Just defended my PhD thesis in medical physics. Worked on radiation therapy treatment planning, which combines optimisation theory with the physics of Monte Carlo particle transport engines (and more macro energy deposition modeling as well) to simulate millions of different radiation dose distributions in patients and figure out which combination will lead to the right outcome based on what the radiation oncologist prescribes.
People in my field are fairly fortunate as there is a career track as a clinical medical physicist that is highly paid and pretty low stress, so most people end up going there. The work consists of maintaining and calibrating the radiation therapy machines, along with implementing new technologies in the clinic, and fixing problems that don't fall within the job description of the radiation therapists. Like what to do when a radioactive seed falls on the floor instead of going inside the patient where it's supposed to go. There's also a separate track as an imaging physicist where you maintain and QA the diagnostic imaging machines.
I'm personally doing a postdoc at the junction between optimisation, machine learning and radiation therapy. Just starting out though. Basically just extending my PhD work to automate the treatment planning process and remove the variability in treatment plan quality due to the level of experience of the people making the plans.