Back in the day it was not nearly as difficult for FMGs to become certified to practice medicine in the US. Now it is extraordinarily difficult (excepting Canada as stephew said, but getting a RadOnc residency there is rough for Canadian citizens).
Even if you are a world-reknowned expert in field "X" you will have to redo an entire residency plus pass Step I-III to practice medicine in the US. One interesting example I have run into over the course of my training (though I know of many more):
One of my classmates was an head and neck surgeon in Vietnam and practiced for about 7 years until he emigrated here. He really wanted to practice medicine but realized he had to complete a residency and understood that it would ve virtually impossible to get an ENT residency as an FMG. So, he re-enrolled in medical school. While the rest of my classmates (and me!) were fumbling with patients during the 1st two years, he seriously had an attending level understanding (obviously) of patient care. Interestingly, he ended up going into EM.
A lot of FMG RadOncs would love to get an attending level position here. The chances of successfully doing so are very, very low. Sometimes they are "stuck" in a kind of a fellowship position for years trying to convince the powers that be to give them a residency slot. What usually happens is that the hospital is able to "farm" them for their attending-level expertise while paying them crap.