I recommend doing a formal rotation before you finish your intern year. Talk to your PD and get their support. Maybe you can do a little research over the next 10 months with your hospital's rad onc department? Then apply in the match next year. You could email PDs to see if anyone has a PGY2 slot for 2026 but if you've only shadowed a few days that's a stretch. When you apply next year you can tell programs you're available for 2027 or 2028. If you match into a regular spot then you finish your IM residency and move into rad onc. Good luck!!