An invitation to interview is confirmation that you're considered a competitive candidate for a residency position. However, your work is just starting. You'll need to be extremely thorough and diligent in your interview preparation to maximize your chances of success.
In the most recent NRMP Program Director Survey of radiation oncology residency programs, "interactions with faculty during interview" and "interpersonal skills" were the two most important factors for ranking applicants. These two factors both were given a mean importance rating of 4.9, out of 5.0 (scale from 1 [not at all important] to 5 [very important]).