I believe there are a few actually, but I can't remember which ones I had seen. Here are a couple of things you could do. First, a lot of programs accept applications through VSAS. Using VSAS, you can sort the externships by length (2-4 weeks). Secondly, you can contact programs that are not on VSAS and find out through their Student Affairs (usually, since Rad Onc departments typically don't know) whether the institution offers 2-week electives. I was trying to do one of these 2-weekers actually, and I found that many institutions wouldn't even allow it - minimum 4 weeks. Someone else here may be able to name a couple places for you tho... Good luck!