We've had a lot of rotating students this year asking this question...
The answer is: there is nothing you can do to prepare. You're right, nobody expects you to know anything about radiation. You can do a history and physical, and that's about all you will be able to do.
If you know what body site you're going to be on, you could read and try to memorize the NCCN guidelines to get the basics of the cancers you're going to be working on. At the medical student level you should focus on things like: what is the staging. What are the survivals like for those stages. Which stages or scenarios get surgery, chemo, and/or radiation. If you don't know what service you'll be on, or if you'll be on multiple services, this may not actually be feasible.
There are three things I tell the rotating students.
1) Be punctual
2) Be prepared -- i.e. read up on your patients beforehand. When you see the patient, be thorough. You'd rather be too slow or give presentations that are too lengthy, rather than them thinking you are stupid or lazy. Hopefully your residents can guide you in this regard.
3) Most importantly, be affable. Program directors want residents they think will do the most, learn the most without actually being taught, and will complain the least when there are problems.
Between the NCCN guidelines, the wikibooks, and the blue book (Hansen & Roach), you will have all the reference materials you will ever need as a medical student.
