps to have a well rounded experience as a resident although whether this helps to get on the program or not is questionable.
Good references from resident jobs is helpful as well. If you could get a term as a resident in radiation oncology and get referees from that term it would be beneficial. I don't think more than one term would be helpful.
I don't know how it works in other countries.
Sorry about the extremely late reply, I must have missed your post while I was studying for exams :)