Well, you probably know that you have an uphill battle in front of you. Very few IMG's are able to obtain spots in radiation oncology in the US.
I would think you will need exceptional scores on your USMLE steps to even be considered. Beyond that, I think the best things you could do to improve your chances would be:
1) Get an MD/PhD..perhaps consider a PhD program in the US after your MD degree. PhD's are a sought-after commodity in this field.
2) Rotate through several radonc departments in the US to make contacts. Like everything else in this world, it's who you know. You may be able to impress a program enough to overcome the IMG stigma.
Best of luck to you! I'm an optimist & think it is possible for you..but you have to know how difficult it will be. If you want it bad enough, you'll find a way in. ND.