Wow, all of those are really good options. I was really drawn to rads and rad onc too (before I discovered that path wasn't just autopsies.

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I think it would depend on what type of research you wanted to do. Most of the research that is going on in radiology departments is very applied such as developing digital imaging systems, etc. You wouldn't need a Ph.D. for this as you would probably get all the training you needed in your residency. Rad onc research can run the gamut all the way from basic cancer research to again very applied things. Rad onc is extremely competitive and has a very very high percentage of MD/PhDs in it (not that that is any reason to do a Ph.D.).
A Ph.D. in med physics is a very viable option - Ph.Ds actually do most of the calculations in clinical radiation oncology. Personally, I am not a huge fan of MD/MS programs because I didn't find graduate school classes to be very useful - the useful part of graduate school was struggling to produce data to get a Ph.D.