Hi Cassia and welcome to PF!
This Insights Article should give you some of the answers you're looking for.
And if you haven't already you could check out the
ACPSEM web page.
I can offer a perspective as a Canadian
medical physicist, though I'm not too familiar with how things are done in Asian countries. Generally speaking, about 80% or more of practicing
medical physicists work in radiation oncology. And when it comes to treatment planning a lot can depend on local conditions. In larger centres, it's quite common to have dosimetrists do the bulk of the treatment planning with
medical physicists free to work on the more challenging cases. In smaller centres,
medical physicists may take on more of the regular planning workload. So a lot can depend on where you end up. And things can change over the course of your career too. I suspect that over the next decade or so, we're going to see a lot of treatment planning tasks become automated and the role of the person responsible for the planning will shift towards quality control. The Insights article above should give you a good idea of what radiation oncology physicists do, but just as if there's anything more specific you want to know about.
Diagnostic imaging physicists are generally responsible for ensuring that imaging equipment is performing as well as it needs to. So they perform a lot of image quality control tests as well as a lot of radiation safety testing. From what I've seen, a lot of them work independently, essentially functioning as consultants. They can also work in research and development either academically or for the bigger imaging companies. A lot of the research in this area now and in the coming years is likely to focus on things like machine learning and computer-assisted diagnosis, automated image deformation and registration, automated organ delineation, imaging dose reduction, and combined modality imaging. I would also note that MRI specialists tend to fall into their own category, just because the MR-imaging process is so different from ionizing-radiation-based modalities. The MRI specialists tend to share the field with biomedical engineers specializing in MRI.
The nuclear medicine specialists that I know specialize in PET and SPECT imaging. Day-to-day, I think a lot of the work may be quite similar to the DI guys, with the added concerns about tracking and storing unsealed sources or radiation. So there's likely a much larger radiation safety component to that field.