there's not a quantifiable, exact answer to this question. In a med school orientation meeting, we saw a study on program directors and what they considered "very, somewhat, little, or not at all" important; I think 150 or so program directors responded, and the answers were across the board.
the most important areas, on average, were clerkship grades (MS3 and possibly MS4, to the extent MS4 clerkships are complete prior to residency interviews) and USMLE step 1 scores. Grades were important to some, not important to others. Other than that, who knows; you can play the "all else being equal" game, but that always seemed silly to me. (all else being equal, if I'm wearing the same brand shoes as the interviewer maybe I'd have a leg up on the competition!).
That said, if you've never shown any interest, or done any research, or shadowed any physicians, or obtained any letters of reference from any radonc docs and are applying to a residency in that field, I cannot imagine the interviewer would think you have solidly planned for a career in rad onc.