I'm not sure it's common but there are a couple ways to go about doing this.
First, you could "bookend" it to a clinical away rotation. Basically, you spend two months at one place, with one month designated as a "clinical" month and a second month as a "research" month. You would have to contact programs you're interested in to see if they can accommodate such an arrangement, but this would give you a longer period of time to collect/analyze data while getting to know faculty and residents. The downside is that lots of programs won't be able to get you set up for two months. You'd also have to arrange this early in your clinical aways, which most people do during the summer between third and fourth year. This would give you the greatest chance at generating something publishable in time for the application submission period (September of your 4th year).
A second approach would be to find a dedicated research elective. Again, you would want to do this early in order to have a publication or poster come out of it. This is what I did, and it worked well for me. I ended up doing an away at a major research institution at the mid-point of my MS3 year (January), and was able to submit something to ASTRO and get on a couple papers as well.
In my opinion, the part of all this that will make your life the easiest is finding a good PI/research mentor who will help you find a good project that is doable in the time you have. If people at your med school have matched into rad onc in previous years, ask them where they did their research and who was helpful. Networking goes a long way in getting involved in research.
Good luck!