Clinical Medical Physics in a Field Other than Radiotherapy
The research the medical physicsts do can vary considerably. Naturally, a lot of the research is directly related to their clinical work, and it's not difficult to see why this is. To branch out into a "non-clinical" area usually requires significant outside funding and equipment, whereas for projects directly related to radiation therapy you already have much of the equipment you need at your disposal, provided you are willing to stay late on a few nights.
There is however no rule that dictates "thall shalt perform research in field X." My experience is that as long as I get my clinical duties done, I'm basically free to pursue whatever research projects capture my interest, so long as I'm being reasonably productive.