Increasing the N. Used the same PS and letters, got plenty of interviews. From the feedback at the prelims/TYs, it seemed they offered me interviews based off three factors: performance on IM rotation, scores, and publications (oddly enough; that is. the number of pubs etc, rather than the actual strength of the pubs (like radonc programs cared about)). If you have competitive scores and did well on your IM rotation, you will get plenty of interviews using the same PS and letters. If you don't, ask your IM chair or PD to email a particular program or two for you, write a prelim/TY specific ending to the PS, and you'll do fine. You're likely very competitive with these programs if you're applying to radonc.
Definitely do not tailor PSs to individual programs - you may scare away some with tactic. If you really want one in particular, ask a chair or PD to email them, or email the student coordinator yourself. Good luck.