At my institution, residents are typically called upon to teach the radiation oncology lectures to the second year medical students - I had that opportunity recently and thought it was a lot of fun. Granted this is only 2 hours of instruction, but the process of preparing a syllabus, writing exam questions and lecturing is nonetheless valuable. Many medical schools do not offer lectures in radonc - if you end up going to an institution where that is the case, I imagine you may have the opportunity if you express interest and can substantiate a need for the curriculum.