I don't know if I can give any more advice that is more specific to rad onc, but for personal statements in general I think it is reasonable to bring up some of your accomplishments as long as it is not in a way that makes you sound arrogant. E.g. bring up your research in the context of how it strengthened your decision to pursue this field or how it was otherwise a good experience for you.
I think I have also read somewhere that there should be something in there about what you expect from a program or what it is you are looking for. The purpose of this whole process, after all, is to get the best match of applicants in programs where they will best fit.
I think the best advice you can get about personal statements is that it is better to have a bland boring one than one that makes you look like an arrogant jackass. I would be very hesitant about the very stylized "grab your attention" anecdotes and wacky stories like the ones many of us put in our med school applications. Just my opinion, I'd be interested to see if anyone cares to differ.