There have been a couple people from SGU who have matched in Radiation Oncology in the past, but unfortunately you are correct: Rad Onc is one of the more competitive fields out there. This is where coming from a Caribbean school will really hold you back. Not saying that it can't be done, but it does tend to be more difficult.
The moderator of the Radiation Oncology forum (stephew) is now an attending at Johns Hopkins I believe (where she did her Rad Onc residency, by the way)--she is also a graduate from St. George's University. I would contact her, as she will be a more accurate source of information.
Good Luck!!
Brian
MS-III, St. George's University School of Medicine