There seems to be a misconception among pre-meds and med students that a PhD in Physics is the "best" way to go for RadOnc. Definitely not the case.
Among the current MSTP grads as residents there is a broad diversity of fields represented. Of course it would be nice to be involved in oncology-related research or perhaps even radiobiology. Ultimately, it does matter to do some related work in the field but a PhD is a PhD -- particularly if you have good pubs.
EDIT: One interesting thing I've noticed on the trail -- ERAS does not distinguish b/w full-length articles and abstracts. All of them are simply listed under the heading of "publications." Some attendings don't seem to recognize this. I've been told "you sure have a lot of publications" even though many were abstracts.