I think the main difference is the focus on oncology in rad onc, as opposed to an image-guided proceduralist focus in IR. I don't want to paint all interventionalists with the same brush (which means I'm going to), but two talks I have recently attended dealt with cryo for breast tumors and RFA for lung tumors respectively. The speakers seemed almost willfully ignorant of basic oncologic principles such as margin status, nodal evaluation, and patterns of microscopic tumor spread, although they showed plenty of nice pictures demonstrating how they can fry/freeze the bejeezus out of a radiographic abnormality. This does not IMO equate to good cancer care.
Oncology should be thought of in a very broad sense, as multi-disciplinary care is by far the best care that can be provided. This means that surg-oncs, rad-oncs, and med-oncs need some degree of education in all three disciplines, so they can speak intelligently to their patients about treatment options which may not be under their particular umbrella. This is a fact which is likely under-appreciated by med students, but don't feel flamed because it's also underappreciated by a lot of residents and attendings who don't take care on cancer patients on a regular basis. IR, to my knowledge, does not currently have a provision in their GME for dedicated oncologic training. If they want true inclusion in multi-disciplinary cancer care, they have to include that in their education.
Let me conclude by saying that IR is a very nice field on it's own merits, and one that should be worthy of your consideration if you like imaging and procedures. And no, PDs would likely be none the wiser if you didn't say anything, unless the rads and rad onc PDs at a particular institution are pals who like to talk a lot about the applicants they meet. Just wanted to point out my take on the differences.
