Let me preface this by saying that I'm not 100% positive that the below is totally correct, so someone please fix any mistakes..... But here's how I see it:
One can always check out FREIDA to see who has open slots; however, programs typically do a very poor job of keeping this info up-to-date. This seems to be especially true of whether or not programs have open slots for the upcoming cycle. There were a couple of programs (Northwestern and Wake, if memory serves) who listed open slot(s) when in fact they have none.
Once you register for ERAS, you can go to the Programs page and search Radiation Oncology by state. Searching North Carolina, for example, shows a link for UNC (they participate) and greyed-out sections for Duke and Wake, stating that they don't participate in ERAS. What they don't say is that Wake has no spots. Same for Northwestern. You really just need to phone the non-ERAS programs to find out about open spots & to obtain up-to-date apps.
Mostly, this is due to PDs who don't know what's going on in their programs. I'm currently doing an offsite rotation at Northwestern and was told that there would be 2 open slots for this cycle. At the end of the first week, the PD told me that she had miscalculated and that there were NONE....
Hope this helped....