MD/PhDs are not immune to burnout and likely suffer from it in larger proportions than MD-"only" counterparts and choose to exit the rat race. That, coupled with the fact MD/PhDs are older and more often subject to family pressures/obligations, it's easy to see how PP is often their destination.
I don't like the phrase "burn out" because I don't feel like it reflects reality. There simply aren't enough positions out there for all the MD/PhDs in radiation oncology to continue in research that would require PhD-level training. The ones who are getting those positions nowadays are a combination of lucky, persistent, and flexible.
You need to be lucky enough to have any opportunity to pursue the research you believe in.
You need to be persistent to get through high levels of uncertainty, rejection, and often the requirement for a temporary, low paying position such as in a fellowship in the *hope* that you will someday get a real research position.
You need to be flexible, because these positions are uncommon, and that may mean moving *anywhere*. Can your family relocate *anywhere* ? How long do you feel comfortable not only going *anywhere* but also living on resident level salaries? There aren't many people who meet that kind of threshold.
There aren't even unlimited fellowship opportunities out there for future physician-scientists, even if every MD/PhD was required to do a fellowship to pursue significant research careers. I constantly hear that you need to do a fellowship to go out and get funding. A lot of departments will tell you "we don't hire physician-scientists without funding". Yeah, except my buddy applied for a K grant and the study section SRO told him, off the record of course, that they aren't even considering grants from fellows given that the funding rates are so low. The sentiment is: why should they give grants to fellows when they have assistant professors with real permanent positions and real institutional support (many of whom are in med onc...) applying for those grants? What a catch-22.
Where's the burn out here? You can't be burnt out doing a job that you can't even get.
At the end of the day, research is the basis of our specialty. I believe that it's important. I believe that my particular area of interest based on a skillset earned from years of obtaining a closely related PhD could transform how we do radiation oncology in the next 10 years. I believe that there are a lot of opportunities out there. But, everyone's heard that before. The money isn't there. Grant funding continues to decline. Clinical reimbursements are not increasing. Departments, and particularly research programs, are in survival mode. So in an ideal world, we should have lots of research that utilize all these MD/PhDs towards the goal of improving cancer cure. But in the real world, the money just isn't there to support us.
So what percentage of residents should have PhDs? I don't know, it depends who you ask. When I applied to residency, my experience was certainly that, subjectively and objectively, my PhD was not valued. I felt like the sentiments from MegaVoltagePhoton were overwhelmingly more common than the sentiments from ReOxygenation, even at the big name academic programs. At least MD/PhDs typically have no fear of pushing a few buttons in SPSS to generate the stats for their retrospective chart reviews. Hooray.