What happens if we match in TY.. but not the specialty [Rad Onc]? Since I read somewhere that only a few specialties accept TY.. since the specialties are pretty competitive
The only way this will happen if if you don't match into Rad Onc and you DID rank a prelim year (IM, surgery, TY) program on your primary match list. The match really only "cares" about your primary match list, and it matches you based on that. If you match to say, your third choice, it will then, and only then, look at your supplemental list for that third choice, and go through a secondary match for the preliminary year, based on how you ranked prelims for choice #3. You can use the same supplemental list for every program, or you can rank prelims separate based on each program.
If you didn't list enough prelims in your supplemental list or you do and you just don't match into any, then you're stuck because you already matched into your primary specialty--the match won't look at any of your other supplemental lists. So you would then need to scramble for a prelim-year.
If you're concerned about matching, and not willing to apply to back-up programs (which for Rad Onc is probably just about any other specialty...), then you would want to add prelim positions at the end of your primary match list (after ALL of your rad-onc positions). That way, you hopefully still match somewhere, albeit just for your intern year.
However, if you do this and don't match into Rad Onc but match into a prelim year, you will be unable to scramble into any categorical programs (I'm unsure if Rad Onc has any)--you can only scramble into advanced programs. This also means that you would be ineligible to scramble into almost any other specialty if you decided to switch, as most other specialties are categorical.
Because of this, I wouldn't recommend ranking prelims on your primary list--the odds are if you don't match Rad Onc this year, you won't next year, so you'd want the opportunity to scramble into other specialties.
Regarding your last point--to my understanding every advanced program will accept a TY. The only thing is some have specific requirements--I think neurology, if they do a TY, has to do 8 months of medicine (so they might as well apply to prelim-IM programs). PM&R on the other hand, doesn't have any limits beyond the ACGME's TY requirements, which every program meets.
