AFAIK, ASTRO has multiple levels of membership. One is obviously for licensed RadOncs, but another is for PhDs who've published in the field. Having an abstract accepted is not enough for full membership.
RSNA membership is free for students, members get to go to the annual conference for no charge, and their conference in Chicago every year accepts a high percentage of abstracts, so get that one.
AACR membership for students is 250 bucks and is a waste of money, as you still get shafted on conference registration.